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arXiv2013

Superconductivity in Topological Insulator Sb2Te3 Induced by Pressure

Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature.

cond-mat.mtrl-sci↗

arXiv2013

Crystal, magnetic, and electronic structures, and properties of new BaMnPnF (Pn = As, Sb, Bi)

New fluoropnictides BaMnPnF with Pn = As, Sb, Bi, are synthesized by stoichiometric reaction of elements with BaF\_2. The compounds crystallize in the tetragonal P4/nmm (No. 129, Z = 2) space group, with the ZrCuSiAs-type structure, as indicated by single crystal and powder X-ray diffraction results. Electrical resistivity results indicate that Pn = As, Sb, and Bi are semiconductors with band gaps of E\_g = 0.73 eV, E\_g = 0.48 eV and E\_g = 0.003 eV, respectively. Powder neutron diffraction reveals a G-type antiferromagnetic order below T\_N = 338(1) K for Pn = As, and below T\_N = 272(1) K for Pn = Sb. Magnetic susceptibility increases with temperature above 100 K for all the materials. Density functional calculations also find semiconducting antiferromagnetic compounds with strong in-plane and weaker out-of-plane exchange coupling that may result in non-Curie Weiss behavior above T\_N. There is strong covalency between Mn and pnictogen elements. The ordered magnetic moments are 3.65(5) μB/Mn for Pn = As, and 3.66(3) μB/Mn for Pn = Sb at 4 K, as refined from neutron diffraction experiments.

cond-mat.str-el↗

arXiv2013

Electronic superlattice revealed by resonant scattering from random impurities in Sr3Ru2O7

Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive tool for the study of subtle charge, orbital, and spin superlattice orders driven by the valence electrons, which therefore escape detection in conventional x-ray diffraction (XRD). Although the power of REXS has been demonstrated by numerous studies of complex oxides performed in the soft x-ray regime, the cross section and photon wavelength of the material-specific elemental absorption edges ultimately set the limit to the smallest superlattice amplitude and periodicity one can probe. Here we show -- with simulations and REXS on Mn-substituted Sr$_3$Ru$_2$O$_7$ -- that these limitations can be overcome by performing resonant scattering experiments at the absorption edge of a suitably-chosen, dilute impurity. This establishes that -- in analogy with impurity-based methods used in electron-spin-resonance, nuclear-magnetic resonance, and Mössbauer spectroscopy -- randomly distributed impurities can serve as a non-invasive, but now momentum-dependent probe, greatly extending the applicability of resonant x-ray scattering techniques.

cond-mat.str-el↗

arXiv2012

Strong correlation induced charge localization in antiferromagnets

The fate of an injected hole in a Mott antiferromagnet is an outstanding issue of strongly correlated physics. It provides important insights into doped Mott insulators closely related to high-temperature superconductivity in cuprates. Here, we report a systematic numerical study based on the density matrix renormalization group (DMRG). It reveals a remarkable novelty and surprise for the single hole's motion in otherwise well-understood Mott insulators. Specifically, we find that the charge of the hole is self-localized by a novel quantum interference mechanism purely of strong correlation origin, in contrast to Anderson localization due to disorders. The common belief of quasiparticle picture is invalidated by the charge localization concomitant with spin-charge separation: the spin of the doped hole is found to remain a mobile object. Our findings unveil a new paradigm for doped Mott insulators that emerges already in the simplest single hole case.

cond-mat.str-el↗

arXiv2013

Emergence of nanoscale inhomogeneity in the superconducting state of a homogeneously disordered conventional superconductor, NbN

The notion of spontaneous formation of an inhomogeneous superconducting state is at the heart of most theories attempting to understand the superconducting state in the presence of strong disorder. Using scanning tunneling spectroscopy and high resolution scanning transmission electron microscopy, we experimentally demonstrate that under the competing effects of strong homogeneous disorder and superconducting correlations, the superconducting state of a conventional superconductor, NbN, spontaneously segregates into domains. Tracking these domains as a function of temperature we observe that the superconducting domains persist across the bulk superconducting transition, Tc, and disappear close to the pseudogap temperature, T*, where signatures of superconducting correlations disappear from the tunneling spectrum and the superfluid response of the system.

