arXiv subjects

Tanmoy Das

Publications and source records attributed to Tanmoy Das.

At least 73 records · Page 4Linked to original sources

arXiv2016

A pedagogic review on designing model topological insulators

Following the centuries old concept of the quantization of flux through a Gaussian curvature (Euler characteristic) and its successive dispersal into various condensed matter properties such as quantum Hall effect, and topological invariants, we can establish a simple and fairly universal understanding of various modern topological insulators (TIs). Formation of a periodic lattice (which is a non-trivial Gaussian curvature) of 'cyclotron orbits' with applied magnetic field, or 'chiral orbits' with fictitious 'momentum space magnetic field' (Berry curvature) guarantees its flux quantization, and thus integer quantum Hall (IQH), and quantum spin-Hall (QSH) insulators, respectively, occur. The bulk-boundary correspondence associated with all classes of TIs dictates that some sort of pumping or polarization of a 'quantity' at the boundary must be associated with the flux quantization or topological invariant in the bulk. Unlike charge or spin polarizations at the edge for IQH and QSH states, the time-reversal (TR) invariant Z2 TIs pump a mathematical quantity called 'TR polarization' to the surface. This requires that the valence electron's wavefunction (say, ψ_{\uparrow} (k)) switches to its TR conjugate (ψ_{\downarrow}^* (-k)) odd number of times in half of the Brillouin zone. These two universal features can be considered as 'targets' to design and predict various TIs. For example, we demonstrate that when two adjacent atomic chains or layers are assembled with opposite spin-orbit coupling (SOC), serving as the TR partner to each other, the system naturally becomes a Z2 TI. This review delivers a holistic overview on various concepts, computational schemes, and engineering principles of TIs.

cond-mat.mes-hall↗

arXiv2016

Colloquium: Topological Band Theory

The first-principles band theory paradigm has been a key player not only in the process of discovering new classes of topologically interesting materials, but also for identifying salient characteristics of topological states, enabling direct and sharpened confrontation between theory and experiment. We begin this review by discussing underpinnings of the topological band theory, which basically involves a layer of analysis and interpretation for assessing topological properties of band structures beyond the standard band theory construct. Methods for evaluating topological invariants are delineated, including crystals without inversion symmetry and interacting systems. The extent to which theoretically predicted properties and protections of topological states have been verified experimentally is discussed, including work on topological crystalline insulators, disorder/interaction driven topological insulators (TIs), topological superconductors, Weyl semimetal phases, and topological phase transitions. Successful strategies for new materials discovery process are outlined. A comprehensive survey of currently predicted 2D and 3D topological materials is provided. This includes binary, ternary and quaternary compounds, transition metal and f-electron materials, Weyl and 3D Dirac semimetals, complex oxides, organometallics, skutterudites and antiperovskites. Also included is the emerging area of 2D atomically thin films beyond graphene of various elements and their alloys, functional thin films, multilayer systems, and ultra-thin films of 3D TIs, all of which hold exciting promise of wide-ranging applications. We conclude by giving a perspective on research directions where further work will broadly benefit the topological materials field.

cond-mat.mes-hall↗

arXiv2016

Quantum oscillations from `open' Fermi surface in quasi-one-dimensional lattices: Application to YBa2Cu3O6 cuprates, organic salts, ladder compounds, and related systems

According to the celebrated Onsagar-Lifshitz paradigm, the observation of Shubnikov de-Haas and de-Haas van Alphen (SdHvA) oscillations is an indication of the presence of `closed' orbit Fermi surface in the bulk. We present a real-space based calculation of SdHvA oscillations in generalized quasi-one-dimensional lattices by relaxing the quasi-classical approximations embedded in this decades old Onsagar-Lifshitz paradigm. We find that sizable quantum oscillation can arise from `open' Fermi surfaces as long as cyclotron orbits can form in real-space with finite, but not necessarily equal, electron hopping along both x- and y-directions. Our results quantitatively explain the puzzling emergence of SdHvA oscillation in various quasi-one-dimensional materials, including the chain state of YBa2Cu3O6 cuprates, organic materials, various ladder compounds, weakly coupled linear chains, or quantum wires, and other related systems.

