Virtual Conference - Past Seminars

List of Previous talks

Various Topics in Condensed Matter Physics

Keshav Dani (Register here)

Okinawa Institute of Science & Technology, Japan

9 November, 11:00 IST 2021

Imaging the internal structure of the exciton

In the 1930s, Frenkel and Wannier described the existence of a composite two particle state that was made up of a negatively charged photoexcited electron bound to a hole – an exciton. As a fundamental optical excitation in semiconductors and insulators, excitons have exhibited a variety of interesting phenomena in condensed matter physics and materials systems. However, they have largely been studied using only optical techniques, which leave an important degree of freedom of the exciton inaccessible – its momentum. For decades, accessing the momentum coordinates of an exciton has been a grand challenge.

Over the past decade, we’ve developed novel time-resolved photoemission spectroscopy tools in my lab at OIST to access the momentum coordinates of the exciton in two-dimensional semiconductor structures. In this talk, I will discuss what one learns from such measurements – from being able to visualize the formation dynamics of dark excitons [1], to imaging the distribution of the electron around the hole [2], to measuring the localization of the center of mass coordinate of the exciton in the moiré potential [3].

[1] J. Madeo^*, M. K. L. Man^*, et al. Science 370, 1199 (2020).

[2] M. K. L. Man^*, J. Madeo^*, et al. Science Advances 7, eabg0192 (2021).

[3] O Karni^*, E. Barr^*, V. Pareek^*, J. D. Georgaras^*, M. K. L. Man^*, C. Sahoo^*, et al. arXiv:2108.01933 (2021)

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Arun Paramekanti

University of Toronto,

5 October, 18:30 IST 2021 (Register here)

Topology in Magnets

Topology plays an important role in many new phases discovered in condensed matter systems. In this talk, I will discuss two aspects of topology in magnetism of solids. Momentum space topology is important in understanding spin waves in ordered magnets or triplons in quantum paramagnets, and they can realize unusual Chern bands or higher order topological states with corner modes. I will discuss examples of honeycomb magnets where such physics plays a role. Real space topology enters in the characterization of magnetic spin textures such as skyrmions and skyrmion crystals. I will discuss how skyrmions impart nontrivial real space Berry phases leading to Hofstadter bands and how their signature may be indirectly seen in transport and optical probes. Time permitting, I will touch upon how quantum melting of skyrmion crystals could potentially stabilize unusual chiral quantum spin liquids with anyon excitations.

JAECK Berthold

The Hong Kong University of Science and Technology

15 Sep 2021, 11:00 IST

Visualizing emergent phenomena in quantum materials with scanning tunnelling microscopy

Tuning the complex interplay of electronic interactions, spin-spin correlations, and geometric phases of wavefunctions at microscopic length scales can endow materials with new macroscopic quantum properties. These include fractionalized quantum excitations and new forms of superconductivity, many of which promise applications such as topological quantum computation and energy-efficient electronic devices. Scanning tunneling microscopy experiments with their ability visualise the electronic and magnetic degrees of freedom atomically resolved are particularly well-suited to obtain microscopic insights on these phenomena. In this talk I will present some of our recent results on two-dimensional epitaxial topological heterostructures and moire-superlattices, where we used such measurements to establish the presence of Majorana zero modes in the topological edge state of bismuth(111) [1] and to demonstrate strong electronic correlations in the flat bands of magic-angle twisted bilayer graphene [2]. I will conclude the talk with an outlook on how (scanning) tunnelling spectroscopy experiments can be used to demonstrate the existence of gap-less Majorana modes as the fractionalized spin excitations of quantum spin liquids [3].

[1] Science 364, 1255-1259 (2019)

[2] Nature 572, 101–105 (2019)

[3] Phys. Rev. Lett. 125, 267206 (2020)

Interfacing trapped-ion quantum systems and integrated photonics

Karan K. Mehta, ETH Zürich, 2 September 2021, 17:30 IST

Abstract: Practical quantum information processing requires significant advances over current systems in error and robustness of basic operations, and in scale. The fundamental qualities of trapped atomic ion qubits are promising for long-term systems, but the optics required pose a major challenge in scaling. Interfacing low-noise atomic qubits with scalable integrated photonics [1] is a promising route forward, enabling practical extensibility while simultaneously lending robustness to noise. I will describe recent experiments utilizing ion trap devices with integrated waveguide optical delivery, which have allowed us to realize multi-ion entangling quantum logic with fidelities competitive with the highest achieved across qubit platforms [2]. I will discuss how such techniques further allow generation of optical field profiles enabling new physical operations, including recent results from our lab on ions manipulated in passively phase-stable optical standing waves, and possibilities for systems at this interface to advance future experiments in areas including sensing and precision metrology.

[1] K.K. Mehta, C.D. Bruzewicz, R. McConnell, R.J. Ram, J.M. Sage, and J. Chiaverini. “Integrated optical addressing of an ion qubit.” Nature Nanotechnology 11, 1066-1070 (2016).

[2] K.K. Mehta, C. Zhang, M. Malinowski, T.-L. Nguyen, M. Stadler, and J.P. Home. “Integrated optical multi-ion quantum logic.” Nature 586, 533-537 (2020).

Materials Modelling for Next Generation Spintronics in 2D Materials

Kapildeb Dolui, Lomare Technolgies Ltd., London, United Kingdom.

17 August 2021, 18.30 IST

The discovery of the giant magnetoresistance effect, celebrated by the 2007 Nobel Prize, has generated a revolutionary impact on the data storage technologies such as magnetoresistive random access memory (MRAM). This triggered the rise of Spintronics, an interdisciplinary field which involves the study of active

control and manipulation of spin degrees of freedom in solid-state systems. Spin-orbit-torques (SOTs), which rely on spin current generation from charge current in a nonmagnetic material, promise an energy-efficient scheme for manipulating magnetization in Spintronic devices. A critical topic for spintronic devices using

SOTs is to enhance the charge to spin conversion efficiency. In this talk, I will discuss how the different classes of 2D materials, namely transition-metal chalcogenides (CrI3, WSe2, TaSe2 etc.), topological insulators (Bi2Se3), and Weyl semimetals (WTe2, MoTe2), could emerge as viable source of current-driven

SOT [1,2,3]. Subsequently, the current-driven magnetization dynamics is studied by combining calculated complex angular dependence of SOT with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization. In addition, I will discuss the quintessence of first-principle based spin-transport calculations

for these modelling with quantum materials. In particular, I will show our development of computational methodology using noncollinear density functional theory (ncDFT) Hamiltonian combined with charge conserving Floquet-nonequilibrium Green function formalism (Floquet-NEGF) [4] and with density matrix approach [5] to quantify several spintronics phenomena such as spin-torque, spin pumping, and spin memory loss.

