arXiv subjects

M. Z. Hasan

Publications and source records attributed to M. Z. Hasan.

At least 37 records · Page 2Linked to original sources

arXiv2018

Planar Hall-effect, Anomalous planar Hall-effect, and Magnetic Field-Induced Phase Transitions in TaAs

We evaluate the topological character of TaAs through a detailed study of the angular, magnetic-field and temperature dependence of its magnetoresistivity and Hall-effect(s), and of its bulk electronic structure through quantum oscillatory phenomena. At low temperatures, and for fields perpendicular to the electrical current, we extract an extremely large Hall angle $Θ_H$ at higher fields, that is $Θ_H \sim 82.5^{\circ}$, implying a very pronounced Hall signal superimposed into its magnetoresistivity. For magnetic fields and electrical currents perpendicular to the \emph{c}-axis we observe a very pronounced planar Hall-effect, when the magnetic field is rotated within the basal plane. This effect is observed even at higher temperatures, i.e. as high as $T = 100$ K, and predicted recently to result from the chiral anomaly among Weyl points. Superimposed onto this planar Hall, which is an even function of the field, we observe an anomalous planar Hall-signal akin to the one reported for that is an odd function of the field. Below 100 K, negative longitudinal magnetoresistivity (LMR), initially ascribed to the chiral anomaly and subsequently to current inhomogeneities, is observed in samples having different geometries and contact configurations, once the large Hall signal is subtracted. Our measurements reveal a phase transition upon approaching the quantum limit that leads to the reconstruction of the FS and to the concomitant suppression of the negative LMR indicating that it is intrinsically associated with the Weyl dispersion at the Fermi level. For fields along the \emph{a}-axis it also leads to a pronounced hysteresis pointing to a field-induced electronic phase-transition. This collection of unconventional tranport observations points to the prominent role played by the axial anomaly among Weyl nodes.

cond-mat.mes-hall↗

arXiv2017

Recently synthesized (Zr1-xTix)2AlC (0 - x - 1) solid solutions: Theoretical study of the effects of M mixing on physical properties

The effects of M atomic species mixing on the physical properties of newly synthesized MAX phase (Zr1-xTix)2AlC solid solutions have been studied by means of density functional theory (DFT) calculations. The lattice constants in good accord with the experimental results, are found to decrease with Ti content. The elastic constants, Cij, and the other polycrystalline elastic moduli have been calculated. The elastic constants satisfy the mechanical stability conditions of these solid solutions. The constants C11, C33 and C44 are found to increase with Ti contents up to x = 0.67, thereafter these decrease slightly. A reverse trend is followed by C12 and C13. The elastic moduli are also found to increase up to x = 0.67, beyond which these moduli go down slightly. Pughs ratio and Poissons ratio both confirm the brittleness of (Zr1-xTix)2AlC. Different anisotropy factors revealed the anisotropic character of these solid solutions. A non-vanishing value of the electronic energy density of states (EDOS) at the Fermi level suggests that (Zr1-xTix)2AlC are metallic in nature. A mixture of covalent, ionic and metallic bonding has been indicated from the electronic structure with dominant covalent bonding due to hybridization of Zr-4d states and C-2p states. The variation of elastic stiffness and elastic parameters with x is seen to be correlated with partial DOS (PDOS) and charge density distribution. The calculated Debye temperature and minimum thermal conductivity are found to increase with Ti contents, while melting temperature is the highest for x = 0.67. The solid solution with x = 0.67 shows improved mechanical and thermal properties compared to that of the two end members Zr2AlC and Ti2AlC. The study of charge transport properties of (Zr1-xTix)2AlC reveals the metallic nature with saturated resistivity. The maximum power factor is obtained at 400 K for (Zr1-xTix)2AlC.

cond-mat.mtrl-sci↗

arXiv2015

An experimental algorithm for identifying the topological nature of Kondo and mixed valence insulators

Possible topological nature of Kondo and mixed valence insulators has been a recent topic of interest in condensed matter physics. Attention has focused on SmB6, which has long been known to exhibit low temperature transport anomaly, whose origin is of independent interest. We argue that it is possible to resolve the topological nature of surface states by uniquely accessing the surface electronic structure of the low temperature anomalous transport regime through combining state-of-the-art laser- and synchrotron-based angle-resolved photoemission spectroscopy (ARPES) with or without spin resolution. A combination of low temperature and ultra-high resolution (laser) which is lacking in previous ARPES studies of this compound is the key to resolve the possible existence of topological surface state in SmB6. Here we outline an experimental algorithm to systematically explore the topological versus trivial or mixed (topological and trivial surface state admixture as in the first 3D TI Bi$_{1-x}$Sb$_x$) nature of the surface states in Kondo and mixed valence insulators. We conclude based on this methodology that the observed topology of the surface Fermi surface in our low temperature data considered within the level of current resolution is consistent with the theoretically predicted topological picture, suggesting a topological origin of the dominant in-gap ARPES signal in SmB6.}

cond-mat.str-el↗

arXiv2014

A Neutron Scattering Study of the H-T Phase Diagram of the Bond Frustrated ZnCr2S4