cond-mat.supr-con↗

arXiv2015

Observation of correlated spin-orbit order in a strongly anisotropic quantum wire system

Quantum wires with spin-orbit coupling provide a unique opportunity to simultaneously control the coupling strength and the screened Coulomb interactions where new exotic phases of matter can be explored. Here we report on the observation of an exotic spin-orbit density wave in Pb-atomic wires on Si(557) surfaces by mapping out the evolution of the modulated spin-texture at various conditions with spin- and angle-resolved photoelectron spectroscopy. The results are independently quantified by surface transport measurements. The spin polarization, coherence length, spin dephasing rate, and the associated quasiparticle gap decrease simultaneously as the screened Coulomb interaction decreases with increasing excess coverage, providing a new mechanism for generating and manipulating a spin-orbit entanglement effect via electronic interaction. Despite clear evidence of spontaneous spin-rotation symmetry breaking and modulation of spin-momentum structure as a function of excess coverage, the average spin-polarization over the Brillouin zone vanishes, indicating that time-reversal symmetry is intact as theoretically predicted.

cond-mat.str-el↗

arXiv2015

Tuning the metal-insulator transition in NdNiO3 heterostructures via Fermi surface instability and spin-fluctuations

We employed {\it in-situ} pulsed laser deposition (PLD) and angle-resolved photoemission spectroscopy (ARPES) to investigate the mechanism of the metal-insulator transition (MIT) in NdNiO$_3$ (NNO) thin films, grown on NdGaO$_3$(110) and LaAlO$_3$(100) substrates. In the metallic phase, we observe three dimensional hole and electron Fermi surface (FS) pockets formed from strongly renormalized bands with well-defined quasiparticles. Upon cooling across the MIT in NNO/NGO sample, the quasiparticles lose coherence via a spectral weight transfer from near the Fermi level to localized states forming at higher binding energies. In the case of NNO/LAO, the bands are apparently shifted upward with an additional holelike pocket forming at the corner of the Brillouin zone. We find that the renormalization effects are strongly anisotropic and are stronger in NNO/NGO than NNO/LAO. Our study reveals that substrate-induced strain tunes the crystal field splitting, which changes the FS properties, nesting conditions, and spin-fluctuation strength, and thereby controls the MIT via the formation of an electronic order parameter with Q$_{AF}\sim$(1/4, 1/4, 1/4$\pm$$δ$).

cond-mat.str-el↗

arXiv2015

Two distinct topological phases in the mixed valence compound YbB6 and its differences from SmB6

We discuss the evolution of topological states and their orbital textures in the mixed valence compounds SmB6 and YbB6 within the framework of the generalized gradient approximation plus onsite Coulomb interaction (GGA+U) scheme for a wide range of values of U. In SmB6, the topological Kondo insulator (TKI) gap is found to be insensitive to the value of U, but in sharp contrast, Kondo physics in isostructural YbB6 displays a surprising sensitivity to U. In particular, as U is increased in YbB6, the correlated TKI state in the weak-coupling regime transforms into a d-p-type topological insulator phase with a band inversion between Yb-5d and B-2p orbitals in the intermediate coupling range, without closing the insulating energy gap throughout this process. Our theoretical predictions related to the TKI and non-TKI phases in SmB6 and YbB6 are in substantial accord with recent angle-resolved photoemission spectroscopy (ARPES) experiments.

cond-mat.mtrl-sci↗

arXiv2015

Theory of Weyl orbital semimetals and predictions of several materials classes

Graphene, topological insulators, and Weyl semimetals are three widely studied materials classes which possess Dirac or Weyl cones arising from either sublattice symmetry or spin-orbit coupling. In this work, we present a theory of a new class of bulk Dirac and Weyl cones, dubbed Weyl orbital semimetals, where the orbital polarization and texture inversion between two electronic states at discrete momenta lend itself into protected Dirac or Weyl cones without spin-orbit coupling. We also predict several families of Weyl orbital semimetals including V$_3$S$_4$, NiTi3S6, BLi, and PbO$_2$ via first-principle band structure calculations. We find that the highest Fermi velocity predicted in some of these materials is even larger than that of the existing Dirac materials. The synthesis of Weyl orbital semimetals will not only expand the territory of Dirac materials beyond the quintessential spin-orbit coupled systems and hexagonal lattice to the entire periodic table, but it may also open up new possibilities for orbital controlled electronics or `orbitronics'.

