Low-temperature thermal conductivity measurements were performed on single crystal of ${\mathrm{PbTaSe}}_2$, a noncentrosymmetric superconductor with topological bulk nodal lines in the electronic band structure. It is found that the residual linear term ${\kappa }_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of ${\kappa }_0/T$ exhibits an $\mathsf{S}$-shaped curve. These results suggest that ${\mathrm{PbTaSe}}_2$ has multiple nodeless superconducting gaps. Therefore, the spin-triplet state with gap nodes does not play an important role in this noncentrosymmetric superconductor with strong spin-orbital coupling. The fully gapped superconducting state also meets the requirement of a topological superconductor, if ${\mathrm{PbTaSe}}_2$ is indeed the case.

A first-principles nonequilibrium molecular dynamics (NEMD) study employing the color-diffusion algorithm has been conducted to obtain the bulk ionic conductivity and the diffusion constant of gadolinium-doped cerium oxide (GDC) in the 850–1150 K temperature range. Being a slow process, ionic diffusion in solids usually requires simulation times that are prohibitively long for ab initio equilibrium molecular dynamics. The use of the color-diffusion algorithm allowed us to substantially speed up the oxygen-ion diffusion. The key parameters of the method, such as field direction and strength as well as color-charge distribution, have been investigated and their optimized values for the considered system have been determined. The calculated ionic conductivity and diffusion constants are in good agreement with available experimental data.

Static dc magnetization $M\left(H\right)$ measurements were performed for a single crystal of ${\mathrm{UBe}}_{13}$ down to 0.1 K in magnetic fields up to 80 kOe for $H\parallel \langle 100\rangle$ and $H\parallel \langle 110\rangle$. For both field directions, an unusual magnetic anomaly was observed at $H_{\mathrm{Mag}}^{*}\sim 20$–30 kOe in the superconducting mixed state. This anomaly was seen in the thermal-equilibrium-magnetization curves as well as the increasing- and decreasing-field processes of the raw magnetization curves, implying a change of the thermodynamic property at $H_{\mathrm{Mag}}^{*}$. Furthermore, the magnetic anisotropy of the superconducting diamagnetic response is found to be significant above $H_{\mathrm{Mag}}^{*}$, possibly associated with the anisotropy of the upper critical field $H_{\mathrm{c}2}$. Considering the suppression of reduced upper critical field $h^{*}$ and the dramatic decrease of Maki parameter ${\kappa }_2$ around $0.9T_{\mathrm{c}}$, the paramagnetic effect is present in ${\mathrm{UBe}}_{13}$. The paramagnetic effect in ${\mathrm{UBe}}_{13}$ could become anisotropic at low temperature below $\sim T_{\mathrm{c}}/2$. An alternative explanation of the magnetic anisotropy is however that the anisotropy of the nonlinear susceptibility for the heavy electrons in the vortex cores becomes significant below $\sim T_{\mathrm{c}}/2$.

We report on the electronic structure of the 111-type iron pnictide superconductor LiFeAs as a function of temperature using angle-resolved photoemission spectroscopy. Below approximately 50 K, both the $d_{yz}$ hole band at the $Z$ point and the $d_{xz/yz}$ electron band at the $A$ point shift to a higher binding energy side. However, at the high-symmetry points $Z,A,\mathrm{\Gamma }$, and $M$, the remaining bands are almost independent of temperature. One of the possible scenarios for these observations is that a strong, three-dimensional orbital-dependent correlation exists in the normal state of LiFeAs in relation to short-range spin fluctuations.

