arXiv subjects

David J. Singh

Publications and source records attributed to David J. Singh.

At least 19 recordsLinked to original sources

arXiv2023

Optical anisotropy and nonlinearity in deep ultraviolet fluorooxoborates

Optical anisotropy and nonlinearity are two tantalizingly important and enticing properties of an optical crystal. Combining these two features will have a miraculous effect. The up conversion can extend solid state laser sources to the ultraviolet and deep ultraviolet (DUV) ranges through harmonic generation and for down conversion needed for quantum information technology, but only a few suitable materials are known as the medium because of the combination of properties that are required. These include suitable band gaps, moderate optical anisotropy for phase matching and strong nonlinear optical (NLO) response. Fluorooxoborates are a new ideal platform for this effect in DUV. Here we demonstrate that fluorooxoborate is the optimal framework for DUV NLO material and show that the significance of the incorporation of fluorine in borates. The NLO performance of fluorooxoborates is strongly improved in terms of local crystal structure and distribution of electronic states. Importantly, the role of fluorine is to control the structure, while maintaining high band gaps but does not directly provide large contributions to birefringence and the second harmonic generation as the conventional assumptions. This is a consequence of the microscopic electron distribution and the energy position of the fluorine states well below the valence band maxima. Based on our understandings, we constructed two artificial structure and they all behave as anticipated.

physics.optics↗

arXiv2022

Conventional Half-Heusler Alloys Advance State-of-the-Art Thermoelectric Properties

Half-Heusler (HH) phases have garnered much attention as thermally stable and non-toxic thermoelectric materials for power conversion. The most studied alloys to date utilize Hf, Zr, and Ti as the base components. These alloys can achieve a moderate dimensionless figure of merit, ZT, near 1. Recent studies have advanced the thermoelectric performance of HH alloys by employing nanostructures and novel compositions to achieve larger ZT, reaching as high as 1.5. Herein, we report that traditional alloying techniques applied to the conventional HfZr-based half-Heusler alloys can also lead to exceptional ZT. Specifically, we present the well-studied p-type Hf0.3Zr0.7CoSn0.3Sb0.7, previously reported to have a ZT~0.8, resonantly doped with less than 1 at. % metallic Al on the Sn/Sb site, touting a remarkable ZT near 1.5 at 980 K. This is achieved through a significant increase in power factor, by ~65%, and a notable but smaller decrease in thermal conductivity, by ~13%, at high temperatures. These favorable thermoelectric properties are discussed in terms of a local anomaly in the density of states near the Fermi energy designed to enhance the Seebeck coefficient, as revealed by first-principles calculations, as well as the emergence of a highly heterogeneous grain structure that can scatter phonons across different length scales, effectively suppressing the thermal conductivity. Consequently, the effective mass is significantly enhanced from ~ 7 to 10me within a single parabolic band model, consistent with the result from first-principles calculations. The discovery of high ZT in a commonly studied half-Heusler alloy through a conventional and non-complex approach opens a new path for further discoveries in similar types of alloys. Furthermore, it is reasonable to believe that the study will reinvigorate effort in exploring high thermoelectric performance in conventional alloy systems.

cond-mat.mtrl-sci↗

arXiv2022

Violation of the Rule of Parsimony: Mixed Local Moment and Itinerant Fe Magnetism in Fe$_{3}$GeN

Ternary iron nitrides are of considerable interest due to their diverse magnetic properties. We find, based on first principles calculations, that the relatively minor structural distortion from the cubic antiperovskite structure in Fe$_3$GeN leads to unusual magnetic behavior. In particular, there is a separation into Fe sites with very different magnetic behaviors, specifically a site with Fe atoms having a stable local moment and a site where the Fe shows characteristics of much more itinerant behavior. This shows a remarkable flexibility of the Fe magnetic behavior in these nitrides and points towards the possibility of systems where minor structural and chemical changes can lead to dramatic changes in magnetic properties. The results suggest that, analogously to oxide perovskite materials, modulation of magnetic properties via chemical or strain control of octahedral rotation may be feasible. This may then lead to approaches for tuning magnetism to realize properties of interest, for example tuning magnetic transitions to quantum critical regimes or to proximity to metamagnetic transitions of interest for devices.

cond-mat.mtrl-sci↗

arXiv2022

Gapless spin-excitations in the superconducting state of a quasi-one-dimensional spin-triplet superconductor