cond-mat.supr-con↗

arXiv2013

Robust Surface Hall Effect and Nonlocal Transport in SmB6: Indication for an Ideal Topological Insulator

A topological insulator (TI) is an unusual quantum state in which the insulating bulk is topologically distinct from vacuum, resulting in a unique metallic surface that is robust against time-reversal invariant perturbations. These surface transport properties, however, remains difficult to be isolated from the bulk in existing TI crystals (Bi2Se3, Bi2Te3 and Sb2Te3) due to impurity caused bulk conduction. We report in large crystals of topological Kondo insulator (TKI) candidate material SmB6 the thickness-independent surface Hall effects and non- local transport, which are robust against perturbations including mechanical abrasion. These results serve as proof that at low temperatures SmB6 has a robust metallic surface that surrounds a truly insulating bulk, paving the way for transport studies of the surface state in this proposed TKI material. (xia.jing@uci.edu).

cond-mat.str-el↗

arXiv2013

A unified theory of spin-relaxation due to spin-orbit coupling in metals and semiconductors

Spintronics is an emerging paradigm with the aim to replace conventional electronics by using electron spins as information carriers. Its utility relies on the magnitude of the spin-relaxation, which is dominated by spin- orbit coupling (SOC). Yet, SOC induced spin-relaxation in metals and semiconductors is discussed for the seemingly orthogonal cases when inversion symmetry is retained or broken by the so-called Elliott-Yafet and Dyakonov-Perel spin-relaxation mechanisms, respectively. We unify the two theories on general grounds for a generic two-band system containing intra- and inter-band SOC. While the previously known limiting cases are recovered, we also identify parameter domains when a crossover occurs between them, i.e. when an inversion symmetry broken state evolves from a Dyakonov-Perel to an Elliott-Yafet type of spin-relaxation and conversely for a state with inversional symmetry. This provides an ultimate link between the two mechanisms of spin-relaxation.

cond-mat.str-el↗

arXiv2014

High-density two-dimensional small polaron gas in a delta-doped Mott insulator

Heterointerfaces in complex oxide systems open new arenas in which to test models of strongly correlated material, explore the role of dimensionality in metal-insulator-transitions (MITs) and small polaron formation. Close to the quantum critical point Mott MITs depend on band filling controlled by random disordered substitutional doping. Delta-doped Mott insulators are potentially free of random disorder and introduce a new arena in which to explore the effect of electron correlations and dimensionality. Epitaxial films of the prototypical Mott insulator GdTiO3 are delta-doped by substituting a single (GdO)+1 plane with a monolayer of charge neutral SrO to produce a two-dimensional system with high planar doping density. Unlike metallic SrTiO3 quantum wells in GdTiO3 the single SrO delta-doped layer exhibits thermally activated DC and optical conductivity that agree in a quantitative manner with predictions of small polaron transport but with an extremely high two-dimensional density of polarons, ~ 7E14 cm-2

cond-mat.str-el↗

arXiv2013

Optical conductivity of nodal metals

Fermi liquid theory is remarkably successful in describing the transport and optical properties of metals; at frequencies higher than the scattering rate, the optical conductivity adopts the well-known power law behavior $σ_1(ω) \propto ω^{-2}$. We have observed an unusual non-Fermi liquid response $σ_1(ω) \propto ω^{-1\pm 0.2}$ in the ground states of several cuprate and iron-based materials which undergo electronic or magnetic phase transitions resulting in dramatically reduced or nodal Fermi surfaces. The identification of an inverse (or fractional) power-law behavior in the residual optical conductivity now permits the removal of this contribution, revealing the direct transitions across the gap and allowing the nature of the electron-boson coupling to be probed. The non-Fermi liquid behavior in these systems may be the result of a common Fermi surface topology of Dirac cone-like features in the electronic dispersion.

