Mayolana fermions, which are defined as fermions that are themselves antiparticles, unlike Dirac fermions such as electrons, have been searched for a long time in the field of high-energy physics but have not yet been realized. In recent years, the search for Majorana fermions has shifted the stage to condensed matter, especially superconductors, where active research has been going on. In topological superconducting states with the non-trivial topology of quasiparticle wave functions, topological edge states appear at zero energy. Due to the particle-hole symmetry of the superconductor, this edge state has the characteristic "particle=antiparticle" property of a Majorana particle, and Majorana quasiparticles appear as Majorana zero modes (zero-energy states) at the surface (edge) and vortex core. Since these states are not affected by local noise, they are considered to be promising candidates for quantum information storage and fault-tolerant quantum communication. Furthermore, since these states obey non-Abelian statistics rather than Bose- or Fermi-statistics, various novel phenomena are expected to be exploited in quantum computing.

One possible way to realize topological superconductivity is to use p-wave superconductors, which are intrinsic topological superconductors. However, p-wave superconductivity is limited in the number of candidate materials and very sensitive to disorder, making it very difficult not only to confirm its topological edge state experimentally but also to use it for applications. Another way to achieve topological superconductivity is to realize s-wave superconductivity in helical spin states by placing a topological insulator or a semiconductor with a Rashba spin-splitting state under proximity to a BCS superconductor. By designing materials based on this guideline, experimental evidence for the Mayolana bound state has been reported. However, this method requires a long superconducting coherence length, which in principle precludes the use of high-temperature superconductors. In addition, it involves a complex heterostructure, which does not facilitate research development and application. The third stage of realizing topological superconductivity is a superconductor with a topologically non-trivial band structure and a topological surface state. In this talk, I will introduce the topological properties of iron-based superconductors, which are such promising candidates, revealed by laser-excited high-energy, high-momentum-resolution ARPES and spin-resolved ARPES [1,2].

Reference

[1] P. Zhang et al., Science 360, 182–186 (2018).

[2] P. Zhang et al., Nature Physics 15, 41–47 (2019).

Topological superconductors attract significant interest because they may realize Majorana qubits of the topological quantum computer. While spin-triplet superconductors are intrinsically topological, conventional s-wave superconductors also can be topological [1-5]. A scenario with spin-orbit interaction and a Zeeman field is now widely used to Majorana excitation in s-wave superconductors [3-5]. In this talk, I present an alternative scenario for topological　superconductivity in s-wave superconductors [6]. With spin-dependent dissipation, an s-wave superconductor shows an instability-induced topological phase transition. We establish the bulk-boundary correspondence intrinsic to a system with dissipation by identifying the bulk topological invariant. I also discuss an experimental realization of the system by using the spin-current injection to a quantum wire.

[1] M.Sato, Phys. Lett. B575, 126 (2003).

[2] L. Fu and C. Kane, Phys. Rev. Lett 100, 096407 (2008).

[3] M. Sato, Y. Takahashi, S. Fujimoto, Phys. Rev. Lett. 103, 020401 (2009).

[4] R. M. Lutchyn, J. D. Sau, S. Das Sarma, Phys. Rev. Lett. 105, 077001 (2010).

[5] Y. Oreg, G. Rafael, F. von Oppen, Phys. Rev. Lett. 105, 177002 (2010).

[6] N. Okuma. M. Sato, Phys. Rev. Lett. 123, 097701 (2019).

We investigate pressure and field induced superconducting states and their novel characteristics. First, we study the spin orbit Mott insulator GaTa4Se8, which belongs to lacunar spinels, and discuss the pressure induced superconductivity as a realization of a new topological phase. Applying pressure and strong Hund coupling trigger the virtual interband tunneling process and result in stabilizing a particular d-wave quintet channel. To observe such pairing, we also suggest Josephson junction transport and scanning tunneling microscope as a unique experimental signature. Second, we focus on quasi-two dimensional materials with a tilted magnetic field and study pairing mechanism in Hofstadter regime. It turns out that the van-Hove singularities of the butterfly flat bands greatly elevate the superconducting critical temperature and the quantum geometry of the Landau minibands play a crucial role, offering a new mechanism of field-enhanced superconductivity. We discuss the relevance of our results to recently discovered re-entrant superconductivity of UTe2 in strong magnetic fields.

