arXiv subjects

Y. S. Hor

Publications and source records attributed to Y. S. Hor.

At least 19 recordsLinked to original sources

arXiv2023

Chemically induced ferromagnetism near room temperature in single crystal (Zn$_{1-x}$Cr$_{x}$)Te half-metal

Magnetic semiconductors are at the core of recent spintronics research endeavors. Chemically doped II-VI diluted magnetic semiconductors, such as (Zn$_{1-x}$Cr$_{x}$)Te, provide promising platform in this quest. However, a detailed knowledge of the microscopic nature of magnetic ground state is necessary for any practical application. Here, we report on the synergistic study of (Zn$_{1-x}$Cr$_{x}$)Te single crystals using elastic neutron scattering measurements and density functional calculations. For the first time, our research unveils the intrinsic properties of ferromagnetic state in macroscopic specimen of (Zn$_{0.8}$Cr$_{0.2}$)Te. The ferromagnetism is onset at $T_c \sim$ 290 K and remains somewhat independent to modest change in the substitution coefficient x. We show that magnetic moments on Zn/Cr site develop ferromagnetic correlation in a-c plane with large ordered moment of $μ$ = 3.08 $μ_B$. Magnetic moment across the lattice is induced via the mediation of Te site, uncoupled to the number of dopant carriers as inferred from the density functional calculation. Additionally, the ab-initio calculations also reveal half-metallicity in x = 0.2 composition. These properties are highly desirable for future spintronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Local orbital degeneracy lifting as a precursor to an orbital-selective Peierls transition

Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the $d$-electron orbitals and the influence of this filling on structural configurations and responses. Here we use a sensitive local structural technique, x-ray atomic pair distribution function analysis, to reveal the presence of fluctuating local-structural distortions at high temperature of one such compound, \cis . We show that this hitherto overlooked fluctuating symmetry lowering is electronic in origin and will significantly modify the energy-level spectrum and electronic and magnetic properties. The explanation is a local, fluctuating, orbital-degeneracy-lifted state. The natural extension of our result would be that this phenomenon is likely to be widespread amongst diverse classes of partially filled nominally degenerate d-electron systems, with potentially broad implications for our understanding of their properties.

cond-mat.mtrl-sci↗

arXiv2017

An increase in $T_c$ under hydrostatic pressure in the superconducting doped topological insulator Nb$_{0.25}$Bi$_2$Se$_3$

We report an unexpected positive hydrostatic pressure derivative of the superconducting transition temperature in the doped topological insulator \NBS via $dc$ SQUID magnetometry in pressures up to 0.6 GPa. This result is contrary to reports on the homologues \CBS and \SBS where smooth suppression of $T_c$ is observed. Our results are consistent with recent Ginzburg-Landau theory predictions of a pressure-induced enhancement of $T_c$ in the nematic multicomponent $E_u$ state proposed to explain observations of rotational symmetry breaking in doped Bi$_2$Se$_3$ superconductors.

cond-mat.supr-con↗

arXiv2017

Robust odd-parity superconductivity in the doped topological insulator Nb$_x$Bi$_2$Se$_3$

We present resistivity and magnetization measurements on proton-irradiated crystals demonstrating that the superconducting state in the doped topological superconductor Nb$_x$Bi$_2$Se$_3$ (x = 0.25) is surprisingly robust against disorder-induced electron scattering. The superconducting transition temperature $T_c$ decreases without indication of saturation with increasing defect concentration, and the corresponding scattering rates far surpass expectations based on conventional theory. The low-temperature variation of the London penetration depth $Δλ(T)$ follows a power law ($Δλ(T)\sim T^2$) indicating the presence of symmetry-protected point nodes. Our results are consistent with the proposed robust nematic $E_u$ pairing state in this material.

cond-mat.supr-con↗

arXiv2016

Spin dynamics and magnetic interactions of Mn dopants in the topological insulator Bi$_2$Te$_3$

The magnetic and electronic properties of the magnetically doped topological insulator Bi$_{\rm 2-x}$Mn$_{\rm x}$Te$_3$ were studied using electron spin resonance (ESR) and measurements of static magnetization and electrical transport. The investigated high quality single crystals of Bi$_{\rm 2-x}$Mn$_{\rm x}$Te$_3$ show a ferromagnetic phase transition for $x\geq 0.04$ at $T_{C}\approx 12$ K. The Hall measurements reveal a p-type finite charge-carrier density. Measurements of the temperature dependence of the ESR signal of Mn dopants for different orientations of the external magnetic field give evidence that the localized Mn moments interact with the mobile charge carriers leading to a Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling between the Mn spins of order 2-3 meV. Furthermore, ESR reveals a low-dimensional character of magnetic correlations that persist far above the ferromagnetic ordering temperature.

cond-mat.str-el↗

arXiv2016

Evidence of nodes in the order parameter of the superconducting doped topological insulator Nb$_x$Bi$_2$Se$_3$ via penetration depth measurements

