arXiv subjects

Robert J. Cava

Publications and source records attributed to Robert J. Cava.

At least 127 records · Page 7Linked to original sources

arXiv2017

S-shaped suppression of the superconducting transition temperature in Cu-intercalated NbSe2

2H-NbSe2 is the prototype and most frequently studied of the well-known transition metal dichalcogenide (TMDC) superconductors. Widely acknowledged to be a conventional superconductor, its transition temperature to the superconducting state (Tc) is 7.3 K - a Tc that is substantially higher than those seen for the majority of TMDCs, where Tcs between 2 and 4 K are the norm. Here we report the intercalation of Cu into 2H-NbSe2 to make CuxNbSe2. As is typically found when chemically altering an optimal superconductor, Tc decreases with increasing x, but the way that Tc is suppressed in this case is unusual - an S-shaped character is observed, with an inflection point near x = 0.03 and, at higher x, a leveling off of the Tc near 3 K - down to the usual value for a layered TMDC. Electronic characterization reveals corresponding S-like behavior for many of the materials parameters that influence Tc. To illustrate its character, the superconducting phase diagram for CuxNbSe2 is contrasted to those of FexNbSe2 and NbSe2-xSx.

cond-mat.supr-con↗

arXiv2017

Prediction of Nontrivial Band Topology and Superconductivity in Mg$_2$Pb

The interplay of BCS superconductivity and nontrivial band topology is expected to give rise to opportunities for creating topological superconductors, achieved through pairing spin-filtered boundary modes via superconducting proximity effects. The thus-engineered topological superconductivity can, for example, facilitate the search for Majorana fermion quasiparticles in condensed matter systems. Here we report a first-principles study of Mg$_2$Pb and predict that it should be a superconducting topological material. The band topology of Mg$_2$Pb is identical to that of the archetypal quantum spin Hall insulator HgTe, while isostructural and isoelectronic Mg$_2$Sn is topologically trivial; a trivial to topological transition is predicted for Mg$_2$Sn$_{1-x}$Pb$_x$ for x~0.77. We propose that Mg$_2$Pb-Mg$_2$Sn quantum wells should generate robust spin-filtered edge currents in analogy to HgTe/CdTe quantum wells. In addition, our calculations predict that Mg$_2$Pb should become superconducting upon electron doping. Therefore, Mg$_2$Pb is expected to provide a practical material platform for studying emergent phenomena arising from the interplay of superconductivity and band topology.

cond-mat.supr-con↗

arXiv2017

Gate Tunable Magneto-resistance of Ultra-Thin WTe2 Devices

In this work, the magneto-resistance (MR) of ultra-thin WTe2/BN heterostructures far away from electron-hole equilibrium is measured. The change of MR of such devices is found to be determined largely by a single tunable parameter, i.e. the amount of imbalance between electrons and holes. We also found that the magnetoresistive behavior of ultra-thin WTe2 devices is well-captured by a two-fluid model. According to the model, the change of MR could be as large as 400,000%, the largest potential change of MR among all materials known, if the ultra-thin samples are tuned to neutrality when preserving the mobility of 167,000 cm2V-1s-1 observed in bulk samples. Our findings show the prospects of ultra-thin WTe2 as a variable magnetoresistance material in future applications such as magnetic field sensors, information storage and extraction devices, and galvanic isolators. The results also provide important insight into the electronic structure and the origin of the large MR in ultra-thin WTe2 samples.

cond-mat.mtrl-sci↗

arXiv2017

High-Pressure Synthesis and Characterization of $β$-GeSe - A Semiconductor with Six-Rings in an Uncommon Boat Conformation