References

[1] J. M. Marmolejo-Tejada, et al., Nano Lett. 17, 5626 (2017).

[2] K. Dolui et al., Nano Lett. 20, 2020 (2020).

[3] K. Dolui and B. K. Nikolic, Phys. Rev. Materials 4, 104007 (2020) (Editor’s suggestion).

[4] K. Dolui, U. Bajpai, and B. K. Nikolic, Phys. Rev. Materials 4, 121201(R) (2020).

[5] K. Dolui and B. K. Nikolic, Phys. Rev. B 96, 220403 (R) (2017).

Are there Upper Bounds on the Superconducting Transition Temperature?

Mohit Randeria, Ohio State University, 18:00 IST, 03 August 2021

Understanding limits on the superconducting transition temperature Tc is a question of fundamental and practical importance. I will begin by describing developments in quantum materials and ultracold atoms that challenge conventional ideas on what controls Tc. I will then describe recent progress [1] on deriving exact upper bounds on Tc for two-dimensional (2D) systems that are valid for any pairing mechanism or strength. The bound takes a particularly simple form for parabolic dispersion in 2D: Tc cannot exceed one-eighth the Fermi temperature. This has recently been realized in gate-tuned Li:ZrNCl [2.3]. I will next present results for arbitrary multi-band systems and discuss applications to monolayer FeSe/STO and magic-angle twisted bilayer graphene. I will then turn to the problem of 2D flat bands, both trivial and topological, where the Tc bounds [4] involve the quantum geometry of electronic wave functions. Finally, I will discuss the open question of deriving bounds on Tc in 3D.

[1] T. Hazra, N. Verma, M. Randeria, Phys. Rev. X 9, 031049 (2019)

[2] Y. Nakagawa et al., Science 372, 190 (2021)

[3] M. Randeria, Science 372,132 (2021)

[4] N. Verma, T. Hazra, M. Randeria, arXiv:2103.08540; Proc. Nat. Acad. Sci. (to appear)

Topological Superconductivity in Transition Metal Dichalcogenide

Abhay Kumar Nayak, Weizmann Institute of Science, Israel, 18.30 IST, 20 July 2021

Topological superconductors are an essential resource for topological quantum computation and information processing. While signatures of topological superconductivity have been reported in heterostructures, material realizations of intrinsic topological superconductors are rare. Here, we use scanning tunneling microscopy and spectroscopy to directly image the Majorana edge modes localized to the boundaries of an intrinsic topological superconductor realized in a transition metal dichalcogenide. The edge modes exhibit crystallographic anisotropy, that together with a finite in-gap density of states throughout the 1H layers, allude to the presence of a topological nodal-point superconducting state. We also visualize zero bias states in vortex cores. Our observations are accommodated by a theoretical model of a two-dimensional nodal superconducting state, which ensues from inter-orbital Cooper pairing. The observation of an intrinsic topological nodal superconductivity in a layered material will pave the way for further studies of Majorana edge modes and its applications in quantum information processing.

PIQUE: a new framework for quantum systems engineering

Archana Kamal, University of Massachusetts Lowell, USA, 18.30 IST 06 July 2021

Generating and characterizing quantum correlations is at the heart of any quantum information processing application. These tasks, however, are particularly challenging in the presence of noise which dominates the current multi-qubit platforms. In this talk I will describe a new paradigm to overcome these challenges called PIQUE (for Parametrically-Induced QUantum Engineering), which employs strong parametrically-modulated interactions to realize critical QIP functionalities, such as high-fidelity state stabilization, high-contrast tunable readout, and quantum-limited nonreciprocal amplification. I will also discuss on how such parametric systems are also enabling new opportunities for fundamental open system studies, ranging from qubits to the cosmos.

Majorana fermions and topological superconductivity in semiconductor-superconductor heterostructures

Sumanta Tewari, Clemson University, USA

10.30 am IST, 08 June 2021

Abstract: After a decade of intensive search for Majorana fermions in semiconductor-superconductor (SM-SC) heterostructures, the field is right now at a crossroads. In this talk, I will introduce the concept of Majorana fermions - or rather Majorana zero modes or Majorana bound states - as it applies to condensed matter systems, and discuss the latest experimental and theoretical developments and future experimental prognosis in this rapidly evolving field.

Moiré Magic

Allan H. MacDonald, University of Texas at Austin,

18.00 IST 25 May 2021

Moiré patterns are ubiquitous in layered van der Waals materials and can now be fabricated with considerable control by combining mechanical exfoliation of van der Waals layers with tear and stack device fabrication techniques. I will explain why the electronic and optical properties of two-dimensional semiconductors and semimetals are strongly altered in long-period moiré superlattices, focusing in particular on the remarkable example of twisted bilayer graphene. When twisted to a magic [1] relative orientation angle near 1 degree, the typical velocity of an electron slows by many orders of magnitude. Experimental studies [2] of magic-angle twisted bilayer graphene (MATBG) have demonstrated that the electronic ground state under these circumstances can be a superconductor, a metal, or an insulator, depending on the filling of the magic angle flat bands. In some cases, the insulating states are purely orbital ferromagnets that exhibit a quantum anomalous Hall effect, and have superlattice bands with non-zero topological Chern indices C. I will discuss some of the progress [3,4] that has been made toward understanding these remarkable properties.

[1] Moire bands in twisted double-layer graphene, R. Bistritzer and A.H. MacDonald, PNAS 108, 12233 (2011).

[2] Magic-angle graphene superlattices: a new platform for unconventional superconductivity, Y. Cao et al. Nature (2018).

[3] Graphene bilayers with a twist, E.Y. Andrei and A.H. MacDonald, Nature Materials 19, 1265 (2020).

[4] The marvels of moiré materials, E.Y. Andrei et al., Nature Reviews Materials (2021).