Reference

[1] Moon Jip Park, GiBaik Sim, Min Yong Jeong, Archana Mishra, Myung Joon Han and SungBin Lee, npj Quantum materials 5, 41 (2020)

[2] Moon Jip Park, Yong Baek Kim and SungBin Lee, ArXiv 2007.16205 (2020)

The Bardeen-Cooper-Schrieffer (BCS) condensation and the Bose-Einstein condensation (BEC) are the two extreme limits of the ground state of the paired fermion systems, which are theoretically predicted to continuously connected through an intermediate regime [1]. We report the two-dimensional (2D) BCS-BEC realized in a gate-controlled superconductor, electron doped layered material ZrNCl. To observe this phenomenon, we utilized an ionic gating method, which is well known as a powerful tool to control the carrier density in a large scale and induced 2D superconductivity [2].

We have succeeded in controlling the carrier density by nearly two-orders of magnitude, and established an electronic phase diagram through the simultaneous experiments of resistivity and tunneling spectra on the ionic gating devices. We found Tc exhibits dome-like behavior, and more importantly, a wide pseudogap phase in the low doping regime. In the low carrier density limit, Tc scales as Tc/TF = 0.12, where TF is the Fermi temperature [3], which shows fair agreement with the theoretical prediction of the upper limit Tc/TF = 1/8 for the 2D BEC-BEC crossover [4, 5].

[1] M. Randeria and E. Taylor, Annu. Rev. Condens. Matter Phys. 5, 209 (2014).

[2] Y. Saito, T. Nojima and Y. Iwasa, Nat. Rev. Mater. 2, 16094 (2017). .

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

[4] S. S. Botelho and C. A. R. Sa´ de Melo, Phys. Rev. Lett. 96, 040404 (2006).

After decades of explorations, suffering from the subtle nature, whether a metallic ground state exists in a two-dimensional (2D) superconducting system is still a mystery. In high-Tc superconducting films with patterned nanopores, we reveal how quantum phase coherence evolves across bosonic superconductor-metal-insulator transitions via magneto-conductance quantum oscillations. A robust intervening anomalous metallic state characterized by both resistance and oscillation amplitude saturations in the low temperature regime is detected, which suggests that the saturation of phase coherence plays a prominent role in the formation of the anomalous metallic state (i.e. quantum metal or Bose metal). [1] In macro-size ambient-stable ultrathin crystalline PdTe2 films grown by molecular beam epitaxy, the high quality filters are used for a systematic transport study to exclude the influence from external high frequency noise. Remarkably, at ultralow temperatures, the film undergoes superconducting state and anomalous metallic state with increasing perpendicular magnetic field. [2] Furthermore, our electrical transport measurements on 4Ha-TaSe2 flakes indicate that at ultralow temperatures the anomalous metallic state can be experimentally revealed in 2D crystalline transition metal dichalcogenide superconductors as the quantum ground state by precise measurements with high-attenuation low-pass filters although at higher temperatures the metallic state induced by the external noise can be detected.[3] Our findings offer the reliable evidences on the existence of anomalous quantum metallic ground states in 2D superconducting systems and shed light on the origin of anomalous metallic states.

References:

[1] Science 366, 1505 (2019) Accompanied with a perspective paper: Science 366, 1450 (2019)

[2] Nano Letters 20, 5728 (2020) Highlighted by Editors’ Choice in Science: Science 369, 388 (2020)

[3] arXiv:2007.10843

The two-dimensional crystalline superconductors possess a variety of exotic properties [1]. For instance, their Cooper pairs can sustain a large in-plane magnetic field owning to the spin-orbital locking. Here we report two-dimensional superconductivity in few layer stanene—ultrathin gray tin (111)—with an enhanced in-plane upper critical field. The emergence of superconductivity in stanene is unexpected because bulk gray tin is non-superconductive. We found superconductivity in few-layer stanene on PbTe/Bi2Te3/Si(111) substrate grown by molecular beam epitaxy [2,3]. The superconducting properties can be modulated by varying the substrate thickness. The band structure of this system is consistent with first-principles calculations, suggesting topologically non-trivial properties. Furthermore, few-layer stanene hosts enhanced in-plane upper critical fields that greatly exceed the conventional limit. The established Ising superconductivity for transition metal dichalcogenide with broken inversion symmetry does not apply here because few-layer stanene is centrosymmetric and its electronic bands center around the Γ point. We propose a novel type of spin locking mechanism—dubbed type-II Ising pairing, which accounts for the large in-plane upper critical magnetic field in centrosymmetric superconductor with multiple degenerate orbitals [4,5].