The low-temperature variation of the London penetration depth $λ(T)$ in the candidate topological superconductor Nb$_x$Bi$_2$Se$_3$ (x = 0.25) is reported for several crystals. The measurements were carried out by means of a tunnel-diode oscillator (TDO) technique in both field orientations ($H_{rf} \parallel$ $c$ and $H_{rf} \parallel$ $ab$ planes). All samples exhibited power law behavior at low temperatures ($Δλ\sim T^2$) clearly indicating the presence of point nodes in the superconducting order parameter. The results presented here are consistent with a nematic odd-parity spin-triplet $E_u$ pairing state in Nb$_x$Bi$_2$Se$_3$.

cond-mat.supr-con↗

arXiv2016

Robust Fabry-Perot interference in dual-gated Bi$_2$Se$_3$ devices

We study Fabry-Perot interference in hybrid devices, each consisting of a mesoscopic superconducting disk deposited on the surface of a three-dimensional topological insulator. Such structures are hypothesized to contain protected zero modes known as Majorana fermions bound to vortices. The interference manifests as periodic conductance oscillations of magnitude $\sim 0.1$ $e^2/h$. These oscillations show no strong dependence on bulk carrier density or sample thickness, suggesting that they result from phase coherent transport in surface states. However, the Fabry-Perot interference can be tuned by both top and back gates, implying strong electrostatic coupling between the top and bottom surfaces of topological insulator.

cond-mat.mes-hall↗

arXiv2016

Rotational Symmetry Breaking in a Trigonal superconductor Nb-doped Bi2Se3

The search for unconventional superconductivity has been focused on materials with strong spin-orbit coupling and unique crystal lattices. Doped bismuth selenide (Bi$_2$Se$_3$) is a strong candidate given the topological insulator nature of the parent compound and its triangular lattice. The coupling between the physical properties in the superconducting state and its underlying crystal symmetry is a crucial test for unconventional superconductivity. In this paper, we report direct evidence that the superconducting magnetic response couples strongly to the underlying 3-fold crystal symmetry in the recently discovered superconductor with trigonal crystal structure, niobium (Nb)-doped bismuth selenide (Bi$_2$Se$_3$). More importantly, we observed that the magnetic response is greatly enhanced along one preferred direction spontaneously breaking the rotational symmetry. Instead of a simple 3-fold crystalline symmetry, the superconducting hysteresis loop shows dominating 2-fold and 4-fold symmetry. This observation confirms the breaking of the rotational symmetry and indicates the presence of nematic order in the superconducting ground state of Nb-doped Bi$_2$Se$_3$. Further, heat capacity measurements display an exponential decay in superconducting state and suggest that there is no line node in the superconducting gap. These observations provide strong evidence of odd-parity topological superconductivity.

cond-mat.supr-con↗

arXiv2016

The hole Fermi surface in Bi$_{2}$Se$_{3}$ probed by quantum oscillations

Transport and torque magnetometry measurements are performed at high magnetic fields and low temperatures in a series of p-type (Ca-doped) Bi$_{2}$Se$_{3}$ crystals. The angular dependence of the Shubnikov-de Haas and de Haas-van Alphen quantum oscillations enables us to determine the Fermi surface of the bulk valence band states as a function of the carrier density. At low density, the angular dependence exhibits a downturn in the oscillations frequency between $0^\circ$ and $90^\circ$, reflecting a bag-shaped hole Fermi surface. The detection of a single frequency for all tilt angles rules out the existence of a Fermi surface with different extremal cross-sections down to $24$~meV. There is therefore no signature of a camel-back in the valence band of our bulk samples, in accordance with the direct band gap predicted by $GW$ calculations.

cond-mat.mes-hall↗

arXiv2015

Evidence for an anomalous current phase relation in topological insulator Josephson junctions

Josephson junctions with topological insulator weak links can host low energy Andreev bound states giving rise to a current phase relation that deviates from sinusoidal behaviour. Of particular interest are zero energy Majorana bound states that form at a phase difference of $π$. Here we report on interferometry studies of Josephson junctions and superconducting quantum interference devices (SQUIDs) incorporating topological insulator weak links. We find that the nodes in single junction diffraction patterns and SQUID oscillations are lifted and independent of chemical potential. At high temperatures, the SQUID oscillations revert to conventional behaviour, ruling out asymmetry. The node lifting of the SQUID oscillations is consistent with low energy Andreev bound states exhibiting a nonsinusoidal current phase relation, coexisting with states possessing a conventional sinusoidal current phase relation. However, the finite nodal currents in the single junction diffraction pattern suggest an anomalous contribution to the supercurrent possibly carried by Majorana bound states, although we also consider the possibility of inhomogeneity.

cond-mat.mes-hall↗

arXiv2014

Phase Coherence and Andreev Reflection in Topological Insulator Devices

Topological insulators (TIs) have attracted immense interest because they host helical surface states. Protected by time-reversal symmetry, they are robust to non-magnetic disorder. When superconductivity is induced in these helical states, they are predicted to emulate p-wave pairing symmetry, with Majorana states bound to vortices. Majorana bound states possess non-Abelian exchange statistics which can be probed through interferometry. Here, we take a significant step towards Majorana interferometry by observing pronounced Fabry-Perot oscillations in a TI sandwiched between a superconducting and normal lead. For energies below the superconducting gap, we observe a doubling in the frequency of the oscillations, arising from the additional phase accumulated from Andreev reflection. When a magnetic field is applied perpendicular to the TI surface, a number of very sharp and gate-tunable conductance peaks appear at or near zero energy, which has consequences for interpreting spectroscopic probes of Majorana fermions. Our results demonstrate that TIs are a promising platform for exploring phase-coherent transport in a solid-state system.