Geometrical photon drag and bulk photovoltaic photocurrent in centrosymmetric materials

Justin Song, Nanyang Technological University, Singapore,

16.00 IST, 11 May 2021

Symmetry in a crystal, Berry curvature and geometric-phase related phenomena have an intimate relationship. For instance in the presence of time-reversal symmetry, Berry flux across a Fermi surface is zero, and anomalous Hall effects vanish. Similarly, even though geometric phases can be large in centrosymmetric crystals, inversion symmetry leads to a vanishing non-linear bulk photovoltaic effect. In this talk, I will discuss how such symmetry requirements can be circumvented. First, I will discuss how a nonlinear shift current (a type of bulk photovoltaic effect) can be activated in centrosymmetric crystals. In particular, I will discuss how non-vertical transitions (enabled by a photon drag processes) produce finite shift currents even in the presence of crystal inversion symmetry. While arising from a finite momentum transfer, such photon drag shift current is intrinsic and geometric with a magnitude controlled by a “shift-current dipole" that captures the interband geometry of a material. Strikingly, photon-drag shift current can manifest a purely transverse response underscoring its geometric phase origin. If time permits, I will also discuss other ways in which symmetry requirements in a crystal can be circumvented.

Electrical Transport Properties of Topological Materials

Chandra Shekhar, Max Planck Institute for Chemical Physics of Solids, Germany

27 April 2021 18.30 IST (Link for the talk here)

Topological materials have newly been identified as quantum materials and their properties are highlighted by topology of bands. These materials are further classified into three groups and these are topological insulator, Dirac semimetal, and Weyl semimetal. Topological insulators

(TIs) are insulating in bulk (interior) but metallic (conducting) on the surface or edge. This conducting surface originates from the inversion of bulk bands as a result of strong spin-orbit coupling. In materials, not necessarily bulk is always an insulator; if it is gapless, materials turn into a semimetal. Other two topological materials have semimetallic bulk where valence and conduction bands touch at the Fermi level at the same point or different points. Depending on whether the bands are doubly degenerate (same point) or nondegenerate (different point) at the touching point, such topological material is called a topological Dirac semimetal or a topological Weyl semimetal, respectively. Peculiar properties of topological materials indicate the existence of Majorana, Dirac, and Weyl fermions in condensed matter. My talk will cover the electrical properties of topological materials, for example, extremely large magnetoresistance and mobility, strong Berry phase induced anomalous Hall conductivity, chiral and gravitational anomalies, axion particle etc.

References:

[1] Hasan et al., Rev. Mod. Phys. 82, (2010) 3045.

[2] Shekhar et al., Phys. Rev. B 90 (2014) 165140; Phys. Rev. B 93 (2016) 241106(R).

[3] Shekhar et al., Nat. Phys. 11 (2015) 645.

[4] Arnold & Shekhar et al., Nat. Commun. 7 (2016) 11615.

[5] Liu & Shekhar et al., Nat. Phy.14 (2018)1125.

[6] Gooth & Shekhar et al., Nature 547 (2017) 324.

[7] Gooth & Shekhar et al., Nature 575 (2019) 315.

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Quasi-1D Hexagonal Chalcogenides: From Giant Optical Anisotropy to Ultralow Glassy Thermal Conductivity

Jayakanth Ravichandran, University of Southern California, USA, 13 April 2021 19.00 IST

Perovskite Chalcogenides are a new class of semiconductors with very high absorption coefficient, giant optical anisotropy and tunable band gaps in the visible to infrared energies. In this talk, I will briefly summarize advances made both in my research group and in the research community on the theory, synthesis of these materials and understanding their physical properties. Among these materials, I will discuss the unconventional properties of quasi-1D hexagonal chalcogenides with face shared transition metal – chalcogen octahedra. First, I will outline the large linear optical anisotropy in these materials in terms of birefringence, dichroism, and linear dichroism conversion properties in the mid- to long-wave infrared energies. Second, I will discuss the observation of ultra-low glassy thermal conductivity in single crystals of these materials and a possible explanation for this observation. Third, I will elucidate our efforts to understand electronic transitions arising from the quasi-1D structure, how these observations can lead to novel electronic and photonic functionalities. Finally, I will provide a general outlook for future studies and applications of these exciting new class of materials.

References:

1. Nature Communications, 11, 6039 (2020).

2. Advanced Materials, 31 (33), 1902118 (2019).

3. Nature Photonics, 12, 392-396 (2018).

4. Chemistry of Materials, 30 (15), 4897-4901 (2018).

5. Chemistry of Materials, 30 (15), 4882-4886 (2018).

Van der Waals Heterostructure Magnetic Josephson Junction

Prof. Phillip Kim, Harvard University, USA, 30 March 2021 18.30 IST

When two superconductors are connected through a ferromagnet, spin configuration of the transferred Cooper pairs can be modulated due to the exchange interaction. The resulting supercurrent can reverse its sign across the Josephson junction, depending on the thickness of the ferromagnetic weak link. In this talk, we present Josephson phase engineering in van der Waals heterostructures of atomically thin magnetic insulator Cr2Ge2Te6 sandwiched between NbSe2 van der Waals superconductors. Employing a superconducting quantum interference device based on our magnetic insulator Josephson Junctions, we reveal a doubly degenerate nontrivial Josephson phase originating from the magnetic barrier. We find that these unusual magnetic Josephson junctions are formed by momentum conserving tunneling of Ising Cooper pairs of NbSe2 across the magnetic domains of atomically thin Cr2Ge2Te6. The doubly degenerate ground states in magnetic insulator Josephson junction provide a quantum two-level system which can be utilized as a new component for superconducting quantum devices.

Ten-years-journey: From classical to quantum regime of topological surface states

Prof. Seongshik Oh, Rutgers University, USA 02 March 2021, 19:30 IST

Since the notion of topological insulator (TI) was envisioned about a decade ago, topology has become a new paradigm in condensed matter physics. Realization of topology as a generic property of materials has led to numerous predictions of classical and quantum topological effects. Although most of the classical topological effects, directly resulting from the presence of the spin-momentum-locked topological surface states, were experimentally confirmed soon after the discovery of TIs, topological quantum effects remained elusive. It turns out that defects, especially interfacial defects, have been the main culprit behind this impasse. With a series of interface engineering schemes, however, the density of these interface defects and the corresponding residual carrier densities have decreased by 300 times over the past ten years. Subsequently, a series of topological quantum effects such as quantized Faraday/Kerr rotations, quantum Hall effects, topological quantum phase transitions, zeroth Landau level physics etc. started to emerge. Here, I will overview this ten-years-of-journey toward the extreme quantum regime of topological surface states.

