arXiv subjects

D. J. Singh

Publications and source records attributed to D. J. Singh.

At least 19 recordsLinked to original sources

arXiv2025

Metallic layered materials with magnetic frustration: An ARPES view of the SmAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$

Compounds of the new materials class LnTAl$_4$X$_2$ (Ln = lanthanide, X = tetrel, T = transition metal) host exotic magnetic phenomena due to geometric frustration induced by their triangular lattice. Complex spin arrangements, magnetic fluctuations and double magnetic transitions have been well observed by means of magneto-transport. Nevertheless, the experimental electronic structure of this family of materials has been poorly studied. We have investigated the experimental electronic structure of two members of this class of materials: SmAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$. By means of Angle-Resolved PhotoEmission Spectroscopy (ARPES) accompanied by Density Functional Theory calculations (DFT), we reveal common trends and features, the important effect of localized spin moments on the electronic structure, the presence of surface-localized electronic states and the nature of the surface termination layer. Low-dimensionality, exchange interaction, and spin-orbit coupling are all important ingredients of the electronic structure.

cond-mat.str-el↗

arXiv2024

Unconventional Pressure Dependent Interorbital and Interlayer Doping in Superconducting Nickelates

The discovery of nickelate superconductivity provided the first example of a non-copper-based material with superconductivity strongly analogous to the cuprates, but recent findings raise questions and inconsistencies around the electron counts and doping phase diagrams. We show using superconducting La$_4$Ni$_3$O$_{10}$ that there are unconventional interlayer and interorbital intrinsic doping effects that render the $d_{x^2-y^2}$ orbital occupation similar to the cuprates. The results enable a consistent framework for nickelate superconductivity, while maintaining the connection between cuprate and nickelate superconductors.

cond-mat.supr-con↗

arXiv2022

KCo$_2$As$_2$: A New Portal for the Physics of High-Purity Metals

High-quality single crystals of KCo$_2$As$_2$ with the body-centered tetragonal ThCr$_2$Si$_2$ structure were grown using KAs self flux. Structural, magnetic, thermal, and electrical transport were investigated. No clear evidence for any phase transitions was found in the temperature range 2 to 300 K. The in-plane electrical resistivity $ρ$ versus temperature $T$ is highly unusual, showing a $T^4$ behavior below 30 K and an anomalous positive curvature up to 300 K which is different from the linear behavior expected from the Bloch-Grüneisen theory for electron scattering by acoustic phonons. This positive curvature has been previously observed in the in-plane resistivity of high-conductivity layered delafossites such as PdCoO$_2$ and PtCoO$_2$. The in-plane $ρ(T\to0) = 0.36~μΩ$ cm of KCo$_2$As$_2$ is exceptionally small for this class of compounds. The material also exhibits a nearly linear magnetoresistance at low $T$ which attains a value of about 40% at $T=2$K and magnetic field $H= 80$ kOe. The magnetic susceptibility $χ$ of KCo$_2$As$_2$ is isotropic and about an order of magnitude smaller than the values for the related compounds SrCo$_2$As$_2$ and BaCo$_2$As$_2$. The $χ$ increases above 100 K which is found from our first-principles calculations to arise from a sharp peak in the electronic density of states just above the Fermi energy $E_{\rm F}$. Heat capacity $C_{\rm p}(T)$ data at low $T$ yield an electronic density of states $N(E_{\rm F})$ that is about 36% larger than predicted by the first-principles theory. The $C_{\rm p}(T)$ data near room temperature suggest the presence of excited optic vibration modes which may also be the source of the positive curvature in $ρ(T)$. Our results show that KCo$_2$As$_2$ provides a new avenue for investigating the physics of high-purity metals.

cond-mat.str-el↗

arXiv2020

Competing magnetic orders in quantum critical Sr$_3$Ru$_2$O$_7$

We investigated Sr$_3$Ru$_2$O$_7$, a quantum critical metal that shows a metamagnetic quantum phase transition and electronic nematicity, through density functional calculations. These predict a ferromagnetic ground state in contrast to the experimentally observed paramagnetism, raising the question of competing magnetic states and associated fluctuations that may suppress magnetic order. We did a search to identify such low energy antiferromagnetically ordered metastable states. We find that the lowest energy antiferromagnetic state has a striped order. This corresponds to the E-type order that has been shown to be induced by Mn alloying. We also note significant transport anisotropy in this E-type ordered state. These results are discussed in relation to experimental observations.

cond-mat.str-el↗

arXiv2019

Density Functional Methods for the Magnetism of Transition Metals: SCAN in Relation to Other Functionals