Two-dimensional materials have significant potential for the development of new devices. Here we report the electronic and structural properties of $β$-GeSe, a previously unreported polymorph of GeSe, with a unique crystal structure that displays strong two-dimensional structural features. $β$-GeSe is made at high pressure and temperature and is stable under ambient conditions. We compare it to its structural and electronic relatives $α$-GeSe and black phosphorus. The $β$ form of GeSe displays a boat conformation for its Ge-Se six-ring, while the previously known $α$ form, and black phosphorus, display the more common chair conformation for their six-rings. Electronic structure calculations indicate that $β$-GeSe is a semiconductor, with an approximate bulk band gap of $Δ~\approx$ 0.5 eV, and, in its monolayer form, $Δ~\approx$ 0.9 eV. These values fall between those of $α$-GeSe and black phosphorus, making $β$-GeSe a promising candidate for future applications. The resistivity of our $β$-GeSe crystals measured in-plane is on the order of $ρ\approx$ 1 $Ω$cm, while being essentially temperature independent.

cond-mat.mtrl-sci↗

arXiv2017

Magnetoresistance and Quantum Oscillations of an Electrostatically Tuned Semimetal-to-Metal Transition in Ultra-Thin WTe2

We report on electronic transport measurements of electrostatically gated nano-devices of the semimetal WTe\textsubscript{2}. High mobility metallic behavior is achieved in the 2D limit by encapsulating thin flakes in an inert atmosphere. At low temperatures, we find that a large magnetoresistance can be turned on and off by electrostatically doping the system between a semimetallic state and an electron-only metallic state, respectively. We confirm the nature of the two regimes by analyzing the magnetoresistance and Hall effect with a two-carrier model, as well as by analysis of Shubnikov-de Haas oscillations, both of which indicate depletion of hole carriers via the electrostatic gate. This confirms that semiclassical transport of two oppositely charged carriers accurately describes the exceptional magnetoresistance observed in this material. Finally, we also find that the magnetoresistance power law is sub-quadratic and density-independent, suggesting new physics specifically in the semimetallic regime.

cond-mat.mes-hall↗

arXiv2016

Time-Reversal-Breaking Weyl Fermions in Magnetic Heusler Alloys

Weyl fermions have recently been observed in several time-reversal-invariant semimetals and photonics materials with broken inversion symmetry. These systems are expected to have exotic transport properties such as the chiral anomaly. However, most discovered Weyl materials possess a substantial number of Weyl nodes close to the Fermi level that give rise to complicated transport properties. Here we predict, for the first time, a new family of Weyl systems defined by broken time-reversal symmetry, namely, Co-based magnetic Heusler materials XCo2Z (X = IVB or VB; Z = IVA or IIIA). To search for Weyl fermions in the centrosymmetric magnetic systems, we recall an easy and practical inversion invariant, which has been calculated to be -1, guaranteeing the existence of an odd number of pairs of Weyl fermions. These materials exhibit, when alloyed, only two Weyl nodes at the Fermi level - the minimum number possible in a condensed matter system. The Weyl nodes are protected by the rotational symmetry along the magnetic axis and separated by a large distance (of order 2$π$) in the Brillouin zone. The corresponding Fermi arcs have been calculated as well. This discovery provides a realistic and promising platform for manipulating and studying the magnetic Weyl physics in experiments.

cond-mat.mtrl-sci↗

arXiv2016

Imaging electronic states on topological semimetals using scanning tunneling microscopy

Following the intense studies on topological insulators, significant efforts have recently been devoted to the search for gapless topological systems. These materials not only broaden the topological classification of matter but also provide a condensed matter realization of various relativistic particles and phenomena previously discussed mainly in high energy physics. Weyl semimetals host massless, chiral, low-energy excitations in the bulk electronic band structure, whereas a symmetry protected pair of Weyl fermions gives rise to massless Dirac fermions. We employed scanning tunneling microscopy/spectroscopy to explore the behavior of electronic states both on the surface and in the bulk of topological semimetal phases. By mapping the quasiparticle interference and emerging Landau levels at high magnetic field in Dirac semimetals Cd$_3$As$_2$ and Na$_3$Bi, we observed extended Dirac-like bulk electronic bands. Quasiparticle interference imaged on Weyl semimetal TaAs demonstrated the predicted momentum dependent delocalization of Fermi arc surface states in the vicinity of the surface-projected Weyl nodes.