Advances in 2D Quantum Materials

Highly Tunable Junctions in Magic Angle Twisted Bilayer Graphene Tunneling Devices

Speaker: Daniel Rodan Legrain, MIT, USA Date: 26 January 2021 Time: 19.00 IST

The recent observation of superconductivity and correlated insulating states in ‘magic-angle’ twisted bilayer graphene (MATBG) featuring nearly-flat bands at twist angles close to 1.1 degrees presents a highly tunable two-dimensional material platform capable of behaving as a metal, an insulator, or a superconductor. Local electrostatic control over these phases may enable the creation of versatile quantum devices that were previously not achievable in other single material platforms. In this talk, I will introduce MATBG as a new arena to investigate strongly correlated physics. I will then show how we can exploit the electrical tunability of MATBG to engineer Josephson junctions and tunneling transistors all within one material, defined solely by electrostatic gates. Our multi-gated device geometry offers complete control over the Josephson junction, with the ability to independently tune the weak link, barriers, and tunneling electrodes. Utilizing the intrinsic bandgaps of MATBG, we also demonstrate monolithic edge tunneling spectroscopy within the same MATBG devices and measure the energy spectrum of MATBG in the superconducting phase. Furthermore, by inducing a double barrier geometry, the devices can be operated as a single-electron transistor, exhibiting Coulomb blockade. These MATBG tunneling devices, with versatile functionality encompassed within a single material, may find applications in graphene-based tunable superconducting qubits, on-chip superconducting circuits, and electromagnetic sensing in next-generation quantum nanoelectronics.

Edge transport to Andreev edge transport in Graphene Quantum Hall

Speaker: Anindya Das, IISc, India

Transport through edge-channels is responsible for conduction in quantum Hall (QH) phases. The equilibration among the edges dictates the electrical and thermal transport coefficients, and its robust quantization relies on the nature of equilibration: coherent vs. incoherent process. In the first part of the talk, we will discuss the classic example of symmetry broken graphene p-n junction in the QH regime, where the equilibration is selective based on the polarization of the edge states. The quantum noise studies as a function of both p and n side filling factors reveal that the equilibration is fully tunable from incoherent to the coherent regime with the increasing number of QH edges at the p-n junction, shedding crucial insights into graphene-based electron interferometer. In the second part of the talk, we will discuss the thermal equilibration for the hole-conjugate fractional QH states having counter-propagating edge states. Our findings suggest the vanishing thermal equilibration for hole-conjugate fractional QH states in graphene. This is striking, given that, at the same time, our results for the electrical conductance indicate efficient charge equilibration. These results are pointing to a divergent thermal equilibration length in the limit of strong electrostatic interaction. Such a regime would implement a long-sought goal, namely, ballistic flow of downstream charge along with a ballistic and coherent upstream neutral mode. In the last part of the talk, we will consider the equilibration issue of the chiral QH edge states in proximity with a superconductor. Due to the Andreev processes, the hybridized electron-hole states called Andreev edge states (AESs) to carry the current at the QH-SC interface, and we experimentally show that the uantum noise generated from the junction can uniquely distinguish the AESs mediated transport from the normal quasi-particle transport in the presence of disorders and dissipations. These results on equilibration of the edge states pave the way forward to explore the exotic topological excitations in quantum devices based on new kind of quantum materials.

References: Physical Review B 98 (15), 155421 (2018) Physical review letters 121 (8), 086809 (2018) Science Advances 5 (eaaw5798), 1-5 (2019) Communications Physics 3 (171), 1-7 (2020) arXiv:2010.01090 (2020)

Spin interactions in van der Waals heterostructures

Prof. Jaroslav Fabian, University of Regensburg, Germany Date: 22 Dec 2020 18.00 IST

Two dimensional materials offer unprecedented opportunities for spintronics research. The main advantages of van der Waals heterostructures are (i) the possibility to control the spin properties of electrons electrically very efficiently by gating, and (ii) tailoring the spin properties---spin-orbit and exchange couplings---by the proximity effect. In this talk, I will present the current understanding of the spin-orbit coupling in graphene-based heterostructures, and introduce some new ideas how to turn the spin-orbit coupling on and off, and even how to swap spin-orbit and exchange interactions in ex-so-tic heterostructures which comprise strong spin-orbit as well as ferromagnetic layers [1]. Finally, I will discuss ramifications of the spin proximity effects on topological transport in graphene [2].

[1] K Zollner et al, Phys. Rev. Lett. 125, 196402 (2020) [2] P. Högl et al, Phys. Rev. Lett. 124, 136403 (2020)

Thermoelectricity in twisted bilayer graphene

Arindam Ghosh, IISc, India Date: 15 Dec 2020

Link to the recording of the talk

Twisted bilayer graphene (TBLG) is a new and versatile platform to realize effects of strong electron-electron interaction as the mis-orientation angle between the graphene lattices profoundly affects the electronic structure of the combined system. While the layers behave independently at large angles (> 5 degrees), new electronic bands emerge when the angle is decreased, including nearly flat dispersion at the magic angle of 1.1 degree that has been shown the harbor superconductivity, magnetism and other many-body phases. In addition of direct electrical transport, thermoeletric properties are also highly sensitive to electronic correlations, and often manifest in departure from the well-established Mott semiclassical framework. In this seminar I shall present measurement of thermoelectric power in TBLG over a wide range of mis-orientation angles in high quality van der Waals stacks of twisted bilayer graphene. We have shown that thermoelectricity in TBLG at large angles (> 5 degrees) is expectedly determined by independent electronic structures in the two graphene layers [1]. Even at moderate angles (~ 2 – 5 degrees), the thermoelectricity can be described by the semiclassical Mott relation [2]. At low angles (< 2 degree), however, we observed a strong departure from the semi-classical description, which is most pronounced at the half-filling of the underlying Moire lattice, and persists up to temperatures as high as 40 K. In accordance with the strong enhancement in the electronic interactions at half filling, our experiments provide a new route towards probing the novel interaction-driven effects in TBLG.

