Research Highlights

La2O3Mn2Se2: A correlated insulating layered d-wave altermagnet

Altermagnets represent a new class of magnetic phases without net magnetization, invariant under a combination of rotation and time reversal. Unlike conventional collinear antiferromagnets (AFM), altermagnets could lead to new correlated states and important material properties deriving from their nonrelativistic spin-split band structure. Indeed, they serve as the magnetic analogue of unconventional superconductors and can yield spin-polarized electrical currents in the absence of external magnetic fields, making them promising candidates for next-generation spintronics. Here, we report altermagnetism in the correlated insulator, magnetically ordered tetragonal oxychalcogenide, La2O3Mn2Se2. Symmetry analysis reveals a dx2−y2 -wave-like spin-momentum locking arising from the Mn2O Lieb lattice, supported by density functional theory (DFT) calculations. Magnetic measurements confirm the AFM transition below∼166 K while neutron pair distribution function analysis reveals a 2D short-range magnetic order that persists above the Néel temperature. Single crystals are grown and characterized using x-ray diffraction, optical and electron microscopy, and micro-Raman spectroscopy to confirm the crystal structure, stoichiometry, and uniformity. Our findings establish La2O3Mn2Se2 as a model altermagnetic system realized on a Lieb lattice.

Engineering Anomalously Large Electron Transport in Topological Semimetals

Anomalous transport of topological semimetals has generated significant interest for applications in optoelectronics, nanoscale devices, and interconnects. Understanding the origin of novel transport is crucial to engineering the desired material properties, yet their orders of magnitude higher transport than single-particle mobilities remain unexplained. This work demonstrates the dramatic mobility enhancements result from phonons primarily returning momentum to electrons due to phonon-electron dominating over phonon-phonon scattering. Proving this idea, proposed by Peierls in 1932, requires tuning electron and phonon dispersions without changing symmetry, topology, or disorder. This is achieved by combining de Haas - van Alphen (dHvA), electron transport, Raman scattering, and first-principles calculations in the topological semimetals MX2 (M=Nb, Ta and X=Ge, Si). Replacing Ge with Si brings the transport mobilities from an order magnitude larger than single particle ones to nearly balanced. This occurs without changing the crystal structure or topology and with small differences in disorder or Fermi surface. Simultaneously, Raman scattering and first-principles calculations establish phonon-electron dominated scattering only in the MGe2 compounds. Thus, this study proves that phonon-drag is crucial to the transport properties of topological semimetals and provides insight to engineer these materials further.

Topological phonons and electronic structure of Li2BaSi class of semimetals

Extension of the topological concepts to the bosonic systems has led to the prediction of topological phonons in materials. Here we discuss the topological phonons and electronic structure of Li2BaX (X = Si, Ge, Sn, and Pb) materials using first-principles theoretical modelling. A careful analysis of the phonon spectrum of Li2BaX reveals an optical mode inversion with the formation of nodal line states in the Brillouin zone. Our electronic structure results reveal a double band inversion at the Γ point with the formation of inner nodal-chain states in the absence of spin–orbit coupling (SOC). Inclusion of the SOC opens a materials-dependent gap at the band crossing points and transitions the system into a trivial insulator state. We also discuss the lattice thermal conductivity and transport properties of Li2BaX materials. Our results show that coexisting phonon and electron nontrivial topology with robust transport properties would make Li2BaX materials appealing for device applications.

Fractional spin fluctuations and quantum liquid signature in Gd2ZnIrO6

Hitherto, the discrete identification of quantum spin liquid phase, holy grail of condensed-matter physics, remains a challenging task experimentally. However, the precursor of quantum spin liquid state may reflect in the spin dynamics even in the paramagnetic phase over a wide temperature range as conjectured theoretically. Here we report comprehensive inelastic light- (Raman) scattering measurements on the Ir-based double perovskite, Gd2ZnIrO6, as a function of different incident photon energies and polarization in a broad temperature range. Our results evidence the spin fractionalization within the paramagnetic phase reflected in the emergence of a polarization-independent quasielastic peak at low energies with lowering temperature. Also, the fluctuating scat- tering amplitude measured via dynamic Raman susceptibility increases with lowering temperature and decreases mildly upon entering into long-range magnetic ordering phase, below 23 K, suggesting the magnetic origin of these quantum fluctuations. This anomalous scattering response is thus indicative of fluctuating fractional spin evincing the proximate quantum spin liquid phase in a three-dimensional double-perovskite system.