cond-mat.supr-con↗

arXiv2013

Goldstone and Higgs modes of photons inside a cavity

Goldstone and Higgs modes have been detected in various condensed matter, cold atom and particle physics experiments. Here, we demonstrate that the two modes can also be observed in optical systems with only a few (artificial) atoms inside a cavity. We establish this connection by studying the $ U(1)/Z_2 $ Dicke model where $ N $ qubits (atoms) coupled to a single photon mode. We determine the Goldstone and Higgs modes inside the super-radiant phase and their corresponding spectral weights by performing both $ 1/J=2/N $ expansion and exact diagonization (ED) study at a finite $ N $. We find nearly perfect agreements between the results achieved by the two approaches when $ N $ gets down even to $ N = 2 $. The quantum finite size effects at a few qubits make the two modes quite robust against an effectively small counter-rotating wave term. We present a few schemes to reduce the critical coupling strength, so the two modes can be observed in several experimental systems of (artificial) atoms inside a cavity by just conventional optical measurements.

quant-ph↗

arXiv2013

Experimental observation of ferrielectricity in multiferroic DyMn2O5: revisiting the electric polarization

The electric polarization and its magnetic origins in multiferroic RMn2O5, where R is rare-earth ion, are still issues under debate. In this work, the temperature-dependent electric polarization of DyMn2O5, the most attractive member of this RMn2O5 family, is investigated using the pyroelectric current method upon varying endpoint temperature of the electric cooling, plus the positive-up-negative-down (PUND) technique. It is revealed that DyMn2O5 at low temperature does exhibit the unusual ferrielectricity rather than ferroelectricity, characterized by two interactive and anti-parallel ferroelectric sublattices which show different temperature-dependences. The two ferroelectric sublattices are believed to be generated from the symmetric exchange-striction mechanisms associated with the Mn-Mn spin interactions and Dy-Mn spin interactions, respectively. The path-dependent electric polarization reflects the first-order magnetic transitions in the low temperature regime. The magnetoelectric effect is mainly attributed to the Dy spin order which is sensitive to magnetic field. The present experiments may be helpful for clarifying the puzzling issues on the multiferroicity in DyMn2O5 and probably other RMn2O5 multiferroics.

cond-mat.mtrl-sci↗

arXiv2014

Complex structures of different CaFe$_{2}$As$_{2}$ samples

The interplay between magnetism and crystal structures in three CaFe$_{2}$As$_{2}$ samples is studied. For the nonmagnetic quenched crystals, different crystalline domains with varying lattice parameters are found, and three phases (orthorhombic, tetragonal, and collapsed tetragonal) coexist between T$_{S}$ = 95 K and 45 K. Annealing of the quenched crystals at 350°C leads to a strain relief through a large (~1.3 %) expansion of the c-parameter and a small (~0.2 %) contraction of the a-parameter, and to local ~0.2 Å displacements at the atomic-level. This annealing procedure results in the most homogeneous crystals for which the antiferromagnetic and orthorhombic phase transitions occur at T$_{N}$/T$_{S}$ = 168(1) K. In the 700°C-annealed crystal, an intermediate strain regime takes place, with tetragonal and orthorhombic structural phases coexisting between 80 to 120 K. The origin of such strong shifts in the transition temperatures are tied to structural parameters. Importantly, with annealing, an increase in the Fe-As length leads to more localized Fe electrons and higher local magnetic moments on Fe ions. Synergistic contribution of other structural parameters, including a decrease in the Fe-Fe distance, and a dramatic increase of the c-parameter, which enhances the Fermi surface nesting in CaFe$_{2}$As$_{2}$, are also discussed.

cond-mat.supr-con↗

arXiv2013

Impact of spin-orbit coupling on the magnetism of Sr3MIrO6 (M = Ni, Co)

Using density functional calculations, we demonstrate that the spin-orbit coupling (SOC) of the Ir4+ ion plays an essential role in determining the antiferromagnetism of the hexagonal spin-chain system Sr3MIrO6 (M = Ni, Co) by tuning the crystal-field level sequence and altering the Ir-M inter-orbital interactions. The SOC splits the e'_{g} doublet of the octahedral Ir4+ ion (t_{2g}^5) in a trigonal crystal field, and the single t_{2g} hole resides on the e'_{g} upper branch and gives rise to the antiferromagnetic superexchange. In absence of the SOC, however, the single t_{2g} hole would occupy the a_{1g} singlet instead, which would mediate an unreal ferromagnetic exchange due to a direct a_{1g} hopping along the Ir-M chain. We also find that the Ni2+ and Co2+ ions are both in a high-spin state and moreover the Co2+ ion carries a huge orbital moment. This work well accounts for the recent experiments and magnifies again the significance of the SOC in iridates.