Reference

[1] Y. Saito, T. Nojima and Y. Iwasa, Nat. Rev. Mater. 2, 16094 (2017).

[2] M. Liao#, Y. Zang#, et al. Nat. Phys. 14, 344-348 (2018).

[3] Y. Zang, et al. Adv. Funct. Mater. 28, 1802723 (2018).

[4] J. Falson, et al. Science 367, 1454 (2020).

[5] C. Wang, et al. Phys. Rev. Lett. 123, 126402 (2019).

To ensure a superconducting instability, key symmetries are required. In three dimensions, these are time-reversal and inversion. Here I discuss how these two symmetries give rise to topologically protected nodes, emphasizing nodal Bogoliubov Fermi surfaces and Weyl point nodes. I further argue that Sr2RuO4 hosts Bogoliubov Fermi surfaces [1], while UTe2 hosts Weyl nodes [2]. In UTe2, these Weyl nodes give rise to surface Fermi arc states that may account for the observation of chiral surface states.

[1] Han Gyeol Suh, H. Menke, P. M. R. Brydon, C. Timm, A. Ramires, and D. F. Agterberg, Phys. Rev. Research 2, 032023(R) (2020).

[2] I.M. Hayes, D.S. Wei, T. Metz, J. Zhang, Y.S. Eo, S. Ran, S.R. Saha, J. Collini, N.P. Butch, D.F. Agterberg, A. Kapitulnik, and J. Paglione, arXiv:2002.02539 (2020).

Recently, major progresses have been made for iron-based superconductors as connate topological superconductors in which there are topological nontrivial bands, coexisting with bands that are responsible for superconductivity. However, these studies have nothing to do with the intrinsic properties of the superconducting pairing. Here we show that different s-wave pairing superconducting states proposed for iron-based superconductors can be distinguished from their nontrivial topological properties. Thus, the presence or absence of the topological states can provide smoking-gun evidence to determine the pairing origin in these materials.

[1] Ning Hao and Jiangping Hu, Nati. Sci. Rev. 6, 227 (2019).


[2] XX Wu, WA Benalcazar, YX Li, R Thomale, CX Liu and JP Hu, Phys. Rev. X 10, 04104 (2020)

[3]. Shengshan Qin, Chen Fang, Fu-Chun Zhang, Jiangping Hu, arxiv 2106.04200 (2021)

As the spin-triplet superconductivity arises from the condensation of spinful Cooper pairs, its full characterization requires not only charge ordering, but also spin ordering. For a two-dimensional (2D) easy-plane spin-triplet superconductor, this naïvely seems to suggest the possibility of two distinct Berezinskii-Kosterlitz-Thouless (BKT) phase transitions, one in the charge sector and the other in the spin sector. However, it has been recognized that there are actually three possible BKT transitions, involving the deconfinement of, respectively, the conventional vortices, the merons and the half-quantum vortices with vorticity in both the charge and the spin current. We show how all the transitions can be characterized by the relation between the voltage drop and the spin-polarized current bias. This study reveals that, due to the hitherto unexamined transport of half-quantum vortices, there is an upper bound on the spin supercurrent in a quasi-long range ordered spin-triplet superconductor, which provides a means for half-quantum vortex detection via transport measurements and deeper understanding of fluctuation effects in superconductor-based spintronic devices.