cond-mat.mes-hall↗

arXiv2014

Quantum Oscillations in Cu$_x$Bi$_2$Se$_3$ in High Magnetic Fields

Cu$_x$Bi$_2$Se$_3$ has drawn much attention as the leading candidate to be the first topological superconductor and the realization of coveted Majorana particles in a condensed matter system. However, there has been increasing controversy about the nature of its superconducting phase. This study sheds light on present ambiguity in the normal state electronic state, by providing a complete look at the quantum oscillations in magnetization in Cu$_x$Bi$_2$Se$_3$ at intense high fields up to 31T. Our study focuses on the angular dependence of the quantum oscillation pattern in a low carrier concentration. As magnetic field tilts from along the crystalline c-axis to ab-plane, the change of the oscillation period follows the prediction of the ellipsoidal Fermi surface. As the doping level changes, the 3D Fermi surface is found to transform into quasi-cylindrical at high carrier density. Such a transition is potentially a Lifshitz transition of the electronic state in Cu$_x$Bi$_2$Se$_3$.

cond-mat.str-el↗

arXiv2014

Dynamical Gate Tunable Supercurrents in Topological Josephson Junctions

Josephson junctions made of closely-spaced conventional superconductors on the surface of 3D topological insulators have been proposed to host Andreev bound states (ABSs) which can include Majorana fermions. Here, we present an extensive study of the supercurrent carried by low energy ABSs in Nb/Bi$_2$Se$_3$/Nb Josephson junctions in various SQUIDs as we modulate the carrier density in the Bi$_2$Se$_3$ barriers through electrostatic top gates. As previously reported, we find a precipitous drop in the Josephson current at a critical value of the voltage applied to the top gate. This drop has been attributed to a transition where the topologically trivial 2DEG at the surface is nearly depleted, causing a shift in the spatial location and change in nature of the helical surface states. We present measurements that support this picture by revealing qualitative changes in the temperature and magnetic field dependence of the critical current across this transition. In particular, we observe pronounced fluctuations in the critical current near total depletion of the 2DEG that demonstrate the dynamical nature of the supercurrent transport through topological low energy ABSs.

cond-mat.supr-con↗

arXiv2013

Cu(Ir$_{1-x}$Cr$_x$)$_2$S$_4$: a model system for studying nanoscale phase coexistence at the metal-insulator transition

Increasingly, nanoscale phase coexistence and hidden broken symmetry states are being found in the vicinity of metal-insulator transitions (MIT), for example, in high temperature superconductors, heavy fermion and colossal magnetoresistive materials, but their importance and possible role in the MIT and related emergent behaviors is not understood. Despite their ubiquity, they are hard to study because they produce weak diffuse signals in most measurements. Here we propose Cu(Ir$_{1-x}$Cr$_x$)$_2$S$_4$ as a model system, where robust local structural signals lead to key new insights. We demonstrate a hitherto unobserved coexistence of a Ir$^{4+}$ charge-localized dimer phase and Cr-ferromagnetism. The resulting phase diagram that takes into account the short range dimer order, is highly reminiscent of a generic MIT phase diagram similar to the cuprates. We suggest that the presence of quenched strain from dopant ions acts as an arbiter deciding between the competing ground states.

cond-mat.mtrl-sci↗

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

Crystal Structure and Chemistry of Topological Insulators

Topological surface states, a new kind of electronic state of matter, have recently been observed on the cleaved surfaces of crystals of a handful of small band gap semiconductors. The underlying chemical factors that enable these states are crystal symmetry, the presence of strong spin orbit coupling, and an inversion of the energies of the bulk electronic states that normally contribute to the valence and conduction bands. The goals of this review are to briefly introduce the physics of topological insulators to a chemical audience and to describe the chemistry, defect chemistry, and crystal structures of the compounds in this emergent field.

cond-mat.mtrl-sci↗

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

Point-contact Andreev reflection spectroscopy of candidate topological superconductor Cu0.25Bi2Se3

We perform a point-contact Andreev reflection spectroscopic study of the topological superconducting material, Cu0.25Bi2Se3, in the ballistic regime using a normal-metal gold tip. We observe distinct point-contact spectra on the superconducting and non-superconducting regions of the crystal surface: the former shows a marked zero-bias conductance peak, indicative of unconventional superconductivity, while the latter exhibits a pseudogap-like feature. In both cases the measured differential conductance spectra exhibit a large linear background, preventing direct quantitative comparison with theory. We attribute this background to inelastic scattering at the tip-sample interface, and compare the background-subtracted spectra with a single-band p-wave model.

cond-mat.supr-con↗