cond-mat.mtrl-sci↗

arXiv2016

Effect of Electron Count and Chemical Complexity in the Ta-Nb-Hf-Zr-Ti High-Entropy Alloy Superconductor

High-entropy alloys are made from random mixtures of principal elements on simple lattices, stabilized by a high mixing entropy. The recently discovered BCC Ta-Nb-Hf-Zr-Ti high entropy alloy superconductor appears to display properties of both simple crystalline intermetallics and amorphous materials, e.g. it has a well defined superconducting transition along with an exceptional robustness against disorder. Here we show that the valence-electron count dependence of the superconducting transition temperature in the high entropy alloy falls between those of analogous simple solid solutions and amorphous materials, and test the effect of alloy complexity on the superconductivity. We propose high-entropy alloys as excellent intermediate systems for studying superconductivity as it evolves between crystalline and amorphous materials.

cond-mat.supr-con↗

arXiv2016

MoTe2: A Type-II Weyl Topological Metal

Based on the ab initio calculations, we show that MoTe2, in its low-temperature orthorhombic structure characterized by an X-ray diffraction study at 100 K, realizes 4 type-II Weyl points between the N-th and N+1-th bands, where N is the total number of valence electrons per unit cell. Other WPs and nodal lines between different other bands also appear close to the Fermi level due to a complex topological band structure. We predict a series of strain-driven topological phase transitions in this compound, opening a wide range of possible experimental realizations of different topological semimetal phases. Crucially, with no strain, the number of observable surface Fermi arcs in this material is 2 - the smallest number of arcs consistent with time-reversal symmetry.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity in CaBi$_{2}$

Superconductivity is observed with critical temperature $T_{c}$ = 2.0 K in self-flux-grown single crystals of $CaBi_{2}$. This material adopts the $ZrSi_{2}$ structure type with lattice parameters a = 4.696(1) $Å$, b = 7.081(2) $Å$ and c = 4.611(1) $Å$. The crystals of $CaBi_{2}$ were studied by means of magnetic susceptibility, specific heat and electrical resistivity measurements. The heat capacity jump at $T_{c}$ is $ΔC/γT_{c}$ = 1.41, confirming bulk superconductivity; the Sommerfeld coefficient $γ$ = 4.1 $mJ\: mol^{-1}\, K^{-2}$ and the Debye temperature $Θ_{D}$ = 157 K. The electron-phonon coupling strength is $λ_{el-ph}$ = 0.59, and the thermodynamic critical field $H_{c}$ is low, between 111 and 124 Oe $CaBi_{2}$ is a moderate coupling type-I superconductor. Results of electronic structure calculations are reported and charge densities, electronic bands, densities of states and Fermi surfaces are discussed, focusing on the effects of spin\textendash orbit coupling and electronic property anisotropy. We find a mixed quasi-2D + 3D character in the electronic structure, which reflects the layered crystal structure of the material.

cond-mat.supr-con↗

arXiv2016

A new form of (unexpected) Dirac fermions in the strongly-correlated cerium monopnictides

Discovering Dirac fermions with novel properties has become an important front in condensed matter and materials sciences. Here, we report the observation of unusual Dirac fermion states in a strongly-correlated electron setting, which are uniquely distinct from those of graphene and conventional topological insulators. In strongly-correlated cerium monopnictides, we find two sets of highly anisotropic Dirac fermions that interpenetrate each other with negligible hybridization, and show a peculiar four-fold degeneracy where their Dirac nodes overlap. Despite the lack of protection by crystalline or time-reversal symmetries, this four-fold degeneracy is robust across magnetic phase transitions. Comparison of these experimental findings with our theoretical calculations suggests that the observed surface Dirac fermions arise from bulk band inversions at an odd number of high-symmetry points, which is analogous to the band topology which describes a $\mathbb{Z}_{2}$-topological phase. Our findings open up an unprecedented and long-sought-for platform for exploring novel Dirac fermion physics in a strongly-correlated semimetal.