We report tests of various density functionals for ferromagnetic, Fe, Co and Ni with a focus on characterizing the behavior of the so-called strongly constrained and appropriately normed (SCAN) functional. It is found that SCAN is closer in behavior to functionals that yield localized behavior, such as hybrid functionals, than other semilocal functionals that are tested. The results are understood in terms of a tendency to differentiate orbitals, favoring integer occupation, which is necessary for a correct description of atomic systems, but inappropriate for the open shell metallic ferromagnetic metals studied here.

cond-mat.str-el↗

arXiv2018

Tunability of electronic and optical properties of the Ba-Zr-S system via dimensional reduction

Transition metal sulfide perovskites offer lower band gaps and greater tunability than oxides, along with other desirable properties for applications. Here we explore dimensional reduction as a tuning strategy using the Ruddlesden-Popper phases in the Ba-Zr-S system as a model. The three dimensional perovskite BaZrS$_3$ is a direct band semiconductor, with a band gap of 1.5 eV suitable for solar photovoltaic application. However, the three known members of the Ruddlesden-Popper series, are all indirect gap materials, and additionally have lower fundamental band gaps. This is accompanied in the case of by a band structure that is more favorable for carrier transport for oriented samples. The layered Ruddlesden-Popper compounds, show significantly anisotropic optical properties, as may be expected. The optical spectra show tails at low energy, which may complicate experimental characterization of these materials.

cond-mat.mtrl-sci↗

arXiv2018

Quantum continuum fluctuations in glassy perovskite Ca(Co$_{0.15}$Ru$_{0.85}$)O$_{3}$

The quantum spin continuum and classical spin freezing, associated with a glassy state, represent two opposite extremes of a correlated electronic material. Here, we report the coexistence of a quantum spin continuum with a weak spin glass order in Co-doped CaRuO$_{3}$ perovskite near the chemical doping dependent metal-insulator transition boundary. Inelastic neutron measurements on Ca(Co$_{0.15}$Ru$_{0.85}$)O$_{3}$ at low temperature, $T$ = 1.5 K, reveal a continuum spectrum in the $Q-E$ space due to uncorrelated spin fluctuations. This persists across the glass transition at $T_G \simeq$23 K. Furthermore, scaling of the dynamic susceptibility yields a very small scaling coefficient $α$ $\simeq$ 0.1, suggesting extreme locality of the dynamic properties. The experimental results indicate the realization of a narrow regime where the distinction between continuum dynamic behavior and glass-like regimes is reduced.

cond-mat.str-el↗

arXiv2018

Optical and Electronic Properties of Doped $p$-type CuI: Explanation of Transparent Conductivity from First Principles

We report properties of the reported transparent conductor CuI, including the effect of heavy $p$-type doping. The results, based on first principles calculations, include analysis of the electronic structure and calculations of optical and dielectric properties. We find that the origin of the favorable transparent conducting behavior lies in the absence in the visible of strong interband transitions between deeper valence bands and states at the valence band maximum that become empty with $p$-type doping. Instead, strong interband transitions to the valence band maximum are concentrated in the infrared with energies below 1.3 eV. This is contrast to the valence bands of many wide band gap materials. Turning to the mobility we find that the states at the valence band maximum are relatively dispersive. This originates from their antibonding Cu $d$ - I $p$ character. We find a modest enhancement of the Born effective charges relative to nominal values, leading to a dielectric constant $\varepsilon(0)$=6.3. This is sufficiently large to reduce ionized impurity scattering, leading to the expectation that the properties of CuI can be still can be significantly improved through sample quality.

cond-mat.mtrl-sci↗

arXiv2018

Non-Fermi surface nesting driven commensurate magnetic ordering in Fe-doped Sr$_2$RuO$_4$

Sr$_2$RuO$_4$, an unconventional superconductor, is known to possess an incommensurate spin density wave instability driven by Fermi surface nesting. Here we report a static spin density wave ordering with a commensurate propagation vector $q_c$ = (0.25 0.25 0) in Fe-doped Sr$_2$RuO$_4$, despite the magnetic fluctuations persisting at the incommensurate wave vectors $q_{ic}$ = (0.3 0.3 L) as in the parent compound. The latter feature is corroborated by the first principles calculations, which show that Fe substitution barely changes the nesting vector of the Fermi surface. These results suggest that in addition to the known incommensurate magnetic instability, Sr$_2$RuO$_4$ is also in proximity to a commensurate magnetic tendency that can be stabilized via Fe doping.