Majorana zero modes form as intrinsic defects in an odd-orbital one-dimensional superconductor thus motivating the search for such materials in the pursuit of Majorana physics. Here, we present combined experimental results and first principles calculations which suggest that quasi-one-dimensional K$_2$Cr$_3$As$_3$ may be such a superconductor. Using inelastic neutron scattering we probe the dynamic spin-susceptibilities of K$_2$Cr$_3$As$_3$ and K$_2$Mo$_3$As$_3$ and show the presence of antiferromagnetic spin-fluctuations in both compounds. Below the superconducting transition, these fluctuations gap in K$_2$Mo$_3$As$_3$ but not in K$_2$Cr$_3$As$_3$. Using first principles calculations, we show that these fluctuations likely arise from nesting on one dimensional features of the Fermi surface. Considering these results we propose that while K$_2$Mo$_3$As$_3$ is a conventional superconductor, K$_2$Cr$_3$As$_3$ is likely a spin-triplet, and consequently, topological superconductor.

cond-mat.supr-con↗

arXiv2022

Spectral Evidence for Unidirectional Charge Density Wave in Detwinned BaNi$_2$As$_2$

The emergence of unconventional superconductivity in proximity to intertwined electronic orders is especially relevant in the case of iron-based superconductors. Such order consists of an electronic nematic order and a spin density wave in these systems. BaNi$_2$As$_2$, like its well-known iron-based analog BaFe$_2$As$_2$, also hosts a symmetry-breaking structural transition that is coupled to a unidirectional charge density wave (CDW), providing a novel platform to study intertwined orders. Here, through a systematic angle-resolved photoemission spectroscopy study combined with a detwinning $B_1g$ uniaxial strain, we identify distinct spectral evidence of band evolution due to the structural transition as well as CDW-induced band folding. In contrast to the nematicity and spin density wave in BaFe$_2$As$_2$, the structural and CDW order parameters in BaNi$_2$As$_2$ are observed to be strongly coupled and do not separate in the presence of uniaxial strain. Our measurements point to a likely lattice origin of the CDW in BaNi$_2$As$_2$.

cond-mat.str-el↗

arXiv2022

Complex structure due to As bonding and interplay with electronic structure in superconducting BaNi2As2

BaNi2As2 is a superconductor chemically related to the Fe-based superconductors, with a complex and poorly understood structural phase transition. We show based on first principles calculations that in fact there are two distinct competing structures. These structures are very different from electronic, transport and bonding points of view but are close in energy. These arise due to complex As bonding patterns and drive distortions of the Ni layers. This is supported by photoemission experiments. This leads to an interplay of electronic and structural behavior including induced anisotropic of the electronic transport. The competition between these distortions is associated with the complex behavior observed in BaNi2As2 samples.

cond-mat.supr-con↗

arXiv2021

Identification of a Low Energy Metastable 1$T$-Type Phase for Monolayer VSe2

Elucidating the polymorphism of transition metal dichalcogenide layers and the interplay between structure and properties is a key challenge for the application of these materials. We identify a novel low energy metastable phase of monolayer VSe$_2$ and elucidate its magnetic and electronic properties. This structure is distinct from the previously identified charge density wave (CDW) phase. However, while having rather distinct properties it is very close in energy to the CDW phase and is likely to be realized in experiments. Importantly, local bonding instabilities, as characterized by reconstruction of the electronic structure over a wide energy range, are important for this distortion, which includes both V off-centering in the octahedral coordination cages and a partial disproportionation into two distinct types of V. The phase does not have a ferromagnetic ground state. The results show that the physics of 1$T$ VSe$_2$ are richer than previously known with an interplay of Fermi surface instabilities and local bonding effects.

cond-mat.str-el↗

arXiv2021

Ferromagnetism in a Semiconductor with Mobile Carriers via Low-Level Nonmagnetic Doping

We show that doped cubic iron pyrite, which is a diamagnetic semiconductor, becomes ferromagnetic when $p$-type doped. We furthermore find that this material can exhibit high spin polarization both for tunneling and transport devices. These results are based on first principles electronic structure and transport calculations. This illustrates the use of $p$-type doping without magnetic impurities as a strategy for obtaining ferromagnetic semiconducting behavior, with implications for spintronic applications that require both magnetic ordering and good mobility. This is a combination that has been difficult to achieve by doping semiconductors with magnetic impurities. We show that phosphorus and arsenic may be effective dopants for achieving this behavior.

cond-mat.mtrl-sci↗

arXiv2021

K2Mo3As3 is a Multi-Gap Electron-Phonon Superconductor

We show using density functional calculations that quasi-one-dimensional K2Mo3As3, which is closely related to the K2Cr3As3 and has very similar superconducting properties, is not close to magnetism and has conventional s-wave electron-phonon superconductivity. This superconductivity is of multi-gap character due to different coupling on different Fermi surface sheets. This is discussed in relation to the properties of this family of quasi-one-dimensional pnictide superconductors. The results show that this family of superconductors provides a unique opportunity for studying the interplay of spin-fluctuations and electron-phonon superconductivity in transition metal pnictides and offer a path for sorting out the different proposed superconducting scenarios in this fascinating family of pnictide superconductors.