Superconductivity has its universal origin in the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature Tc. While electron Cooper pairs in most superconductors form anti-parallel spin-singlets with total spin S = 0, they can also form parallel spin-triplet Cooper pairs with S = 1 and an odd parity wavefunction, analogous to the equal spin pairing state in the superfluid 3He. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for fault tolerant quantum computation. However, spin-triplet pairing is rare and has not been unambiguously identified in any solid state systems. Since spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations, uranium based heavy-fermion materials near a FM instability are considered ideal candidates for realizing spin-triplet superconductivity. Indeed, UTe2, which has a Tc = 1.6 K, has been identified as a strong candidate for chiral spin-triplet topological superconductor near a FM instability, although the system also exhibits antiferromagnetic (AF) spin fluctuations. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe2 is coupled with a sharp magnetic excitation at the Brillouin zone (BZ) boundary near AF order, analogous to the resonance seen in high-Tc copper oxide, iron-based, and heavy-fermion superconductors. We find that the resonance in UTe2 occurs below Tc at an energy Er = 7.9kBTc (kB is Boltzmann’s constant) and at the expense of low-energy spin fluctuations. Since the resonance has only been found in spin-singlet superconductors near an AF instability, its discovery in UTe2 suggests that AF spin fluctuations can also induce spin-triplet pairing for superconductivity.

The Kagome metal AV3Sb5 (A=K, Rb, Cs) hosts charge order, topologically nontrivial Dirac band crossings, and a superconducting ground state, providing an ideal platform to investigate the interplay between different electronic states on the Kagome lattice. In this talk, I will present our recent progresses on the studies of AV3Sb5. Measurements of the magnetic penetration depth provides the first evidence for nodeless superconductivity in CsV3Sb5 [1], which is supported by later NMR and STM experiments. On the other hand, two superconducting domes, each one being associated with a distinct charge density wave state/instability, are observed in the pressure-temperature phase diagrams of AV3Sb5 [2].

[1] W. Y. Duan et al., arXiv: 2103.11796.

[2] F. Du et al., Phys. Rev. B 103, L220504 (2021) ; unpublished results.

The temperature-dependent evolution of the Kondo lattice is a long-standing topic of theoretical and experimental investigation and yet it lacks a truly microscopic description of the relation of the basic f-d hybridization processes to the fundamental temperature scales of Kondo screening and Fermi-liquid lattice coherence. In this talk, the temperature-dependence of f-d hybridized band dispersions and Fermi-energy f spectral weight in the Kondo lattice system CeCoIn5 is investigated using first principles dynamical mean field theory (DMFT) calculations containing full realism of crystalline electric field states. All the calculated results are directly compared to f-resonant angle-resolved photoemission (ARPES). Our results reveal f participation in the Fermi surface at temperatures much higher than the lattice coherence temperature, T∗≈ 45 K, commonly believed to be the onset for such behavior. The identification of a T-dependent crystalline electric field will be discussed with its contribution to T∗ as well as local Kondo temperature TK.

[1] Sooyoung Jang, J. D. Denlinger, J. W. Allen, V. S. Zapf, M. B. Maple, Jae Nyeong Kim, Bo Gyu Jang, Ji Hoon Shim, PNAS 117, 23467 (2020).

In 2D materials with atomic-scale thickness, a combination of strong spin-orbit interactions (SOI) with broken inversion symmetry can lead to a plethora of non-trivial superconducting states. Previous studies have been mainly focused on s-wave superconductors. While it has also been theoretically suggested that such phenomena are pronounced in strongly correlated non-s-wave superconductors with strong SOI, no atomically thin-film study has been reported due to technical difficulties. Here we report an in-situ scanning tunneling microscopy study of atomically thin films of CeCoIn5, a d-wave heavy-fermion superconductor, grown with molecular beam epitaxy. Both hybridization and superconducting gaps are clearly resolved even in monolayer CeCoIn5, providing direct evidence of superconductivity of heavy-quasiparticles mediated by purely 2D spin-fluctuations. Remarkably, in these atomically thin films, while Tc is suppressed to nearly half of the bulk, the out-of-plane upper critical field at zero temperature is estimated to be about 30 T, which is about five times larger than the bulk. The enhanced upper critical field significantly exceeds the Pauli and bulk orbital limits, revealing the emergence of exotic superconductivity such as parity-mixed d+p wave state. These results demonstrate that the atomically thin film of CeCoIn5 is a new playground to search for topological superconductivity without fine tuning of the band structure.