cond-mat.str-el↗

arXiv2014

First-order structural transition in the multiferroic perovskite-like formate [(CH3)2NH2][Mn(HCOO)3]

In this work we explore the overall structural behaviour of the [(CH3)2NH2][Mn(HCOO)3] multiferroic compound across the temperature range where its ferroelectric transition takes place by means of calorimetry, thermal expansion measurements and variable temperature powder and single crystal X-ray diffraction. The results clearly proof the presence of structural phase transition at Tt ~187 K (temperature at which the dielectric transition occurs) that involves a symmetry change from R-3c to Cc, twinning of the crystals, a discontinuous variation of the unit cell parameters and unit cell volume, and a sharp first-order-like anomaly in the thermal expansion. In addition, the calorimetric results show a 3-fold order-disorder transition. The calculated pressure dependence of the transition temperature is rather large (dTt/dP = 4.6 $\pm$ 0.1 K/kbar), so that it should be feasible to shift it to room temperature using adequate thermodynamic conditions, for instance by application of external pressure.

cond-mat.str-el↗

arXiv2013

Direct in-situ observation of structural transition driven actuation in VO2 utilizing electron transparent cantilevers

Direct imaging and quantification of actuation in nanostructures that undergo structural phase transitions could advance our understanding of collective phenomena in the solid state. Here, we demonstrate visualization of structural phase transition induced actuation in a model correlated insulator vanadium dioxide by in-situ Fresnel contrast imaging of electron transparent cantilevers. We quantify abrupt, reversible cantilever motion occurring due to the stress relaxation across the structural transition from monoclinic to tetragonal phase with increasing temperature. Deflections measured in such nanoscale cantilevers can be directly correlated with macroscopic stress measurements by wafer curvature studies as well as temperature dependent electrical conduction allowing one to interrogate lattice dynamics across length scales.

cond-mat.mtrl-sci↗

arXiv2014

A hybrid vanadium fluoride with structurally isolated S = 1 kagome layers

A new organically-templated vanadium (III) fluoride, (NH4)2(C2H8N)[V3F12], has been prepared using an ionothermal approach. This compound has a unique layered structure featuring distorted S = 1 kagome planes separated by the cationic species. The compound exhibits magnetic frustration, with a canted antiferromagnetic ground state. On further cooling within the ground state a pronounced change in magnetisation kinetics is observed.

cond-mat.str-el↗

arXiv2009

Multiple conducting carriers generated in LaAlO3/SrTiO3 heterostructures

We have found that there is more than one type of conducting carriers generated in LaAlO3/SrTiO3 heterostructures by comparing the sheet carrier density and mobility from optical transmission spectroscopy with those from dc-transport measurements. When multiple types of carriers exist, optical characterization dominantly reflects the contribution from the high-density carriers whereas dc-transport measurements may exaggerate the contribution of the high-mobility carriers even though they are present at low-density. Since the low-temperature mobilities determined by dc-transport in the LaAlO3/SrTiO3 heterostructures are much higher than those extracted by optical method, we attribute the origin of high-mobility transport to the low-density conducting carriers.

cond-mat.str-el↗

arXiv2013

High-resolution alternating-field technique to determine the magnetocaloric effect of metals down to very low temperatures

The magnetocaloric effect or "magnetic Grüneisen ratio" $Γ_H=T^{-1}(dT/dH)_S$ quantifies the cooling or heating of a material when an applied magnetic field is changed under adiabatic conditions. Recently this property has attracted considerable interest in the field of quantum criticality. Here we report the development of a low-frequency alternating field technique which allows to perform continuous temperature scans of $Γ_H(T)$ on small single crystals with very high precision and down to very low temperatures. Measurements on doped YbRh$_2$Si$_2$ show that $Γ_H(T)$ can be determined with this technique in a faster and much more accurate way than by calculation from magnetization and specific heat measurements.

cond-mat.str-el↗