cond-mat.str-el↗

arXiv2016

Time-Reversal Symmetry Breaking Type-II Weyl State in YbMnBi2

Detection of Dirac, Majorana and Weyl fermions in real materials may significantly strengthen the bridge between high-energy and condensed-matter physics. While the presence of Dirac fermions is well established in graphene and topological insulators, Majorana particles have been reported recently and evidence for Weyl fermions in non-centrosymmetric crystals has been found only a couple of months ago, the 'magnetic' Weyl fermions are still elusive despite numerous theoretical predictions and intense experimental search. In order to detect a time-reversal symmetry breaking Weyl state we designed two materials with Fermi velocities superior to that of graphene and present here the experimental evidence of the realization of such a state in one of them, YbMnBi2. We model the time reversal symmetry breaking observed by magnetization measurements by a canted antiferromagnetic state and find a number of Weyl points both above and below the Fermi level. Using angle-resolved photoemission, we directly observe these latter Weyl points and a hallmark of the exotic state - the arc of the surface states which connects these points. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties.

cond-mat.mes-hall↗

arXiv2016

The New Superconductor tP-SrPd2Bi2: Structural Polymorphism and Superconductivity in Intermetallics

We consider a system where structural polymorphism suggests the possible existence of superconductivity through the implied structural instability. SrPd2Bi2 has two poly-morphs which can be controlled by the synthesis temperature: a tetragonal form (CaBe2Ge2-type) and a monoclinic form (BaAu2Sb2-type). Though the crystallographic difference between the two forms may at first seem trivial, we show that tetragonal SrPd2Bi2 is superconducting at 2.0 K whereas monoclinic SrPd2Bi2 is not. We rationalize this finding and place it in context with other 1-2-2 phases

cond-mat.supr-con↗

arXiv2016

Static and dynamic XY-like short-range order in a frustrated magnet with exchange disorder

A single crystal of the Co2+ based pyrochlore NaCaCo2F7 was studied by inelastic neutron scattering. This frustrated magnet with quenched exchange disorder remains in a strongly correlated paramagnetic state down to one 60th of the Curie-Weiss temperature. Below T_f = 2.4 K, diffuse elastic scattering develops and comprises 30 +/- 10% of the total magnetic scattering, as expected for J_{eff} = 1/2 moments frozen on a time scale that exceeds \hbar/δE=3.8 ps. The diffuse scattering is consistent with short range XY antiferromagnetism with a correlation length of 16 Å. The momentum (Q) dependence of the inelastic intensity indicates relaxing XY-like antiferromagnetic clusters at energies below ~ 5.5 meV, and collinear antiferromagnetic fluctuations above this energy. The relevant XY configurations form a continuous manifold of symmetry-related states. Contrary to well-known models that produce this continuous manifold, order-by-disorder does not select an ordered state in NaCaCo2F7 despite evidence for weak (~12 %) exchange disorder. Instead, NaCaCo2F7 freezes into short range ordered clusters that span this manifold.

cond-mat.str-el↗

arXiv2016

Differences in chemical doping matter - Superconductivity in Ti1-xTaxSe2 but not in Ti1-xNbxSe2

We report that 1T-TiSe2, an archetypical layered transition metal dichalcogenide, becomes superconducting when Ta is substituted for Ti but not when Nb is substituted for Ti. This is unexpected because Nb and Ta should be chemically equivalent electron donors. Superconductivity emerges near x = 0.02 for Ti1-xTaxSe2, while for Ti1-xNbxSe2, no superconducting transitions are observed above 0.4 K. The equivalent chemical nature of the dopants is confirmed by X-ray photoelectron spectroscopy. ARPES and Raman scattering studies show similarities and differences between the two systems, but the fundamental reasons why the Nb and Ta dopants yield such different behavior are unknown. We present a comparison of the electronic phase diagrams of many electron-doped 1T-TiSe2 systems, showing that they behave quite differently, which may have broad implications in the search for new superconductors. We propose that superconducting Ti0.8Ta0.2Se2 will be suitable for devices and other studies based on exfoliated crystal flakes.