[1] P. B. S. Mahapatra et al. Nano Lett. Nano 17, 6822–6827 (2017). [2] P. B. S. Mahapatra et al. arXiv:1910.02614 Physical Review Letters (2020) (In press) [3] J. Aditya et al. arxiv:2003.02880 (under review) [4] B. Ghawri et al. 2004.12356 (to be published)

Spin on 2D Nanoelectronics

Saroj P. Dash, Chalmers University, Sweden Date: 01 December 2020 Time: 18.00 IST

Engineering 2D materials heterostructures by combining the best of different layers in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, we demonstrated electronic creation, transport, and control of spin polarization in 2D materials heterostructures at room temperature. While large-area CVD graphene is shown to be an excellent medium for long-distance spin communicationand fabrication of spintronic circuits [1,2,3], the insulating CVD h-BN has shown a substantial tunnel spin polarization up to 65% [4]. Furthermore, by inducing spin-orbit coupling and spin absorption effects, we demonstrated an electrical gate control of spin-polarization and spin lifetime in graphene/MoS₂ heterostructures [5]. The induction of proximity induced spin-orbit coupling and magnetic exchange interactions in graphene can provide a new electronic state of mater. Recently, we combined graphene with topological insulators (TIs) in van der Waals heterostructures to demonstrate the emergence of a strong proximity-induced spin-orbit coupling in graphene [6], consequently giving rise to a giant and gate-tunable spin galvanic effects at room temperature [7]. Using graphene in heterostructure with a layered magnetic insulator CrGeTe₃, we also demonstrated proximity induced magnetic exchange interaction in graphene [8]. The electrical creation of spin polarization in topological materials is promising for applications in spin-orbit and quantum technologies. By utilizing the electronic band structures of the topological Weyl semimetals and Rashba spin-orbit materials, we demonstrated significant current-induced spin polarization up to room temperature. We reported a both conventional and unconventional charge-spin conversion effects in Weyl semimetal candidate WTe₂ [9,10] and Rashba material BiTeBr [11], and showed its application for spin injection into graphene at room temperature. These findings demonstrate all-electrical spintronic devices at room temperature in van der Waals heterostructure, which can be essential building blocks in future device architectures.

[1] Nature Communications 6, 6766 (2015). [2] Carbon 161, 892-899 (2020). [3] ACS Nano (2020), https://doi.org/10.1021/acsnano.0c07163 [4] Scientific Reports 6, 21168 (2016). [5] Nature Communications 8, 16093 (2017). [6] Science Advances 4:eaat9349 (2018). [7] Nature Communication 11, 3657 (2020). [8] 2D Materials 7 015026 (2020). [9] Physical Review Research 2 (1), 013286 (2020). [10] Advanced Materials, 2000818 (2020). [11] Nano Letters 20, 7, 4782–4791 (2020).

Superconductivity and Magnetism in van der Waals Materials

Mandar M. Deshmukh, TIFR-Mumbai, India Date: 24 November 2020 Time: 18.00 IST

Starting with graphene, 2D materials have been of interest to electronics and optoelectronics. There is a renewed interest in topological, magnetic, and superconducting properties of 2D materials in the last few years as the systems offer unique experimental knobs. I will discuss two examples studied in our lab. High-temperature superconductors (HTS) are essential for potential applications and for understanding the origin of strong correlations. BSCCO, a van der Waals high-temperature superconducting material (Tc of 85 K), offers a platform to probe the physics down to a unit-cell. Modifying superconductivity in HTS locally is of immense interest for superconducting electronics on a small length scale. We develop a simple route to modify superconductivity locally; this opens up avenues for new superconducting devices [1]. Antiferromagnetic materials are intensely studied in the last several years; layered antiferromagnets offer new opportunities. Spin-waves have been studied for data storage, communication, and logic circuits. We probe standing spin-waves in van der Waals antiferromagnetic material, CrCl₃; this is the first direct observation of standing spin-wave modes with optical and acoustic flavors in a van der Waals material [2]. I will show that this system also provides unique opportunities to study hybrid quantum systems by allowing strong magnon-magnon and magnon-photon coupling.

[1] Sanat Ghosh et al. Advanced Materials 32, 2002220 (2020). doi.org/10.1002/adma.202002220 [2] Lucky N. Kapoor*, Supriya Mandal*, et al. https://arxiv.org/abs/2001.05981 (Advanced Materials, In press).

Advances in Quantum Optics and Sensing

Bridging Topological Physics and Quantum Photonics

Sunil Mittal, University of Maryland, USA Date: 03 November 2020 Time: 19.00 IST

A deeper understanding of the role of topology in condensed matter physics has led to a new paradigm of topological phases of matter, for example, the topological insulators. While insulating in their bulk, these systems host conducting states at their boundaries that are chiral and, therefore, robust against disorders. Spurred by this chirality and robustness, photonic systems have recently emerged as an attractive platform for simulations of various condensed matter topological model systems, for example, the integer quantum Hall effect. On one hand, photonic simulations allow for investigations of exotic topological models that are either challenging for fermionic systems or unique to bosonic systems. On the other hand, topological features such as chirality provide a novel and robust route to manipulate the photonic mode structure and quantum photonic processes.

In this talk, I will present two examples of such synergy between topological physics and quantum photonics. First, I will present a topological source of quantum light where topological boundary states are used for robust engineering of the quantum correlations between generated photons. Second, I will present a photonic realization of the quadrupole topological phases that belong to the recently discovered category of higher-order topological insulators. Finally, I will discuss the prospects of generating novel topological phases and quantum states of light.

Integrated Photonic Technologies for Quantum Information Science

Galan Moody, U. C. Santa Barbara, USA

In the last decade, the field of integrated quantum photonics has advanced from devices with just a few photonic components to reconfigurable circuits with nearly 1,000 components that are capable of chip-to-chip quantum teleportation, information processing with multiple qubits, and quantum communications. The rapid and sustained progress in quantum photonics has been driven by innovations in chip-scale quantum light sources and detectors. It this talk, I will discuss two new promising platforms for quantum light generation based on AlGaAs-on-insulator photonic technology and nano-engineered 2D materials. I will also provide an outlook on future directions for all-on-chip quantum photonics combining our sources, reconfigurable photonic circuits, and ultra-fast, ultra-efficient single-photon detectors.