Coupling of lattice, spin, and intraconfigurational excitations of Eu3+ in Eu2ZnIrO6

The detailed lattice-dynamics study of double-perovskite Eu2ZnIrO6 via Raman scattering as a function of temperature and density functional theory-based calculations. We find significant phonon softening and anomalous linewidth narrowing/broadening of the phonon modes well above the spin-solid phase (TN  ~ 12 K) up to ~ 40 K. Our study foretells that quantum magnetic ground state of 5d iridium based double-perovskite materials is the result of intricate coupling of lattice, spin and electronic degrees of freedom. Suggesting that these degrees of freedom should be treated at par with each other to write the ground state Hamiltonian for their understanding. Estimated value of the spin-phonon coupling constant is found to be ranging from ~ 5 to 8 cm-1. The high value of second-order crystal-field parameter, ~50 meV, suggests strong J-mixing of the crystal-field split levels of Eu3+ ion. The density functional theory based calculated zone-centered phonon mode frequencies are observed to be in very good agreement with the experimentally observed values. Our lattice dynamics studies reveal the rich physics associated with optical phonons, and their coupling to electronic, and magnetic degrees of freedom in this system. 

Kramer doublets, phonons, crystal-field excitations and their coupling in Nd2ZnIrO6

We report comprehensive Raman-scattering measurements on a single crystal of double-perovskite Nd2ZnIrO6 in temperature range of 4-330 K, and spanning a broad spectral range from 20 cm-1 to 5500 cm-1. The paper focuses on lattice vibrations and electronic transitions involving Kramer’s doublets of the rare-earth Nd3+ ion with local C1 site symmetry. Temperature evolution of these quasi-particle excitations have allowed us to ascertain the intricate coupling between lattice and electronic degrees of freedom in Nd2ZnIrO6. Strong coupling between phonons and crystal-field excitation is observed via renormalization of the self-energy parameter of the phonons i.e. peak frequency and line-width. The phonon frequency shows abrupt hardening and line-width narrowing below ~ 100 K for the majority of the observed first-order phonons. We observed splitting of the lowest Kramer’s doublets of ground state ( 4I9/2 ) multiplets i.e. lifting of the Kramer’s degeneracy, prominently at low-temperature (below ~ 100 K), attributed to the Nd-Nd/Ir exchange interactions and the intricate coupling with the lattice degrees of freedom. The observed splitting is of the order of ~2-3 meV and is consistent with the estimated value. We also observed a large number of high-energy modes, 46 in total, attributed to the intra-configurational transitions between 4f3 levels of Nd3+ coupled to the phonons reflected in their anomalous temperature evolution. 

Kitaev magnetism and fractionalized excitations in Sm2ZnIrO6

The quest for Kitaev spin liquids in particular three-dimensional solids is a long sought goal in condensed matter physics, as these states may give rise to exotic new types of quasiparticle excitations carrying fractional quantum numbers, namely Majorana fermionic excitations. Here we report the experimental signature of this characteristic feature of the Kitaev spin liquid via Raman measurements. Sm2ZnIrO6 is a strongly spin-orbit-coupled Mott insulator where Jeff=1/2 controls the physics, which provides striking evidence for this characteristic feature of the Kitaev spin liquid. As the temperature is lowered, we find that the spin excitations form a continuum in contrast to the conventional sharp modes expected in ordered antiferromagnets. Our observation of a broad magnetic continuum and anomalous renormalization of the phonon self-energy parameters shows the existence of fractionalization excitations in the double-perovskite structure, as theoretically conjectured in a Kitaev-Heisenberg geometrically frustrated double-perovskite system. 

Correlated paramagnetism and interplay of magnetic and phononic degrees of freedom in 3d-5d coupled  La2CuIrO6 

Conventional paramagnetism–a state with finite magnetic moment per ion sans long range magnetic ordering, but with lowering temperature the moment on each ion picks up a particular direction, breaking spin rotational symmetry, and results into long-range magnetic ordering. However, in systems with competing multiple degrees of freedom this conventional notion may easily break and results into short range correlation much above the global magnetic transition temperature. La2CuIrO6 with complex interplay of spins (s = 1/2) on Cu site and pseudo-spin (j = 1/2) on Ir site owing to strong spin–orbit coupling provides fertile ground to observe such correlated phenomena. By a comprehensive temperature dependent Raman study, we have shown the presence of such a correlated paramagnetic state in La2CuIrO6 much above the long-range magnetic ordering temperature (TN). Our observation of strong interactions of phonons, associated with Cu/Ir octahedra, with underlying magnetic degrees of freedom mirrored in the observed Fano asymmetry, which remarkably persists as high as ~3.5TN clearly signals the existence of correlated paramagnetism hence broken spin rotational symmetry. Our detailed analysis also reveals anomalous changes in the self-energy parameters of the phonon modes, i.e., mode frequencies and linewidth, below TN, providing a useful gauge for monitoring the strong coupling between phonons and magnetic degrees of freedom.