Here we present the results of high-resolution x-ray diffraction experiments along with specific heat, resistivity, and magnetization measurements of chemically well-characterized ${\mathrm{Fe}}_{1.12-x}{M}_x\mathrm{Te}$ ($M=\mathrm{Co}$, Ni) samples. The motivation is to investigate how the two coupled magnetostructural phase transitions in the antiferromagnetic parent compound ${\mathrm{Fe}}_{1.12}\mathrm{Te}$ of chalcogenide superconductors can be tuned. While the two-step magnetostructural transition (tetragonal-to-orthorhombic followed by orthorhombic-to-monoclinic) persists in ${\mathrm{Fe}}_{1.10}{\mathrm{Co}}_{0.02}\mathrm{Te}$, only one, tetragonal-to-orthorhombic transition was observed in ${\mathrm{Fe}}_{1.10}{\mathrm{Ni}}_{0.02}\mathrm{Te}$. Upon increasing the Co and Ni substitution, the structural phase transitions and the long-range magnetic order are systematically suppressed without any sign of superconductivity. For high substitution levels $\left(x\ge 0.05\right)$, a spin-glass-like behavior was observed and the low-temperature structure remains tetragonal. From our results, it can be inferred that the electron doping strongly suppresses the magnetostructural phase transitions.

We investigate the effect of magnetic frustration on the dielectric properties of dimer Mott insulators. We adopt the two-dimensional quarter-filled extended Hubbard model for $\kappa \text{−}\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right){}_{2}X$ with an additional coupling with the dimer lattice motions, and numerically obtain the ground state. In the parameter range for strong magnetic frustration, an interdimer bond-length alternation mode that lifts the quasidegeneracy of classical spin configurations is strongly coupled to the electron system; this alternation mode occurs even with an extremely small electron-lattice coupling constant. Furthermore, the bond-length alternation induces in the dimers electric dipoles, which align along the same direction. A dimer Mott insulator phase with the ferroelectric charge order is obtained in the region. These results are consistent with the relaxorlike dielectric anomaly in $\kappa \text{−}{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$.

Tantalum nitride is considered a promising material for photoelectrochemical water splitting, however, its charge transport properties remain poorly understood. We investigated polaronic and band transport in ${\mathrm{Ta}}_3{\mathrm{N}}_5$ using first-principles calculations. We first studied the formation of small polarons using density-functional theory (DFT) including $\mathrm{DFT}+U$ and hybrid functionals. We found that electron small polarons may occur but hole polarons are not energetically favorable. The estimated polaronic mobility for electrons is at least three orders of magnitude smaller than that measured in ${\mathrm{Ta}}_3{\mathrm{N}}_5$ films, suggesting that the main transport mechanism for both electrons and holes is bandlike. Since band transport is strongly affected by the carrier effective masses, and ${\mathrm{Ta}}_3{\mathrm{N}}_5$ is known to have large electron and hole effective masses, we also investigated whether substitutional impurities or strain may help lower the effective masses. We found a significant reduction in both electron and hole effective masses (up to 17% for electrons and 39% for holes) under applied strain, which may lead to a substantial improvement (up to 30% for electrons and 15% for holes) in the carrier mobilities.

Semiconductor quantum dots in silicon are promising qubits because of long spin coherence times and their potential for scalability. However, such qubits with complete electrical control and fidelities above the threshold for quantum error correction have not yet been achieved. We show theoretically that the threshold fidelity can be achieved with ac gate operation of the quantum dot hybrid qubit. Formed by three electrons in a double dot, this qubit is electrically controlled, does not require magnetic fields, and runs at gigahertz (GHz) gate speeds. We analyze the decoherence caused by $1/f$ charge noise in this qubit, find parameters that minimize the charge noise dependence in the qubit frequency, and determine the optimal working points for ac gate operations that drive the detuning and tunnel coupling.

Optical conformal mapping has been used to construct several isotropic devices with novel functionalities. In particular, a conformal cloak could confer omnidirectional invisibility. However, the maximum values of the refractive indexes needed for current designs are too large to implement, even in microwave experiments. Furthermore, most devices designed so far have had imperfect impedance matching and therefore incomplete invisibility functionalities. Here we describe a perfect conformal invisible device with full impedance matching everywhere. The maximum value of refractive index required by our device is just about five, which is feasible for microwave and terahertz experiments using current metamaterial techniques. To construct the device, we use a logarithmic conformal mapping and a Mikaelian lens. Our results should enable a conformal invisible device with almost perfect invisibility to be made soon.