“Quantum sensor” Magnetic Resonance

Ashok Ajoy, U. C. Berkeley, USA

Nuclear magnetic resonance (NMR) spectroscopy, is renowned for its high chemical specificity, but suffers from low sensitivity and poor spatial resolution. This has largely locked up NMR in “central facilities”, where the measurement paradigm involves taking the sample to the NMR spectrometer. We are innovating a new class of optical NMR probes that can allow one to invert this paradigm, effectively bringing the NMR spectrometer into the sample. This would open possibilities for NMR probes of analytes and materials in their local environment. Our “deployable” NMR sensors rely on a uniquely optically addressable spin platform constructed out of nanoparticles of diamonds, hosting defect centers (NV centers) and 13C nuclei. These electron-nuclear spin hybrids serve dual-roles as optical “polarization injectors” and optical NMR detectors while also being targetable to within the sample of interest. I will focus on the main ingredients of this technology, while alluding to potential frontier applications opened as a result.

The Versatile Electromagnetically Induced Transparency (EIT) Effect:

Fundamentals and Applications

Andal Narayanan, Raman Research Institute, India

The interaction between atoms and photons is inherently quantum mechanical. This is often masked by many atom bulk-effects and high average-photon-number electromagnetic waves. In this talk I will explain the manifestly quantum atom-photon interaction effect called electromagnetically induced transparency (EIT) effect. EIT is the result of atoms being shelved in coherent population-trapped dark states as a result of a specific interaction with electromagnetic waves. I will highlight the far reaching fundamental advances and technological applications made possible by this remarkable effect. These range from quantum engineering of atomic and light states, use of atomic superposition states as quantum memory, production of slow-light and inducing few-atom few-photon non-linearities.

Advances in Quantum Materials

The Unprecedented Thermoelectric Properties of Nodal Semimetals in a Magnetic Field

Brian Skinner, Ohio State University, USA

Date: 07 July 2020 Time: 19.00 IST

Abstract: The thermoelectric effect is a phenomenon in which a temperature difference applied to a conducting material induces a voltage difference. This effect has a range of important applications, since it allows one to convert waste heat into useful electric power. In conventional metals and semiconductors, however, the strength of the thermoelectric effect faces fundamental limitations. In this talk I consider whether these same limitations apply to the three-dimensional nodal semimetals. I show that, surprisingly, the electron-hole symmetry of nodal semimetals allows for a thermopower that grows without bound under the application of a strong magnetic field. This nonsaturating thermopower can be understood in terms of quantum Hall-like edge states, and the corresponding thermoelectric Hall conductivity achieves a universal plateau value at large magnetic field. These effects have been observed experimentally, and they may enable the development of thermoelectric devices with record efficiency.

In pursuit of Higher Order - Hinge Modes a Superconductor

Kenneth S. Burch, Boston College, USA

Date: 14 July 2020 Time: 18.00 IST

Abstract: Combining topology and superconductivity provides a powerful tool for investigating fundamental physics as well as a route to fault-tolerant quantum computing. I will discuss the mounting evidence that the Fe-Based superconductor FeTe0.55Se0.45 (FTS) is also topologically non-trivial. Additionally, I will review our recent results demonstrating the presence of Helical Hinge Zero Modes (HHZM) using exfoliated heterostructures. Time permitting I will briefly touch on other recent advances in our lab including sensing bacteria with graphene, nonlinear responses in Weyl semimetals or a new 2D atomic crystalline acceptor.

Quantum Anomalous Hall Effect in Magnetic Topological Insulator Thin Films

Cui-Zu Chang, Penn State University, USA

Date: 21 July 2020 Time: 19.00 IST

The quantum anomalous Hall (QAH) effect can be considered as the quantum Hall (QH) effect without an external magnetic field, which can be realized by time-reversal symmetry breaking in a topologically non-trivial system [1, 2]. A QAH system carries spin-polarized dissipationless chiral edge transport channels without the need for external energy input, hence may have a huge impact on future electronic and spintronic device applications for ultralow-power consumption. The many decades quest for the experimental realization of QAH phenomenon became a possibility in 2006 with the discovery of topological insulators (TIs). In 2013, the QAH effect was observed in thin films of Cr-doped TI for the first time [3]. Two years later in a near-ideal system, V-doped TI, contrary to the negative prediction from first principle calculations [2], a high-precision QAH quantization with more robust magnetization and a perfectly dissipationless chiral current flow was demonstrated [4]. In this talk, I will also talk about our recent progress on QAH sandwich heterostructures from the axion insulator physics [5] to the concurrence of the QAH and topological Hall effects [6] and the QAH-superconductor devices about the absence of evidence for chiral Majorana fermion excitations [7].

[1] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988). [2] R. Yu et al, Science 329, 61 (2010). [3] C.-Z. Chang et al, Science 340, 167(2013). [4] C.-Z. Chang et al, Nat. Mater. 14, 473(2015). [5] D. Xiao et al, Phys. Rev. Lett. 120, 056801 (2018). [6] J. Jiang et al, Nat. Mater.19,732(2020) [7] M. Kayyalha et al, Science 367, 64(2020)

VCS Special Seminar on 28 July 2020, 19.30 IST

Prof. Bertrand Halperin, Harvard University, USA

Quantum Hall Physics in Coulomb-Coupled Bilayers

Abstract: The subject of “quantum Hall effects” refers to a wide range of peculiar phenomena that occur in two-dimensional electron systems in strong magnetic fields at low temperatures. Perhaps the most spectacular of these phenomena are the fractional quantized Hall states, first seen experimentally in 1982. Understanding these phenomena has required the introduction of radically new theoretical concepts, describing how electron-electron interactions can lead to novel states of matter.

Recent experiments on a system with two parallel layers of graphene separated by a thin insulator have revealed a new set of fractional quantized Hall states, in which the Coulomb interaction between electrons in different layers plays a crucial role, along with interactions between electrons in the same layer. The talk will review the phenomenology and theory of fractional quantized Hall effects in single layer systems as well as the new results for Coulomb-coupled double layers obtained by members of Philip Kim’s group at Harvard and Cory Dean’s group at Columbia. I will also discuss the interlayer-coherent integer quantized Hall state, first seen in GaAs bilayer structures, and more recently studied in graphene double layers.