cond-mat.supr-con↗

arXiv2016

Synthesis, crystal structure, and magnetic properties of RE3Sb3Mg2O14 (RE=La, Pr, Sm, Eu, Tb, Ho): new 2D Kagome materials

We present the crystal structures and magnetic properties of RE3Sb3Mg2O14 (La3Sb3Mg2O14, Pr3Sb3Mg2O14, Sm3Sb3Mg2O14, Eu3Sb3Mg2O14, Tb3Sb3Mg2O14, and Ho3Sb3Mg2O14), a family of novel materials based on a perfect geometry 2D rare earth Kagome lattice. Structure refinements were performed by the Rietveld method using X-ray diffraction data, indicating that the layered compounds are fully structurally ordered. The compounds crystallize in a rhombohedral supercell of the cubic pyrochlore structure, in the space group R-3m. As indicated by magnetic susceptibility measurements, they exhibit predominantly antiferromagnetic interactions between rare earth moments. Except for possibly Pr3Sb3Mg2O14 and Eu3Sb3Mg2O14, none of the compounds show any signs of magnetic ordering above 2 K. This RE3Sb3Mg2O14 family of compounds is similar to that of RE3Sb3Zn2O14, except the series reported here features a fully ordered distribution of cations in both the nonmagnetic antimony and magnesium sites and the magnetic rare earth kagome sites. The compounds appear to be relatively defect-free and are therefore model systems for investigating magnetic frustration on an ideal 2D rare earth Kagome lattice.

cond-mat.str-el↗

arXiv2015

Gigantic surface life-time of an intrinsic topological insulator revealed via time-resolved (pump-probe) ARPES

The interaction between light and novel two-dimensional electronic states holds promise to realize new fundamental physics and optical devices. Here, we use pump-probe photoemission spectroscopy to study the optically-excited Dirac surface states in the bulk-insulating topological insulator Bi2Te2Se, and reveal optical properties that are in sharp contrast to those of bulk-metallic topological insulators. We observe a gigantic optical life-time exceeding 4 micro-sec for the surface states in Bi2Te2Se, whereas the life-time in most topological insulators such as Bi2Se3 has been limited to a few picoseconds. Moreover, we discover a surface photo-voltage in topological materials, a shift of the chemical potential of the Dirac surface states, as large as 100 mV. Our results demonstrate a rare platform to study charge excitation and relaxation in energy and momentum space in a two dimensional quantum system.

cond-mat.mes-hall↗

arXiv2015

Superconductivity in Hf$_5$Sb$_3$$_-$$_x$Ru$_x$: Are Ru and Sb a critical charge-transfer pair for superconductivity?

We address the hypothesis that in intermetallic compounds, contrary to a long-standing view that considers electron count and crystal structure type as the only significant chemical criteria for the occurrence of superconductivity, consideration of the actual elements present is a third equally important factor. The importance of chemical identity may seem obvious, especially to chemists, but it has not previously been explicitly tested for intermetallic superconductors. Here we test the hypothesis by searching for and finding a new superconductor in the tetragonal symmetry Hf5Sb3-xMx solid solution. This phase is ideal for this study, because M can be many 3d, 4d and 5d transition metals in an M-Sb chain and is a minor elemental constituent. We find superconductivity for M = Ru only. This is the case even when the electron count can be adjusted to the same value with a different transition element within the same structure type. This leads us to propose that, like Cu-O (cuprate superconductors) and Fe-As (iron-pnictide superconductors) in different classes of compounds, Ru and Sb may be a critical element pair for superconductivity in intermetallic phases.

cond-mat.supr-con↗