cond-mat.mes-hall↗

arXiv2015

Superconducting dome in MoS2 and TiSe2 generated by quasiparticle-phonon coupling

We use a first-principles based self-consistent momentum-resolved density fluctuation (MRDF) model to compute the combined effects of electron-electron and electron-phonon interactions to describe the superconducting dome in the correlated MoS2 thin flake and TiSe2. We find that without including the electron-electron interaction, the electron-phonon coupling and the superconducting transition temperature (Tc) are overestimated in both these materials. However, once the full angular and dynamical fluctuations of the spin and charge density induced quasiparticle self-energy effects are included, the electron-phonon coupling and Tc are reduced to the experimental value. With doping, both electronic correlation and electron-phonon coupling grows, and above some doping value, the former becomes so large that it starts to reduce the quasiparticle-phonon coupling constant and Tc, creating a superconducting dome, in agreement with experiments.

cond-mat.supr-con↗

arXiv2015

Theory of nodal s+- wave pairing symmetry in the Pu-based 115 superconductor family

The spin-fluctuation mechanism of superconductivity usually results in the presence of gapless or nodal quasiparticle states in the excitation spectrum. Nodal quasiparticle states are well established in copper-oxide, and heavy-fermion superconductors, but not in iron-based superconductors. Here, we study the pairing symmetry and mechanism of a new class of plutonium-based high-Tc superconductors and predict the presence of a nodal $s^{+-}$ wave pairing symmetry in this family. Starting from a density-functional theory (DFT) based electronic structure calculation we predict several three-dimensional (3D) Fermi surfaces in this 115 superconductor family. We identify the dominant Fermi surface ``hot-spots' in the inter-band scattering channel, which are aligned along the wavevector ${\bf Q}\sim(π,π,π)$, where degeneracy could induce sign-reversal of the pairing symmetry. Our calculation demonstrates that the $s^{+-}$ wave pairing strength is stronger than the previously thought $d$-wave pairing; and more importantly, this pairing state allows for the existence of nodal quasiparticles. Finally, we predict the shape of the momentum- and energy-dependent magnetic resonance spectrum for the identification of this pairing symmetry.

cond-mat.supr-con↗

arXiv2014

Intermediate Coupling Model of the Cuprates

We review the intermediate coupling model for treating electronic correlations in the cuprates. Spectral signatures of the intermediate coupling scenario are identified and used to adduce that the cuprates fall in the intermediate rather than the weak or the strong coupling limits. A robust, `beyond LDA' framework for obtaining wide-ranging properties of the cuprates via a GW-approximation based self-consistent self-energy correction for incorporating correlation effects is delineated. In this way, doping and temperature dependent spectra, from the undoped insulator to the overdoped metal, in the normal as well as the superconducting state, with features of both weak and strong coupling can be modeled in a material-specific manner with very few parameters. Efficacy of the model is shown by considering available spectroscopic data on electron and hole doped cuprates from angle-resolved photoemission (ARPES), scanning tunneling microscopy/spectroscopy (STM/STS), neutron scattering, inelastic light scattering, optical and other experiments. Generalizations to treat systems with multiple correlated bands such as the heavy-fermions, the ruthenates, and the actinides are discussed.

cond-mat.supr-con↗

arXiv2014

A Renormalization-Group Study of the Symmetry-Breaking Order Parameters in Spin-Orbit Coupled Iridates and Related Systems