N-type and P-type Ferromagnetic Semiconductors:

A Viable Platform for Topological Quantum Computing

Speaker: Le Duc Anh, University of Tokyo, Japan

Ferromagnetic semiconductors (FMSs) have been drawing a lot of attention for their promising applications to spin-based electronics. These materials, usually made by doping a certain amount of magnetic dopants into conventional semiconductors, are known to exhibit carrier-induced ferromagnetism, which can be tuned effectively via controlling the carrier density using a gate voltage [1]. With the recent surge of interest on Majorana physics in one- and two-dimensional semiconductor structures [2], FMS materials with strong spin-orbit interaction and a well-controlled spontaneous spin-splitting band structure are strongly desired, as they enables a magnetic-field-free control of the topological states for scalable topological quantum computing. In this study, I present a new class of FMSs with high Curie temperature (T_C), Fe-based narrow-gap III-V FMSs. Using low-temperature molecular beam epitaxy, we have successfully grown both p-type FMSs [(Ga,Fe)Sb [3], (Al,Fe)Sb [4]] and n-type FMSs [(In,Fe)As [5], (In,Fe)Sb [6]]. The most notable feature in these Fe-based FMSs is a tendency that T_C is higher as the bandgap is narrower, which enables strong ferromagnetism in narrow-gap semiconductors that possess strong spin-orbit interaction, such as InAs and InSb. With these merits, the Fe-based narrow-gap FMSs are promising materials for studying Majorana physics. In this seminar, I will also present our recent results on proximity superconductivity induced in n-type FMS (In,Fe)As [7], and our findings on a new magnetoresistance in heterostructures of InAs/p-type FMS (Ga,Fe)Sb, which we call proximity magnetoresistance (PMR) [8].

[1] H. Ohno et al., Nature 408, 944–946 (2000). [2] J. Alicea, Rep. Prog. Phys. 75, 076501 (2012). [3] N. T. Tu et al., Appl. Phys. Lett. 108, 192401 (2016). [4] L. D. Anh et al., Appl. Phys. Lett. 107, 232405 (2015). [5] P. N. Hai, L. D. Anh et al., Appl. Phys. Lett. 101, 182403 (2012). [6] N. T. Tu, L. D. Anh et al., APEX 11, 063005 (2018). [7] T. Nakamura, L. D. Anh et al., Phys. Rev. Lett. 122, 107001 (2019). [8] K. Takiguchi, L. D. Anh et al., Nat. Phys. 15, 1134–1139 (2019).

Topological protection of Weyl fermions visualized on the atomic scale

Speaker: Haim Beidenkopf, Weizmann Institute of Science, Israel

Date: 11 August 2020 Time: 18.00 IST

Abstract: Topological electronic materials host exotic boundary modes, that cannot be realized as standalone states, but only at the boundaries of a topologically classified bulk. Topological Weyl semimetals, whose bulk electrons exhibit chiral Weyl-like dispersion, host Fermi-arc states on their surfaces. The Fermi-arc surface bands disperse along open momentum contours terminating at the surface projections of bulk Weyl nodes with opposite chirality. Such reduction of the surface degrees of freedom by their segregation to opposite surfaces of the sample, that reoccurs in all topological states of matter and even exhibited by topological defects [1], provides topological protection from their surface elimination. We have confirmed the Weyl topological classification of both the inversion symmetry broken compound TaAs [2] and the time reversal symmetry broken Co₃Sn₂S₂ [3] by spectroscopic visualization of their Fermi-arc surface states through the interference patterns those electrons embed in the local density of states. This has allowed us to examine their unique nature and level of protection against perturbations. In TaAs the Fermi arc bands are found to be much less affected by the surface potential compared to trivial bands that also exist on its surfaces. In contrast, in Co₃Sn₂S₂ the dispersion of the topological Fermi-arc bands, and even their inter-Weyl node connectivity, are found to vary with the surface termination. A possible resolution of this discrepancy will be discussed.

[1] Abhay Kumar Nayak et al, “Resolving the Topological Classification of Bismuth with Topological Defects” Science Advances 5, eaax6996 (2019) [2] Rajib Batabyal et al, “Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions” Science Advances 2, e1600709 (2016) [3] Noam Morali et al, “Fermi-arc diversity on surface terminations of the magnetic Weyl semimetal Co₃Sn₂S₂”Science 365, 1286 (2019)

Dirac fermions and flat bands in kagome lattice metals

Speaker: Linda Ye, Massachusetts Institute of Technology, USA

Date: 18 August 2020 Time: 19.00 IST

Long known as a platform supporting quantum spin liquid states in the context of magnetic frustration, the kagome lattice is also theoretically anticipated to host Dirac fermions and flat bands with non-trivial topology in its electronic structure. In this talk, I will present our recent collaborative efforts on experimentally establishing the relevance of these exotic band features in a class of transition metal intermetallic compounds termed the “kagome metals”. I will introduce the signatures of bulk massive Dirac fermions in the ferromagnetic kagome metal Fe₃Sn₂ in anomalous transport, photoemission and de Haas-van Alphen effects [1,2]. In addition, I will describe our search of flat bands in structurally related kagome metals FeSn [3] and CoSn [4] and discuss the perspectives of future design of topological electronic states using the kagome lattice.

[1] L. Ye, M. Kang et al., Nature 555, 638-642 (2018). [2] L. Ye et al., Nat. Commun. 10, 4870 (2019).

[3] M. Kang, L. Ye et al., Nat. Mater. 19, 163-169 (2020). [4] M. Kang et al., arXiv/2002.01452

Superconducting Devices and Circuits

R. Vijayaraghavan, TIFR, India

Title: Quantum information processing with multi-modal superconducting circuits

Date: 05 May 2020 Time: 18.00 IST

Abstract: Quantum computers bring in extraordinary capabilities at solving certain problems by taking classically inaccessible paths. A practical quantum computer will need a large number of qubits with good coherence and high fidelity control and measurement. In the superconducting circuit architecture, the transmon qubit is the most popular qubit design and is constructed as a single moderately anharmonic oscillator. In this talk, I will introduce a novel multi-modal circuit nicknamed trimon which implements a novel three-qubit circuit with always-on, all-to-all, longitudinal coupling [1]. This allows simple implementation of high fidelity three-qubit CCNOT gates. We demonstrate the universal programmability of this processor by implementing three-qubit versions of the Grover’s and period finding algorithm [2]. I will also discuss how to construct larger processors using the trimon as a three-qubit building block by adapting the well-known cross-resonance gate [3].