We develop a variational scheme called the “Gutzwiller renormalization group” (GRG), which enables us to calculate the ground state of Anderson impurity models (AIM) with arbitrary numerical precision. Our method exploits the low-entanglement property of the ground state of local Hamiltonians in combination with the framework of the Gutzwiller wave function and indicates that the ground state of the AIM has a very simple structure, which can be represented very accurately in terms of a surprisingly small number of variational parameters. We perform benchmark calculations of the single-band AIM that validate our theory and suggest that the GRG might enable us to study complex systems beyond the reach of the other methods presently available and pave the way to interesting generalizations, e.g., to nonequilibrium transport in nanostructures.

We investigated the electronic structure of $\mathrm{Ir}{\mathrm{O}}_2$ to address the controversy regarding spin-orbit coupling (SOC) effects in metallic $5d$ transition-metal oxides. Two issues have come to the forefront: (1) SOC effects on electronic structure and physical properties of $\mathrm{Ir}{\mathrm{O}}_2$ and (2) the possible formation of a novel ground state in this material, the $J_{\mathrm{eff}}=1/2$ state. To better understand the SOC mechanism, we grew epitaxial $\mathrm{Ir}{\mathrm{O}}_2$ films whose $dc$ resistivity values were comparable with those of a single crystal. We obtained polarization-dependent optical and x-ray absorption spectra (XAS) and compared these results with those acquired using the generalized gradient approximation (GGA) and $\mathrm{GGA}+\mathrm{SOC}$ calculations. From the optical spectra, peak structures were identified at 0.4 and 2.0 eV, which could only be explained using the $\mathrm{GGA}+\mathrm{SOC}$ calculation. This suggests that SOC plays an important role in the electronic structure of $\mathrm{Ir}{\mathrm{O}}_2$. From the polarization-dependent $\mathrm{O}1s$ XAS spectra, we observed that the empty state near the Fermi level lacks involvement of an Ir $d_{xy}$ orbital. Despite the importance of SOC in $\mathrm{Ir}{\mathrm{O}}_2$, the $J_{\mathrm{eff}}=1/2$ state does not form in metallic $\mathrm{Ir}{\mathrm{O}}_2$.

Recently, first principle-based predictions of lattice thermal conductivity $\kappa$ from perturbation theory have achieved significant success. However, it only includes three-phonon scattering due to the assumption that four-phonon and higher-order processes are generally unimportant. Also, directly evaluating the scattering rates of four-phonon and higher-order processes has been a long-standing challenge. In this work, however, we have developed a formalism to explicitly determine quantum mechanical scattering probability matrices for four-phonon scattering in the full Brillouin zone, and by mitigating the computational challenge we have directly calculated four-phonon scattering rates. We find that four-phonon scattering rates are comparable to three-phonon scattering rates at medium and high temperatures, and they increase quadratically with temperature. As a consequence, $\kappa$ of Lennard-Jones argon is reduced by more than 60% at 80 K when four-phonon scattering is included. Also, in less anharmonic materials—diamond, silicon, and germanium—$\kappa$ is still reduced considerably at high temperature by four-phonon scattering by using the classical Tersoff potentials. Also, the thermal conductivity of optical phonons is dominated by the fourth- and higher-orders phonon scattering even at low temperature.

We investigate the contribution of charge puddles to the nonvanishing conductivity minimum in disordered graphene flakes at the charge neutrality point. For that purpose, we study systems with a geometry that suppresses the transmission due to evanescent modes allowing us to single out the effect of charge fluctuations in the transport properties. We use the recursive Green's function technique to obtain local and total transmissions through systems that mimic vanishing density of states at the charge neutrality point in the presence of a local disordered local potential to model the charge puddles. Our microscopic model includes electron-electron interactions via a spin resolved Hubbard mean field term. We establish the relationship between the charge puddle disorder potential and the electronic transmission at the charge neutrality point. We find that electronic interactions do not play a significant role in this setting. We discuss the implications of our findings to high mobility graphene samples deposited on different substrates and provide a qualitative interpretation of recent experimental results.