We study the competition between various forms of density-wave-like order parameters that may arise in multi-orbital correlated materials with strong spin-orbit coupling (SOC) within a renormalization-group (RG) approach. The calculations are restricted to models with two spin-orbit split bands having strong inter-band Fermi surface nesting. We find that for such Fermi surface topology, the interplay between the inter-band nesting and SOC strongly enhances the inter-orbital Coulomb interaction (relative to other interactions) as the system approaches a stable fixed point. This results in an exotic spin-orbit density wave (SODW) to become the energetically favorable symmetry-broken state. While the conclusions are generic to such a Fermi surface topology, the band structure and the numerical results are presented for the iridate systems. We also find that the electronic fingerprints of the SODW are in better agreement with various experimental results than a conventional spin density wave (SDW), explaining the long-standing problem of the origin of the metal-insulator transition in these systems.

cond-mat.str-el↗

arXiv2014

Spin-orbit density wave: A new phase of matter applicable to the hidden order state of URu2Si2

We provide a brief review and detailed analysis of the spin-orbit density wave (SODW), proposed as a possible explanation to the `hidden order' phase of URu2Si2. Due to the interplay between inter-orbital Coulomb interaction and spin-orbit coupling (SOC) in this compound, the SODW is shown to arise from Fermi surface nesting instability between two spin-orbit split bands. An effective low-energy Hamiltonian including single-particle SOC and two-particle SODW is derived, while numerical results are calculated by using density-functional theory (DFT) based band structure input. Computed gapped quasiparticle spectrum, entropy loss and spin-excitation spectrum are in detailed agreement with experiments. Interestingly, despite the fact that SODW governs dynamical spin-excitations, the static magnetic moment is calculated to be zero, owing to the time-reversal invariance imposed by SOC. As a consequence, SODW can be destroyed by finite magnetic field even at zero temperature. Our estimation of the location of the quantum critical point is close to the experimental value of Bc ~ 35 T. Finally, we extend the idea of SODW to other SOC systems including iridium oxides (iridates) and two-dimensional electronic systems such as BiAg2 surface and LaAlO3/SrTiO3 interface. We show hints of quasiparticle gapping, reduction of preexisting magnetic moment, large magneto-resistance etc. in these systems which can be explained consistently within the SODW theory.

cond-mat.str-el↗

arXiv2014

Imprints of spin-orbit density wave in the hidden order state of URu2Si2

The mysterious second order quantum phase transition, commonly attributed to the `hidden-order' (HO) state, in heavy-fermion metal URu2Si2 exhibits a number of paradoxical electronic and magnetic properties which cannot be associated with any conventional order parameter. We characterize and reconcile these exotic properties of the HO state based on a spin-orbit density wave order (SODW), constructed on the basis of a realistic density-functional theory (DFT) band structure. We quantify the nature of the gapped electronic and magnetic excitation spectrum, in agreement with measurements, while the magnetic moment is calculated to be zero owing to the spin-orbit coupling induced time-reversal invariance. Furthermore, a new collective mode in the spin-1 excitation spectrum is predicted to localize at zero momentum transfer in the HO state which can be visualized, for example, by electron spin resonance (ESR) at zero magnetic field or polarized inelastic neutron scattering measurements. The results demonstrate that the concomitant broken and invariant symmetries protected SODW order not only provides insights into numerous nontrivial hidden-order phenomena, but also offers a parallel laboratory to the formation of a topologically protected quantum state beyond the quantum spin-Hall state and Weyl semimetals.

cond-mat.str-el↗

arXiv2013

Origin of pressure induced second superconducting dome in AyFe2-xSe2 [A=K, (Tl,Rb)]

Recent observation of pressure induced second superconducting phase in AyFe2-xSe2 [A=K, (Tl,Rb)] calls for the models of superconductivity that are rich enough to allow for multiple superconducting phases. We propose the model where pressure induced changes in Fermi surface topology lead to appearance of the hole pocket at $Γ$ point at high pressure. We develop low-energy effective model, derived from first-principles band-structure calculation, to suggest the phase assignment where low pressure superconducting state with no hole pocket at $Γ$ point is a nodeless d-wave state. It evolves into a s+- state at higher pressure when the Fermi surface topology changes and hole pocket appears. We analyze the pairing interactions using five band tight binding fitted band structure and find strong pairing strength dependents on pressure. We also evaluate the energy and momentum dependence of neutron spin resonances in each of the phases as verifiable predictions of our proposal.