References:

[1] Tanay Roy et al. Phys. Rev. Applied 7, 054025 (May 2017)

[2] Tanay Roy et al., ArXiv:1809.00668 (2018)

[3] Sumeru Hazra et al., Appl. Phys. Lett. 116, 152601 (2020)

Speaker: Uri Vool, Harvard University, USA

Title: Engineering artificial atoms with superconducting circuits

Date: 19 May 2020 Time: 18.30 IST

Abstract: Coherent quantum effects are the hallmark of atomic systems. The field of circuit quantum electrodynamics also allows for the control of coherent quantum systems. However, these quantum states do no correspond to atomic degrees of freedom, but to the quantized behavior of the electromagnetic field in a macroscopic superconducting circuit. These “artificial atoms” simulate many of the effects in atomic systems, with the added benefits of tunability and fast control and measurement.

This talk will review the different artificial atoms and quantum operations accessible to us using superconducting circuits, and the techniques we can use to create more interesting and complex atoms. We will discuss a method to break selection rules in superconducting circuits by using nonlinear coupling. We use this method to drive forbidden transitions in the fluxonium artificial atom and create a Lambda-type system within it. We demonstrate coherent manipulation of the fluxonium artificial atom at its sweet spot by stimulated Raman transitions. This type of transition enables the creation of new quantum operations, such as the control and readout of physically-protected artificial atoms.

Speaker: Vibhor Singh, Indian Institute of Science, India

Title: Electromechanical System with a Transmon Qubit

Date: 02 June 2020 Time: 18.00 IST

Abstract: Control over the quantum states of a massive oscillator is important for several technological applications and to test the fundamental limits of quantum mechanics. Recently, hybrid electromechanical systems using superconducting qubits, based on electric- charge mediated coupling, have been quite successful in this regard. In this talk, I shall provide a quick overview on the state of the art experiments in this area. In addition, I shall introduce a hybrid device, consisting of a superconducting transmon qubit and a mechanical resonator coupled using the magnetic-flux. Such coupling stems from the quantum-interference of the superconducting phase across the tunnel junctions. Consequently, thermal-motion of the mechanical resonator is detectable by driving the dressed-mode with mean-occupancy well below one photon. In addition, the large coupling between qubit and mechanical resonator is manifested in the observation of the Landau–Zener–Stückelberg effect.

Speaker: Adrian Lupascu, IQC-University of Waterloo, Canada

Title: Implementation of a High-fidelity Walsh-Hadamard Gate with Superconducting Qutrits

Abstract: We have implemented a Walsh-Hadamard gate, which realizes the quantum Fourier transform, in a superconducting qutrit. The qutrit is encoded in the lowest three energy levels of a capacitively shunted flux device. The device design combines high anharmonicity and long coherence times. The Walsh-Hadamard gate is implemented in an optimized way, combining two unitaries, generated by off-diagonal and diagonal Hamiltonians respectively. The gate implementation utilizes simultaneous driving of all three transitions between the three pairs of energy levels of the qutrit, one of which is implemented with a two-photon process. The gate has a duration of 35 ns and an average fidelity of 99.2%, characterized with quantum state tomography. Compensation of ac-Stark and Bloch-Siegert shifts is essential for reaching high gate fidelities. This work outlines interesting prospects for implementing qutrits and higher dimensionality qubits with superconducting devices.

Speaker: Yuval Ronen, Harvard University, USA

Title: Induced Superconductivity in Fractional Quantum Hall Edge

Date: 16 June 2020 Time: 19.00 IST

Abstract: Topological superconductors (TS) represent a phase of matter whose properties are insensitive to local perturbations. This robustness renders TS suitable for application in quantum computing. The past decade has witnessed substantial progress towards a quantum bit using Majorana modes, the well-known non-Abelian modes in TS. However, because Majoranas lack a universal logic gate set, Majorana quantum bits are computationally limited. This important drawback can be overcome by parafermions, a novel set of non-Abelian modes whose array supports universal topological quantum computation. A primary route to synthesize parafermions involves inducing superconductivity in fractional quantum Hall (FQH) edge. Here we use high-quality van der Waals devices coupled to a narrow NbN which remains superconducting in the magnetic fields required for robust FQH. We find crossed Andreev reflection (CAR) across the narrow NbN that separates two counterpropagating FQH edges. Control experiments show that CAR vanishes with increasing temperature, excitation and magnetic field as expected from the theory. These results lay the groundwork for experimental parafermion research in condensed matter.

Speaker: Baladitya Suri, Indian Institute of Science, India

Title: Quantum Acoustics with Artificial Atoms

Date: 23 June 2020 Time: 18.00 IST

Abstract: Superconducting artificial atoms have been used for studying light-matter interaction, the study of "quantum optics". In this talk I will give a brief background to the quantum optics of artificial atoms and then describe a relatively recent set of experiments where these artificial atoms are probed using "sound". A new regime of quantum optics that has been realised using this platform will be discussed.

Speaker: Zlatko Minev, IBM TJ Watson, USA

Title: To Catch and Reverse a Quantum Jump Mid-flight

Date: 02 July 2020 Time: 18.00 IST

Abstract: In quantum physics, measurements can fundamentally yield discrete and random results. Emblematic of this feature is Bohr’s 1913 proposal of quantum jumps between two discrete energy levels of an atom. Experimentally, quantum jumps were first observed in an atomic ion driven by a weak deterministic force while under strong continuous energy measurement. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Despite the non-deterministic character of quantum physics, is it possible to know if a quantum jump is about to occur? Here we answer this question affirmatively: we experimentally demonstrate that the jump from the ground state to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable ‘flight’, by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the evolution of each completed jump is continuous, coherent and deterministic. We exploit these features, using real-time monitoring and feedback, to catch and reverse quantum jumps mid-flight—thus deterministically preventing their completion. Our findings, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory and should provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as the early detection of error syndromes in quantum error correction.

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