cond-mat.supr-con↗

arXiv2020

Prediction of superconductivity and topological aspects in single-layer $β$-Bi$_{2}$Pd

Topological superconductors, characterized by topologically nontrivial states residing in a superconducting gap, are a recently discovered class of materials having Majorana Fermions. The interplay of superconductivity and topological states give rise to opportunities for achieving such topological superconductors in condensed matter systems. Up to now, several single-material topological superconductors in this form have been theoretically predicted and experimentally confirmed. Here, using the first-principles calculations, we study the superconducting single-layer $β$-Bi$_{2}$Pd. The electronic density of states near Fermi level of this monolayer are dominated by the Bi-p and Pd-d orbitals, forming a two-band Fermi surface with multi-class sheets. The presence of soft phonon bands, in cooperation with the electron susceptibility, account for electron-phonon superconductivity of single-layer $β$-Bi$_{2}$Pd. With the centrosymmetric structure, single-layer $β$-Bi$_{2}$Pd possesses a continuous gap over the whole Brillouin zone and topological Dirac-like states at its one-dimensional boundary. The present findings would lead to the expectation of one-dimensional topological superconductivity and Majorana bound states in monolayer candidate of $β$-Bi$_{2}$Pd with intrinsic full-gap superconductivity.

cond-mat.mtrl-sci↗

arXiv2020

Establishing the carrier scattering phase diagram for ZrNiSn-based half-Heusler thermoelectric materials

Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. With ZrNiSn-based half-Heusler materials as an example, we use high-quality single and polycrystalline crystals, various probes, including electrical transport measurements, inelastic neutron scattering measurement, and first-principles calculations, to investigate the underlying electron-phonon interaction. We find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering, but has negligible influence on lattice thermal conductivity. Furthermore, it is possible to establish a carrier scattering phase diagram, which can be used to select reasonable strategies for optimization of the thermoelectric performance.

cond-mat.mtrl-sci↗

arXiv2020

Shortcomings of meta-GGA functionals when describing magnetism

Several recent studies have shown that SCAN, a functional belonging to the meta-generalized gradient approximation (MGGA) family, leads to significantly overestimated magnetic moments in itinerant ferromagnetic metals. However, this behavior is not inherent to the MGGA level of approximation since TPSS, for instance, does not lead to such severe overestimations. In order to provide a broader view of the accuracy of MGGA functionals for magnetism, we extend the assessment to more functionals, but also to antiferromagnetic solids. The results show that to describe magnetism there is overall no real advantage in using a MGGA functional compared to GGAs. For both types of approximation, an improvement in ferromagnetic metals is necessarily accompanied by a deterioration (underestimation) in antiferromagnetic insulators, and vice-versa. We also provide some analysis in order to understand in more detail the relation between the mathematical form of the functionals and the results.

cond-mat.mtrl-sci↗

arXiv2020

Understanding the lattice thermal conductivity of SrTiO3 from an ab initio perspective

We present a detailed analysis of the structure dependence of the lattice thermal conductivity of SrTiO3. We have used both ab initio Molecular Dynamic simulations and Density Functional Theory calculations to decouple the effect of different structural distortions on the thermal conductivity. We have identified two main mechanisms for tuning the thermal conductivity when a distortion is applied. First, the modification of the acoustic-modes energy dispersion when a change in the lattice parameters is imposed and second, the low energy polar modes. In particular and counterintuitively, we have found that an increase in the angle of the oxygen octahedral rotations increases the thermal conductivity due to its coupling to these polar modes.

cond-mat.mtrl-sci↗

arXiv2020

Characterization of rattling in relation to thermal conductivity: ordered half-Heusler semiconductors

The factors that affect the thermal conductivity of semiconductors is a topic of great scientific interest, especially in relation to thermoelectrics. Key developments have been the concept of the phonon-glass-electron-crystal (PGEC) and the related idea of rattling to achieve this. We use first principles phonon and thermal conductivity calculations in order to explore the concept of rattling for stoichiometric ordered half-Heusler compounds. These compounds can be regarded as filled zinc blende materials, and the filling atom could be viewed as a rattler if it is weakly bound. We use two simple metrics, one related to the frequency and the other to bond frustration and anharmonicity. We find that both measures correlate with thermal conductivity. This suggests that both may be useful in screening materials for low thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2019

Electronic and magnetic properties of perovskite selenite and tellurite compounds: CoSeO$_3$, NiSeO$_3$, CoTeO$_3$ and NiTeO$_3$