cond-mat.str-el↗

arXiv2017

Interplay of spin-orbit coupling and hybridization in Ca3LiOsO6 and Ca3LiRuO6

The electronic ground state of Ca3LiOsO6 was recently considered within an intermediate coupling regime that revealed J=3/2 spin-orbit entangled magnetic moments. Through inelastic neutron scattering and density functional theory we investigate the magnetic interactions and probe how the magnetism is influenced by the change in hierarchy of interactions as we move from Ca3LiOsO6 (5d3) to Ca3LiRuO6 (4d3). An alteration of the spin-gap and ordered local moment is observed, however the magnetic structure, Neel temperature and exchange interactions are unaltered. To explain this behavior it is necessary to include both spin-orbit coupling and hybridization, indicating the importance of an intermediate coupling approach when describing 5$d$ oxides.

cond-mat.str-el↗

arXiv2017

Reemergence of high-Tc superconductivity in the (Li1-xFex)OHFe1-ySe under high pressure

The pressure-induced reemergence of the second high-Tc superconducting phase (SC-II) in the alkali-metal intercalated AxFe2-ySe2 (A = K, Rb, Cs, Tl) remains an enigma and proper characterizations on the superconducting- and normal-state properties of the SC-II phase were hampered by the intrinsic inhomogeneity and phase separation. To elucidate this intriguing problem, we performed a detailed high-pressure magnetotransport study on the recently discovered (Li1-xFex)OHFe1-ySe single crystals, which have high Tc~40 K and share similar Fermi surface topology as AxFe2-ySe2, but are free from the sample complications. We found that the ambient-pressure Tc~41 K is suppressed gradually to below 2 K upon increasing pressure to Pc ~5 GPa, above which a SC-II phase with higher Tc emerges and the Tc increases progressively to above 50 K up to 12.5 GPa. Interestingly, our high-precision resistivity data enable us to uncover the sharp transition of the normal state from a Fermi liquid for SC-I phase (0 < P < 5 GPa) to a non-Fermi-liquid for SC-II phase (P > 5GPa). In addition, the reemergence of high-Tc SC-II phase is found to accompany with a concurrent enhancement of electron carrier density. Since high-pressure structural study based on the synchrotron X-ray diffraction rules out the structural transition below 10 GPa, the observed SC-II phase with enhanced carrier density should be ascribed to an electronic origin associated with a pressure-induced Fermi surface reconstruction.

cond-mat.supr-con↗

arXiv2017

Electronic Properties, Screening and Efficient Carrier Transport in NaSbS2

NaSbS2 is a semiconductor that was recently shown to have remarkable efficacy as a solar absorber indicating efficient charge collection even in defected material. We report first principles calculations of properties that show (1) an indirect gap only slightly smaller than the direct gap, which may impede recombination of photoexcited carriers, (2) highly anisotropic electronic and optical properties reflecting a layered crystal structure, (3) a pushed up valence band maximum due to repulsion from the Sb 5s states and (4) cross-gap hybridization between the S p derived valence bands and the Sb 5p states. This latter feature leads to enhanced Born effective charges that can provide local screening and therefore defect tolerance. These features are discussed in relation to the performance of the compound as a semiconductor with efficient charge collection.

cond-mat.mtrl-sci↗

arXiv2016

High-Tc superconductivity in FeSe at high pressure: Dominant hole carriers and enhanced spin fluctuations

The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (Tc). However, high-Tc superconductivity without hole carriers has been suggested in FeSe single-layer films and intercalated iron-selenides, raising a fundamental question whether iron pnictides and chalcogenides have different pairing mechanisms. Here, we study the properties of electronic structure in the high-Tc phase induced by pressure in bulk FeSe from magneto-transport measurements and first-principles calculations. With increasing pressure, the low-Tc superconducting phase transforms into high-Tc phase, where we find the normal-state Hall resistivity changes sign from negative to positive, demonstrating dominant hole carriers in striking contrast to other FeSe-derived high-Tc systems. Moreover, the Hall coefficient is remarkably enlarged and the magnetoresistance exhibits anomalous scaling behaviors, evidencing strongly enhanced interband spin fluctuations in the high-Tc phase. These results in FeSe highlight similarities with high-Tc phases of iron pnictides, constituting a step toward a unified understanding of iron-based superconductivity.

cond-mat.supr-con↗

arXiv2016

Bipolar Conduction is the Origin of the Electronic Transition in Pentatellurides: Metallic vs. Semiconducting Behavior