We report the measurements of c-axis resistivity with rotating in-plane magnetic field, far-infrared reflectance and scanning tunneling microscope with spin polarized tips on the newly found Kagome superconductor CsV3Sb5. The c-axis resistivity with the in-plane rotating magnetic field shows a twofold symmetric superconductivity. Surprisingly, in the normal state, the c-axis resistivity also shows a twofold symmetry when rotating the in-plane magnetic field, but the angle dependence of these two orders are of antiphase with the in-plane angle. Our results may be explained by the C2 symmetric saddle point driven CDW state, which intimately gives influence on the gap structure of the superconducting state [1]. The optical properties of CsV3Sb5 have been examined at dense temperatures above and below TCDW. The normal-state optical conductivity can be well described by two Drude components. Combining with theoretical calculations, we attribute the narrow Drude component to the light Dirac-like bands, while the broad Drude component arises from the heavy bands near the saddle points at M. Below TCDW, the opening of the CDW gap is clearly observed. The spectral weight of the broad Drude is substantially suppressed by the gap, while the narrow Drude component remains unchanged. These observations are in favor of the saddle point driven CDW in CsV3Sb5 [2]. Finally, we will show the scanning tunneling microscope with spin polarized tips. Our preliminary results do not support the chiral flux phase which breaks the time reversal symmetry[3]. However, more detailed structure will be presented concerning the C2 symmetric CDW phase.

In collaboration with Yaomin Dai, Huan Yang, Xiaoxiang Zhou, Ying Xiang, Xinwei Fan, Huazhou Li, Siyuan Wan, et al.

[1] Xiaoxiang Zhou, Yongkai Li, Xinwei Fan, Jiahao Hao, Yaomin Dai, Zhiwei Wang, Yugui Yao, Hai-Hu Wen. arXiv: 2104.01015. PRB Letter in press.

[2] Ying Xiang, Qing Li, Yongkai Li, Wei Xie, Huan Yang, Zhiwei Wang, Yugui Yao, Hai-Hu Wen. arXiv: 2104.06909.

[3] Huazhou Li, Siyuan Wan, Huan Yang, Hai-Hu Wen, et al. To be published.

The effect of non-magnetic impurities on superconductivity is useful to characterize the superconducting pairing symmetry. S-wave superconductivity is unaffected by the presence of non-magnetic impurities. However, unconventional superconductivity responds to the non-magnetic impurities, inducing in-gap states in the superconducting gap. However, this characterization fails if non-magnetic impurities could induce magnetic moments in superconductors. In this study, we used scanning tunneling microscopy and spectroscopy to elucidate if non-magnetic impurities can affect magnetism in FeSe which is one of the Fe-based superconductors. To study this, we grew FeSe film on Pb(111) substrate. We found that FeSe films were proximity-induced s-wave superconductors. The investigation of various non-magnetic impurities and native defects of FeSe verify that these impurities and defects directly induce local magnetism in FeSe.

We present doping and pressure induced optimization of superconductivitiy, particularly focusing on tuning of electronic states in Pd-intercalated TaSe2 PdxTaSe2, in which superconducting transition is optimized from 0.14 (TaSe2) to 3.1 K (x=~0.08) with simultaneous suppression of a commensurate charge density wave (CDW) state. We found that the Pd intercalation can involve a Lifshitz transition in the underlying electronic states at normal states without CDW ground states, which seems to be useful for increasing density of states (DOS) and electron-phonon coupling and in turn two band BCS superconductivity at low temperatures, arbeit its relative importance to enhace superconductivity is not clear yet. Furthermore, an increase of DOS in the vicinity of a doping induced collapse of a commensurate CDW state can certainly help increaseing DOS and opmizing superconducting transition. Although it was not clear with intercalation, with tuning of pressure, we found that a second order quantum phase transition of a CDW transtion can occur around 22 GPa and fluctuating CDW ground states can instigate the superconductivity up to 8.3 K. With systematic Raman and transport results, we discuss how a CDW quantum crticality and related fluctuation can be involved in the process of boosting up superconductivity with pressure.