Orbiton–phonon coupling in double perovskite Ba2YIrO6 

Ba2YIrO6, a Mott insulator, with four valence electrons in Ir5+ d-shell (5d4 ) is supposed to be non-magnetic, with Jeff = 0, within the atomic physics picture. However, recent suggestions of non-zero magnetism have raised some fundamental questions about its origin. We focus on the phonon dynamics, probed via Raman scattering, as a function of temperature and different incident photon energies, as an external perturbation. Our studies reveal strong renormalization of the phonon self-energy parameters and integrated intensity for first-order modes, especially redshift of the few first-order modes with decreasing temperature and anomalous softening of modes associated with IrO6 octahedra, as well as high energy Raman bands attributed to the strong anharmonic phonons and coupling with orbital excitations. The distinct renormalization of second-order Raman bands with respect to their first-order counterpart suggest that higher energy Raman bands have significant contribution from orbital excitations. Our observation indicates that strong anharmonic phonons coupled with electronic/orbital degrees of freedom provides a knob for tuning the conventional electronic levels for 5d-orbitals, and this may give rise to non-zero magnetism as postulated in recent theoretical calculations with rich magnetic phases. 

Iron isotope effect in Fe-based Superconductors

We report the inelastic light scattering studies on SmFeAsO0.65 and SmFeAsO0.77H0.12 with iron isotopes namely 54Fe and 57Fe. In both of these systems under investigation we observed a significant shift in the frequency of the phonon modes associated with the displacement of Fe atoms around ∼ 200 cm-1. The observed shift in the Fe mode (B1g) for SmFeAsO0.65 is ∼ 1.4 % and lower in case of SmFeAsO0.77H0.12, which is ∼ 0.65 %, attributed to the lower percentage of isotopic substitution in case of SmFeAsO0.77H0.12. Our study reveals the significant iron isotope effect in these systems hinting towards the crucial role of electron-phonon coupling in the pairing mechanism of iron based superconductors. 

Structural and Electronic Properties of LaPd2As2

In present work we have studied electronic and structural properties of superconducting LaPd2As2 compound having collapsed tetragonal structure using first-principle calculations. The band structure calculations show that the LaPd2As2 is metallic consistent with the reported experimental observation, and the density of states plots clearly shows that at the Fermi level major contribution to density of states arises from Pd 4d and As 4p states, unlike the Fe-based superconductors where major contribution at the Fermi level comes from Fe 3d states. The estimated value of electronphonon coupling is found to be ~ 0.37, which gives the upper bound of superconducting transition temperature of ~ 5K, suggesting the conventional nature of this superconductor. 

Unconventional Iron-Based Superconductor CsCa2Fe4As4F2

In the present work, we have investigated the structural and electronic properties of newly discovered iron based superconductor CsCa2Fe4As4F2 using first principles calculations. Analysis of the density of states at the Fermi level suggests that Fe-3d states have dominating contribution, and within these 3d states contribution of eg states is significant suggesting multi-band nature of this superconductor. The upper bound of superconducting transition temperature, estimated using electron-phonon coupling constant is found to be ~ 2.6 K. To produce the experimental value of transition temperature (28.2 K), a 4-5 times increase in the electron-phonon constant is necessary, hinting that conventional electron-phonon coupling is not enough to explain the origin of superconductivity. 

Density functional study of ACa2Fe4As4F2 (A = K, Rb): Electronic structure, unconventional superconductors

In this work, we have studied the structural and electronic properties of new-type of iron based superconductors ACa2Fe4As4F2 (A = K, Rb) with first principles density functional theory based calculations. The density of states clearly shows that at Fermi level major contribution comes from iron 3d-orbitals and the contribution from other elements is very small. The electron-phonon coupling constant is found to be ~0.33 and ~ 0.4 for KCa2Fe4As4F2 and RbCa2Fe4As4F2, respectively. The estimated values of the upper bound of superconducting transition temperature is found to be ~ 2.4 K and~ 4.2 K for KCa2Fe4As4F2 and RbCa2Fe4As4F2, respectively, suggesting that superconductingpairing mechanism in these systems have unconventional origin. Band structure calculations shows that ten bands crossesthe Fermi level and the corresponding Fermi surfaces show significant nesting with both hole-like and electron-like pockets.