Detailed neutron scattering measurements on the bond frustrated magnet ZnCr2S4 reveal a rich H-T phase diagram. The field dependence of the two subsequent antiferromagnetic transitions follows closely that of recently reported structural instabilities, providing further evidence for spin driven Jahn-Teller physics. The incommensurate helical ordered phase below TN1 = 15.5 K exhibits gapless spin wave excitations, whereas a spin wave gap (about 2 meV) opens below TN2 = 8 K as the system undergoes a first-order transition to a commensurate collinear ordered phase. The spin wave gap is closed by a strong magnetic field.

cond-mat.str-el↗

arXiv2014

Spin Order by Quantum Frustration in Triangular Lattice Mott Insulator NaCrO2 : A Neutron Scattering Study

We report high resolution neutron scattering measurements on the triangular lattice Mott insulator Na$_x$CrO$_2$ ($x$=1) which has recently been shown to exhibit an unusually broad fluctuating crossover regime extending far below the onset of spin freezing ($T_c\sim$41K). Our results show that below some crossover temperature ($T\sim0.75T_c$) a small incommensuration develops which helps resolve the spin frustration and drives three-dimensional magnetic order supporting coherent spin wave modes. This incommensuration assisted dimensional crossover suggests that inter-layer frustration is responsible for stabilizing the rare 2D correlated phase above 0.75$T_c$. In contrast to the host compound of 2D cobaltate superconductor such as Na$_x$CoO$_2$ ($ξ_c>50$Å), no magnetic long-range order is observed down to 1.5K ($ξ_c<16$Å).

cond-mat.str-el↗

arXiv2014

Saddle point singularity and topological phase diagram in a tunable topological crystalline insulator (TCI)

A topological crystalline insulator (TCI) is a new phase of topological matter, which is predicted to exhibit distinct topological quantum phenomena, since space group symmetries replace the role of time-reversal symmetry in the much-studied Z$_2$ topological insulators. Utilizing high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal the momentum space nature of interconnectivity of the Fermi surface pockets leading to a saddle point singularity within the topological surface state alone in the TCI Pb$_{0.7}$Sn$_{0.3}$Se. Moreover, we show that the measured momentum-integrated density of states exhibits pronounced peaks at the saddle point energies, demonstrating the van Hove singularities (VHSs) in the topological surface states, whose surface chemical potential, as we show, can be tuned via surface chemical gating, providing access to the topological correlated physics on the surface. Our experimental data reveal a delicate relationship among lattice constant, band gap and spin-orbit coupling strength associated with the topological phase transition in Pb$_{1-x}$Sn$_{x}$Se. Furthermore, we explore the robustness of the TCI phase with VHS in Pb$_{1-x}$Sn$_{x}$Se, which shows a variety of distinct topological phase transitions driven by either thermal instability or broken crystalline symmetry, and thus revealing a rich topological phase diagram connectivity in Pb$_{1-x}$Sn$_{x}$Se for the first time.

cond-mat.mes-hall↗

arXiv2013

Observation of a bulk 3D Dirac multiplet, Lifshitz transition, and nestled spin states in Na3Bi

Symmetry or topology protected Dirac fermion states in two and three dimensions constitute novel quantum systems that exhibit exotic physical phenomena. However, none of the studied spin-orbit materials are suitable for realizing bulk multiplet Dirac states for the exploration of interacting Dirac physics. Here we present experimental evidence, for the first time, that the compound Na3Bi hosts a bulk spin-orbit Dirac multiplet and their interaction or overlap leads to a Lifshitz transition in momentum space - a condition for realizing interactions involving Dirac states. By carefully preparing the samples at a non-natural-cleavage (100) crystalline surface, we uncover many novel electronic and spin properties in Na3Bi by utilizing high resolution angle- and spin-resolved photoemission spectroscopy measurements. We observe two bulk 3D Dirac nodes that locate on the opposite sides of the bulk zone center point $Γ$, which exhibit a Fermi surface Lifshitz transition and a saddle point singularity. Furthermore, our data shows evidence for the possible existence of theoretically predicted weak 2D nontrivial spin-orbit surface state with helical spin polarization that are nestled between the two bulk Dirac cones, consistent with the theoretically calculated (100) surface-arc-modes. Our main experimental observation of a rich multiplet of Dirac structure and the Lifshitz transition opens the door for inducing electronic instabilities and correlated physical phenomena in Na3Bi, and paves the way for the engineering of novel topological states using Na3Bi predicted in recent theory.

cond-mat.mes-hall↗

arXiv2013

Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry

The advent of Dirac materials has made it possible to realize two dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity - the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.

cond-mat.mtrl-sci↗

arXiv2013

Photoemission of Bi$_2$Se$_3$ with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation?