We simulate the femtosecond-laser-induced desorption dynamics of a diatomic molecule from a metal surface by including the effect of the electron and phonon excitations created by the laser pulse. Following previous models, the laser-induced surface excitation is treated through the two temperature model, while the multidimensional dynamics of the molecule is described by a classical Langevin equation, in which the friction and random forces account for the action of the heated electrons. In this work we propose the additional use of the generalized Langevin oscillator model to also include the effect of the energy exchange between the molecule and the heated surface lattice in the desorption dynamics. The model is applied to study the laser-induced desorption of ${\mathrm{O}}_2$ from the Ag(110) surface, making use of a six-dimensional potential energy surface calculated within density functional theory. Our results reveal the importance of the phonon mediated process and show that, depending on the value of the electronic density in the surroundings of the molecule adsorption site, its inclusion can significantly enhance or reduce the desorption probabilities.

We study the magnetic response of superconducting $\gamma$-Ga via low temperature scanning tunneling microscopy and spectroscopy. The magnetic vortex cores rely substantially on the Ga geometry, and exhibit an unexpectedly large axial elongation with aspect ratio up to 40 in rectangular Ga nanostrips (width $l<100$ nm). This is in stark contrast with the isotropic circular vortex core in a larger round-shaped Ga island. We suggest that the unusual elongated vortices in Ga nanostrips originate from geometric confinement effect probably via the strong repulsive interaction between the vortices and Meissner screening currents at the sample edge. Our finding provides novel conceptual insights into the geometrical confinement effect on magnetic vortices and forms the basis for the technological applications of superconductors.

We present a versatile scheme for creating topological Bogoliubov excitations in weakly interacting bosonic systems. Our proposal relies on a background stationary field that consists of a kagome vortex lattice, which breaks time-reversal symmetry and induces a periodic potential for Bogoliubov excitations. In analogy to the Haldane model, no external magnetic field or net flux is required. We construct a generic model based on the two-dimensional nonlinear Schrödinger equation and demonstrate the emergence of topological gaps crossed by chiral Bogoliubov edge modes. Our scheme can be realized in a wide variety of physical systems ranging from nonlinear optical systems to exciton-polariton condensates.

The ordered double-perovskite $\mathrm{S}{\mathrm{r}}_2\mathrm{FeMo}{\mathrm{O}}_6$ (SFMO) possesses remarkable room-temperature low-field colossal magnetoresistivity and transport properties which are related, at least in part, to combined structural and magnetic instabilities that are responsible for a cubic-tetragonal phase transition near 420 K. A formal strain analysis combined with measurements of elastic properties from resonant ultrasound spectroscopy reveal a system with weak biquadratic coupling between two order parameters belonging to ${\mathrm{\Gamma }}_4^{+}$ and $m{\mathrm{\Gamma }}_4^{+}$ of parent space group $Fm\overline{3}m$. The observed softening of the shear modulus by ∼50% is due to the classical effects of strain/order parameter coupling at an improper ferroelastic $\left({\mathrm{\Gamma }}_4^{+}\right)$ transition which is second order in character, while the ferromagnetic order parameter $\left(m{\mathrm{\Gamma }}_4^{+}\right)$ couples only with volume strain. The influence of a third order parameter, for ordering of Fe and Mo on crystallographic B sites, is to change the strength of coupling between the ${\mathrm{\Gamma }}_4^{+}$ order parameter and the tetragonal shear strain due to the influence of changes in local strain heterogeneity at a unit cell scale. High anelastic loss below the transition point reveals the presence of mobile ferroelastic twin walls which become pinned by oxygen vacancies in a temperature interval near 340 K. The twin walls must be both ferroelastic and ferromagnetic, but due to the weak coupling between the magnetic and structural order parameters it should be possible to pull them apart with a weak magnetic field. These insights into the role of strain coupling and relaxational effects in a system with only weak coupling between three order parameters allow rationalization and prediction of how static and dynamic properties of the material might be tuned in thin film form by choice of strain contrast with a substrate.

We propose and analyze Maxwell's demon based on a single qubit with avoided level crossing. Its operation cycle consists of adiabatic drive to the point of minimum energy separation, measurement of the qubit state, and conditional feedback. We show that the heat extracted from the bath at temperature $T$ can ideally approach the Landauer limit of $k_{B}Tln2$ per cycle even in the quantum regime. Practical demon efficiency is limited by the interplay of Landau-Zener transitions and coupling to the bath. We suggest that an experimental demonstration of the demon is fully feasible using one of the standard superconducting qubits.