Nearly room temperature superconductivity in several new hydride materials is a hot topic in high-pressure science. The experimental evidence of high critical temperature values in these “superhydrides” is derived mostly from resistivity studies under very high pressure. However, until now the Meissner effect evidence in these new superhydride materials is very limited, which raised concerns about the validity of the resistive probes of the superconducting state and of the mechanism of the superconductivity in these new hydride materials. In our report we present an overview of the Meissner effect studies in these new superconductors, along with our measurements of magnetic susceptibility in several superconducting superhydrides. We also provide a review of essential high-pressure techniques used for magnetic susceptibility studies under high pressure conditions in a diamond anvil cell setup.

There is growing experimental evidence for novel electronic properties, such as superconductivity, of dense hydrogen-rich materials under pressure. However, it is still highly required to search for materials with superconducting critical temperature with near or even above room temperature. We here explored several candidate structures for hydrides under pressure using CALYPSO code. Electron-phonon coupling calculations predict the existence of new superconducting phases, where several compounds exhibit superconductivity in the range of room temperature. Further analysis shows that the hydrogen-dominated density of states at Fermi level in these hydrides played a critical role in improving electron-phonon coupling parameters. Moreover, the computed stabilities indicate these hydrides could be synthesized at pressures that are currently accessible in the laboratory. The results open the prospect for the design, synthesis, and recovery of new high-temperature superconductors with potential practical applications.

Topological superconductors represent a phase of matter with nonlocal properties which cannot smoothly change from one phase to another, providing a robustness suitable for quantum computing. Substantial progress has been made towards a qubit based on Majorana modes, non-Abelian anyons of Ising (Z2) topological order whose exchange—braiding—produces topologically protected logic operations. However, because braiding Ising anyons does not offer a universal quantum gate set, Majorana qubits are computationally limited. This drawback can be overcome by introducing parafermions, a novel generalized set of non-Abelian modes (Zn), an array of which supports universal topological quantum computation. The primary route to synthesize parafermions involves inducing superconductivity in the fractional quantum Hall (fqH) edge. Here we use high-quality graphene-based van der Waals devices with narrow superconducting niobium nitride (NbN) electrodes, in which superconductivity and robust fqH coexist. We find crossed Andreev reflection (CAR) across the superconductor separating two counterpropagating fqH edges which demonstrates their superconducting pairing. Our observed CAR probability of the integer edges is insensitive to magnetic field, temperature, and filling, which provides evidence for spin-orbit coupling inherited from NbN enabling the pairing of the otherwise spin-polarized edges. FqH edges notably exhibit a CAR probability higher than that of integer edges once fully developed. This fqH CAR probability remains nonzero down to our lowest accessible temperature, suggesting superconducting pairing of fractional charges. These results provide a route to realize novel topological superconducting phases with universal braiding statistics in fqH–superconductor hybrid devices based on graphene and NbN.

We report on observing direct signatures of chiral Andreev edge states (CAES) – single particle modes running along the superconductor - quantum Hall interfaces [1]. Semiclassically, the CAES can be interpreted as a result of multiple Andreev reflections of a single electron/hole skipping along the superconducting interface. Experimentally, we observe fluctuations of the signal propagating along the superconducting contacts and interpret them in terms of interference between the CAES.

[1] L. Zhao et al., Interference of chiral Andreev edge states. Nature Physics 16, 862–867 (2020).

https://doi.org/10.1038/s41567-020-0898-5

Engineering quantum states through light-matter interaction has created a new paradigm in condensed matter physics. A representative example is the Floquet-Bloch state, which is generated by time-periodically driving the Bloch wavefunctions in crystals. Previous attempts to realise such states in condensed matter systems have been limited by the transient nature of the Floquet states produced by optical pulses, which masks the universal properties of non-equilibrium physics. Here, we report the generation of steady Floquet Andreev (F-A) states in graphene Josephson junctions by continuous microwave application and direct measurement of their spectra by superconducting tunnelling spectroscopy [1]. We present quantitative analysis of the spectral characteristics of the F-A states while varying the phase difference of superconductors, temperature, microwave frequency and power. The oscillations of the F-A state spectrum with phase difference agreed with our theoretical calculations. Moreover, we confirmed the steady nature of the F-A states by establishing a sum rule of tunnelling conductance, and analysed the spectral density of Floquet states depending on Floquet interaction strength. This study provides a basis for understanding and engineering non-equilibrium quantum states in nano-devices.

[1] S. Park et al., arXiv:2105.00592 (2021)