Topological insulators are characterized by Dirac cone surface states with electron spins aligned in the surface plane and perpendicular to their momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy. We solve this puzzle and show that vacuum ultraviolet photons (50-70 eV) with linear or circular polarization probe indeed the initial state spin texture of Bi$_2$Se$_3$ while circularly polarized 6 eV low energy photons flip the electron spins out of plane and reverse their spin polarization. Our photoemission calculations, considering the interplay between the varying probing depth, dipole selection rules and spin-dependent scattering effects involving initial and final states explain these findings, and reveal proper conditions for light-induced spin manipulation. This paves the way for future applications of topological insulators in opto-spintronic devices.

cond-mat.mes-hall↗

arXiv2013

Chemically gated electronic structure of a superconducting doped topological insulator system

Angle resolved photoemission spectroscopy is used to observe changes in the electronic structure of bulk-doped topological insulator Cu$_x$Bi$_2$Se$_3$ as additional copper atoms are deposited onto the cleaved crystal surface. Carrier density and surface-normal electrical field strength near the crystal surface are estimated to consider the effect of chemical surface gating on atypical superconducting properties associated with topological insulator order, such as the dynamics of theoretically predicted Majorana Fermion vortices.

cond-mat.supr-con↗

arXiv2013

Oscillatory surface dichroism of an insulating topological insulator Bi2Te2Se

Using circular dichroism-angle resolved photoemission spectroscopy (CD-ARPES), we report a study of the effect of angular momentum transfer between polarized photons and topological surface states on the surface of highly bulk insulating topological insulator Bi2Te2Se. The photoelectron dichroism is found to be strongly modulated by the frequency of the helical photons including a dramatic sign-flip. Our results suggest that the observed dichroism and its sign-flip are consequences of strong coupling between the photon field and the spin-orbit nature of the Dirac modes on the surface. Our studies reveal the intrinsic dichroic behavior of topological surface states and point toward the potential utility of bulk insulating topological insulators in device applications.

cond-mat.mes-hall↗

arXiv2013

Tunneling Tuned Spin Modulations in Ultrathin Topological Insulator Films

Quantitative understanding of the relationship between quantum tunneling and Fermi surface spin polarization is key to device design using topological insulator surface states. By using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films across the metal-to-insulator transition, we observe that for a given film thickness, the spin polarization is large for momenta far from the center of the surface Brillouin zone. In addition, the polarization decreases significantly with enhanced tunneling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. Our theoretical model calculations capture this delicate relationship between quantum tunneling and Fermi surface spin polarization. Our results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nano-device.

cond-mat.mes-hall↗

arXiv2013

Surface electronic structure of a topological Kondo insulator candidate SmB6: insights from high-resolution ARPES

The Kondo insulator SmB6 has long been known to exhibit low temperature (T < 10K) transport anomaly and has recently attracted attention as a new topological insulator candidate. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, for the first time, we probe the surface electronic structure of the anomalous conductivity regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within a 4 meV window of the Fermi level on the (001)-surface of this material. The low-lying states are found to form electron-like Fermi surface pockets that enclose the X and the Gamma points of the surface Brillouin zone. These states disappear as temperature is raised above 15K in correspondence with the complete disappearance of the 2D conductivity channels in SmB6. While the topological nature of the in-gap metallic states cannot be ascertained without spin (spin-texture) measurements our bulk and surface measurements carried out in the transport-anomaly-temperature regime (T < 10K) are consistent with the first-principle predicted Fermi surface behavior of a topological Kondo insulator phase in this material.

cond-mat.str-el↗

arXiv2013

Non-trivial spin-texture of the coaxial Dirac cones on the surface of topological crystalline insulator SnTe