The pentatellurides, ZrTe5 and HfTe5 are layered compounds with one dimensional transition-metal chains that show a never understood temperature dependent transition in transport properties as well as recently discovered properties suggesting topological semimetallic behavior. Here we show that these materials are semiconductors and that the electronic transition is due to a combination of bipolar effects and different anisotropies for electrons and holes. We report magneto-transport properties for two kinds of ZrTe5 single crystals grown with the chemical vapor transport (S1) and the flux method (S2), respectively. These have distinct transport properties at zero field: the S1 displays a metallic behavior with a pronounced resistance peak and a sudden sign reversal in thermopower at approximately 130 K, consistent with previous observations of the electronic transition; in strikingly contrast, the S2 exhibits a semiconducting-like behavior at low temperatures and a positive thermopower over the whole temperature range. Refinements on the single-crystal X-ray diffraction and the energy dispersive spectroscopy analysis revealed the presence of noticeable Te-vacancies in the sample S1, confirming that the widely observed anomalous transport behaviors in pentatellurides actually take place in the Te-deficient samples. Electronic structure calculations show narrow gap semiconducting behavior, with different transport anisotropies for holes and electrons. For the degenerately doped n-type samples, our transport calculations can result in a resistivity peak and crossover in thermopower from negative to positive at temperatures close to that observed experimentally. Our present work resolves the longstanding puzzle regarding the anomalous transport behaviors of pentatellurides, and also resolves the electronic structure in favor of a semiconducting state.

cond-mat.mtrl-sci↗

arXiv2016

Candidate Elastic Quantum Critical Point in LaCu$_{6-x}$Au$_x$

The structural properties of LaCu$_{6-x}$Au$_x$ have been studied using neutron diffraction, x-ray diffraction, and heat capacity measurements. The continuous orthorhombic-monoclinic structural phase transition in LaCu$_{6}$ is suppressed linearly with Au substitution until a complete suppression of the structural phase transition occurs at the critical composition, $x_{c}$ = 0.3. Heat capacity measurements at low temperatures indicate residual structural instability at $x_c$ that extends well into the orthorhombic phase. The instability is ferroelastic in nature, with density functional theory (DFT) calculations showing negligible coupling to electronic states near the Fermi level. The data and calculations presented here are consistent with the zero temperature termination of a continuous structural phase transition suggesting that the LaCu$_{6-x}$Au$_x$ series hosts an elastic quantum critical point.

cond-mat.str-el↗

arXiv2016

Spin-orbit coupling control of anisotropy, ground state and frustration in 5d2 Sr2MgOsO6

The influence of spin-orbit coupling (SOC) on the physical properties of the 5d2 system Sr2MgOsO6 is probed via a combination of magnetometry, specific heat measurements, elastic and inelastic neutron scattering, and density functional theory calculations. Although a significant degree of frustration is expected, we find that Sr2MgOsO6 orders in a type I antiferromagnetic structure at the remarkably high temperature of 108 K. The measurements presented allow for the first accurate quantification of the size of the magnetic moment in a 5d2 system of 0.60(2) muB - a significantly reduced moment from the expected value for such a system. Furthermore, significant anisotropy is identified via a spin excitation gap, and we confirm by first principles calculations that SOC not only provides the magnetocrystalline anisotropy, but also plays a crucial role in determining both the ground state magnetic order and the size of the local moment in this compound. Through comparison to Sr2ScOsO6, it is demonstrated that SOC-induced anisotropy has the ability to relieve frustration in 5d2 systems relative to their 5d3 counterparts, providing an explanation of the high TN found in Sr2MgOsO6.

cond-mat.str-el↗

arXiv2016

Evidence for proximity of YFe$_2$Si$_2$ to a magnetic quantum critical point

Calculations of the electronic and magnetic properties of the non-magnetic metallic compound YFe$_2$Si$_2$ are reported. These show that at the density functional level a magnetic state involving ordering along the $c$-axis. The electronic structure is three dimensional, and is similar to that of the unconventional superconductor YFe$_2$Ge$_2$ and as well as that of the high pressure collapsed tetragonal phase of KFe$_2$As$_2$, which is also a superconductor. Based on the results in relation to experiment, we infer that properties of YFe$_2$Si$_2$ are strongly influenced by a nearby antiferromagnetic quantum critical point.

cond-mat.str-el↗

arXiv2016

ThFeAsN in Relation to Other Iron-Based Superconductors

The electronic structure, magnetic and structural properties of the superconductor ThFeAsN are discussed in relation to the Fe-pnictide and Fe-chalcogenide superconductors based on results of first principles calculations. The electronic structure is that of a high density of states semimetal. It shows approximately nested hole and electron Fermi surfaces of Fe $d$ character involving the $xz,yz$ and $xy$ orbitals. There is a strong tendency towards magnetism at the GGA level, and this magnetism is important for describing the Fe-As bonding, but greatly overestimates the magnetic ordering. The lowest energy magnetic state at the GGA level is the stripe order, and this particular order couples strongly to electrons near $E_F$. ThFeAsN is therefore strongly similar to the Fe-pnictide family of superconductors, although it is a particularly anisotropic member.

cond-mat.supr-con↗