cond-mat.supr-con↗

arXiv2013

Distinguishing $s^{\pm}$ and $s^{++}$ electron pairing symmetries by neutron spin resonance in superconducting NaFe$_{0.935}$Co$_{0.045}$As

A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechansim for superconductivity. For iron pnictide superconductors, the $s^\pm$-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies ($E\leq 2Δ$) below $T_c$. On the other hand, the $s^{++}$-pairing symmetry expects a broad spin excitation enhancement at an energy above $2Δ$ below $T_c$. Although the resonance has been observed in iron pnictide superconductors at an energy below $2Δ$ consistent with the $s^\pm$-pairing symmetry, the mode has also be interpreted as arising from the $s^{++}$-pairing symmetry with $E\ge 2Δ$ due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at E=7 meV in SC NaFe$_{0.935}$Co$_{0.045}$As ($T_c = 18$ K). On warming towards $T_c$, the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitations spectra within the $s^{\pm}$ and $s^{++}$-pairing symmetries, we conclude that the ground-state resonance in NaFe$_{0.935}$Co$_{0.045}$As is only consistent with the $s^{\pm}$-pairing, and is inconsistent with the $s^{++}$-pairing symmetry.

cond-mat.supr-con↗

arXiv2013

Weyl semimetals and superconductors designed in an orbital selective superlattice

We propose two complementary design principles for engineering three-dimensional (3D) Weyl semimetals and superconductors in a layer-by-layer setup which includes even and odd parity orbitals in alternating layers - dubbed orbital selective superlattice. Such structure breaks mirror symmetry along the superlattice growth axis which, with the help of either a basal plane spin-orbit coupling or a spinless p+ip superconductivity, stabilizes a 3D Dirac node. To explore this idea, we develop a 3D generalization of Haldane model and a Bogoliubov-de-Gennes (BdG) Hamiltonian for the two cases, respectively, and show that a tunable single or multiple Weyl nodes with linear dispersion in all spatial directions can be engineered desirably in a widespread parameter space. We also demonstrate that a single helical Weyl band can be created at the $Γ$-point at the Fermi level in the superconducting case via gapping out either of the orbital state by violating its particle-hole symmetry but not any other symmetries. Finally, implications of our results for the realization of anomalous Hall effect and Majorana bound state are discussed.

cond-mat.mes-hall↗

arXiv2013

Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures

Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing nontrivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, and with inter-layer quantum tunneling. We demonstrate that in the stack of bilayers grown along a (001)-direction, a nontrivial topological phase transition occurs above a critical number of Rashba-bilayers. In the topological phase we find the formation of a single spin-polarized Dirac cone at the $Γ$-point. This approach offers an accessible way to design artificial topological insulators in a set up that takes full advantage of the atomic layer deposition approach. This design principle is tunable and also allows us to bypass limitations imposed by bulk crystal geometry.

cond-mat.mes-hall↗

arXiv2013

Stability of Weyl metals under impurity scattering

We investigate the effects of bulk impurities on the electronic spectrum of Weyl semimetals, a recently identified class of Dirac-type materials. Using a $T$-matrix approach, we study resonant scattering due to a localized impurity in tight binding versions of the continuum models recently discussed by Burkov, Hook, and Balents, describing perturbed four-component Dirac fermions in the vicinity of a critical point. The impurity potential is described by a strength $g$ as well as a matrix structure $Λ$. Unlike the case in d-wave superconductors, where a zero energy resonance can always be induced by varying the impurity scalar and/or magnetic impurity strength, we find that for certain types of impurity ($Λ$), the Weyl node is protected, and that a scalar impurity will induce an intragap resonance over a wide range of scattering strength. A general framework is developed to address this question, as well as to determine the dependence of resonance energy on the impurity strength.

cond-mat.str-el↗