We present first principles calculations of the nontrivial surface states and their spin-textures in the topological crystalline insulator SnTe. The surface state dispersion on the [001] surface exhibits four Dirac-cones centered along the intersection of the mirror plane and the surface plane. We propose a simple model of two interacting coaxial Dirac cones to describe both the surface state dispersion and the associated spin-texture. While the out-of-the-plane spin polarization is zero due to the crystalline and time-reversal symmetries, the in-plane spin texture shows helicity with some distortion due to the interaction of the two coaxial Dirac cones, indicating a nontrivial mirror Chern number of -2, distinct from the value of -1 in $Z_{2}$ topological insulator such as Bi/Sb alloys or Bi$_2$Se$_3$. The surface state dispersion and its spin-texture would provide an experimentally accessible way to determine the nontrivial mirror Chern number.

cond-mat.mtrl-sci↗

arXiv2013

Adiabatic transformation as a search tool for new topological insulators: distorted ternary Li$_2$AgSb-class semiconductors and related compounds

We demonstrate that first-principles based adiabatic continuation approach is a very powerful and efficient tool for constructing topological phase diagrams and locating non-trivial topological insulator materials. Using this technique, we predict that the ternary intermetallic series Li$_2M'X$ where $M'$=Cu, Ag, Au, or Cd, and $X$=Sb, Bi, or Sn, hosts a number of topological insulators with remarkable functional variants and tunability. We also predict that several III-V semimetallic compounds are topologically non-trivial. We construct a topological phase diagram in the parameter space of the atomic numbers of atoms in Li$_2M'X$ compounds, which places a large number of topological materials presented in this work as well as in earlier studies within a single unified topological framework. Our results demonstrate the efficacy of adiabatic continuation as a useful tool for exploring topologically nontrivial alloying systems and for identifying new topological insulators even when the underlying lattice does not possess inversion symmetry, and the approaches based on parity analysis are not viable.

cond-mat.mtrl-sci↗

arXiv2012

Dirac point spectral weight suppression and surface "gaps" in nonmagnetic and magnetic topological insulators

It is predicted that electrons on the surface of a topological insulator can acquire a mass (massive Dirac fermion) by opening up a gap at the Dirac point when time-reversal symmetry is broken via the out-of-plane magnetization. We report photoemission studies on a series of topological insulator materials focusing on the spectral behavior in the vicinity of the Dirac node. Our results show that the spectral intensity is suppressed resulting in a "gap"-like feature in materials with or without any magnetic impurity or doping. The Zeeman gap in magnetically doped samples, expected to be rather small, is likely masked by the non-magnetic strong spectral weight suppression involving a large energy scale we report. The photoemission spectral weight suppression observed around the Dirac node thus cannot be taken as the sole evidence for a time-reversal symmetry breaking magnetic gap. We discuss a few possible extrinsic and kinematic origins of the Dirac point spectral weight suppression ("gap") observed in many commonly studied topological materials.

cond-mat.mes-hall↗

arXiv2012

Topological Surface States and Dirac point tuning in ternary Bi2Te2Se class of topological insulators

Using angle-resolved photoemission spectroscopy, we report electronic structure for representative members of ternary topological insulators. We show that several members of this family, such as Bi2Se2Te, Bi2Te2Se, and GeBi2Te4, exhibit a singly degenerate Dirac-like surface state, while Bi2Se2S is a fully gapped insulator with no measurable surface state. One of these compounds, Bi2Se2Te, shows tunable surface state dispersion upon its electronic alloying with Sb (SbxBi2-xSe2Te series). Other members of the ternary family such as GeBi2Te4 and BiTe1.5S1.5 show an in-gap surface Dirac point, the former of which has been predicted to show nonzero weak topological invariants such as (1;111); thus belonging to a different topological class than BiTe1.5S1.5. The measured band structure presented here will be a valuable guide for interpreting transport, thermoelectric, and thermopower measurements on these compounds. The unique surface band topology observed in these compounds contributes towards identifying designer materials with desired flexibility needed for thermoelectric and spintronic device fabrication.

cond-mat.mes-hall↗

arXiv2012

A topological crystalline insulator (TCI) phase via topological phase transition and crystalline mirror symmetry

A Z2 topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction driven band inversion. For example, the topological phase in the Bi-Sb system is due to an odd number of band inversions. A related spin-orbit system, the (Pb/Sn)Te class, has been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry protected topological crystalline insulator phase in the (Pb/Sn)Te class of materials which has been theoretically predicted to exist in its non-alloyed version. Our experimental results show that at a finite-Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states (as opposed to odd as observed in Bi-Sb alloy or Bi2Se3) revealing mirror protected topological order distinct from that observed in Bi-Sb or Bi2Se3. Our observation of the spin-polarized Dirac surface states in the inverted (Pb/Sn)Te and their absence in the non-inverted compounds related via a topological phase transition (spin-orbit induced band inversion type) provide the experimental groundwork for opening the research on novel topological order in future quantum devices.

cond-mat.mes-hall↗