Selenium and tellurium are among the few elements that form $AB$O$_3$ perovskite structures with a four valent ion in the $A$ site. This leads to highly distorted structures and unusual magnetic behavior. Here we investigate the Co and Ni selenite and tellurite compounds, CoSeO$_3$, CoTeO$_3$, NiSeO$_3$ and NiTeO$_3$ using first principles calculations. We find an interplay of crystal field and Jahn-Teller distortions that underpin the electronic and magnetic properties. While all compounds are predicted to show an insulating G-type antiferromagnetic ground state, there is a considerable difference in the anisotropy of the exchange interactions between the Ni and Co compounds. This is related to the Jahn-Teller distortion. Finally, we observe that these four compounds show characteristics generally associated with Mott insulators, even when described at the level of standard density functional theory. These are then dense bulk band or Slater, Mott-type insulators.

cond-mat.str-el↗

arXiv2019

Tuning from Frustrated Magnetism to Superconductivity in Quasi-One-Dimensional KCr$_3$As$_3$ Through Hydrogen Doping

We report the charge doping of KCr$_3$As$_3$ via H intercalation. We show that the previously reported ethanol bath deintercalation of K$_2$Cr$_3$As$_3$ to KCr$_3$As$_3$ has a secondary effect whereby H from the bath enters the quasi-one-dimensional Cr$_6$As$_6$ chains. Furthermore, we find that - contrary to previous interpretations - the difference between non-superconducting as-grown KCr$_3$As$_3$ samples and superconducting hydrothermally annealed samples is not a change in crystallinity but due to charge doping, with the latter treatment increasing the H concentration in the CrAs tubes effectively electron-doping the 133 compound. These results suggest a new stoichiometry KH$_x$Cr$_3$As$_3$, that superconductivity arises from a suppressed magnetic order via a tunable parameter and pave the way for the first charge-doped phase diagram in these materials.

cond-mat.supr-con↗

arXiv2019

Dielectric Behavior as a Screen in Rational Searches for Electronic Materials: Metal Pnictide Sulfosalts

Dielectric screening plays an important role in reducing the strength of carrier scattering and trapping by point defects for many semiconductors such as the halide perovskite solar materials. However, it was rarely considered as a screen to find new electronic semiconductors. We performed a material search study using the dielectric properties as a screen to identify potential electronic materials in the class of metal-pnictide ternary sulfosalts, containing Bi or Sb. These salts are basically ionic due to the electronegativity difference between the S and both the metal and pnictogen elements. However, we do find significant cross-gap hybridization between the S p-derived valence bands and pnictogen p-derived conduction bands in many of the materials. This leads to enhanced Born effective charges, and in several cases, highly enhanced dielectric constants. We find a chemical rule for high dielectric constants in terms of the bond connectivity of the pnictogen-chalcogen part of the crystal structure. We additionally find a series of compounds with low effective mass, high dielectric constant and other properties that suggest good performance as electronic materials and also several potential thermoelectric compounds. Experimental optical data and solar conversion efficiency are reported for Sn-Sb-S samples, and results in accord with predicted good performance are found. The results illustrate the utility of dielectric properties as a screen for identifying complex semiconductors.

cond-mat.mtrl-sci↗

arXiv2019

Switchable out-of-plane polarization in two-dimensional LiAlTe$_2$

Covalent-polar semiconductors that show intrinsic two-dimensional (2D) vertical polarization present new device opportunities. These materials differ from ordinary ferroelectrics in that they are able to maintain polarization normal to a surface even with an unscreened depolarization field. Identifying phases that exhibit intrinsic 2D vertical polarization is an ongoing challenge. Here we report via computational material design the discovery of a new promising phase, specifically 2D LiAlTe$_2$. The design idea is developed from the physical understanding of three-dimensional hyperferroelectric covalent polar semiconductors. We used the structure determination method combining swarm intelligence algorithm and first-principles calculations to identify energetically stable structures. In addition to the expected layered version of bulk LiAlTe$_2$, $β$-LiAlTe$_2$, we find a novel 2D structure, $γ$-LiAlTe$_2$. In this phase, the vertical dipole can be switched between 0.07 and -0.11 e$\cdot$angstrom. This switching is triggered by the movement of Li atom between two local energy minima. The associated asymmetric double-well energy profile can be continuously tuned by the applied electric field as well as strain. There is, therefore, a reversible transition between two polar states. This discovered off-plane switchability provides an opportunity for the 2D $γ$-LiAlTe$_2$ based interfacial phase change memory device for example by growing $γ$-LiAlTe$_2$/GeTe heterostructures.

cond-mat.mtrl-sci↗