Layered ${\mathrm{NbSe}}_2$ is a metallic layered transition metal dichalcogenide (TMD) that has attracted much interest for the coexistence of superconductivity and charge-density wave (CDW) down to the monolayer limit. Here we report first-principles calculations of the lattice dynamics, electronic structure, and electron-phonon coupling of Na-intercalated bilayer ${\mathrm{NbSe}}_2$. We show that upon the Na atom intercalation, the CDW instability in the bilayer ${\mathrm{NbSe}}_2$ can be effectively suppressed, accompanied by the removal of the soft phonon modes at ${\mathbf{q}}_{\mathrm{CDW}}=\frac{2}{3}\mathrm{\Gamma }M$. The underlying mechanism for this phenomenon is that a large electron doping from the intercalated Na contracts the Fermi surface of bilayer ${\mathrm{NbSe}}_2$ and reduces the electron-phonon coupling at ${\mathbf{q}}_{\mathrm{CDW}}$. In spite of the disappearance of CDW, the superconductivity still survives in the ${\mathrm{NbSe}}_2$ intercalate, with a predicted superconducting transition temperature $T_c$ of 3 K. Moreover, we find that the biaxial compressive strain can greatly increase density of states near the Fermi surface and soften characteristic phonons contributing to superconductivity, leading to an increase of $T_c$ by more than 100% at a low strain level of 3%. Our results would have significant implications for tuning CDW and superconductivity in ${\mathrm{NbSe}}_2$ and other metallic TMD materials.

We present measurements of the resonant microwave absorption between the Rydberg energy levels of surface-state electrons on the surface of superfluid liquid helium, in the frequency range 165–220 GHz. The resonant frequency was temperature dependent. The experiments are in agreement with recent theoretical calculations of the renormalization of the electron energy levels due to zero-point and thermal ripplons. The temperature-dependent contribution to the linewidth $\gamma \left(T\right)$ for excitation to the first excited state at 189.6 GHz is compared with other measurements and theoretical predictions.

We demonstrate that the use of an optimal initial state of a wave function generated using Lanczos method allows the previously proposed projected Green's function approach to predict the tunneling lifetime of a particle crossing a double quantum well with a considerably improved accuracy. It is further revealed that the electron tunneling lifetime of such system depends linearly or quadratically on the barrier thickness or the well width. We also investigate the asymmetric double quantum well structure and find that the electron tunneling lifetime in it is strongly dependent on barrier thickness. Finally, we discuss the possible experimental designs to observe the above-mentioned tunneling features.

We propose two-dimensional ferromagnetic nanostructures, i.e., single-layer films of MnB as well as MnC, on the basis of first-principles calculations. The ferromagnetic state is stable in a freestanding MnB monolayer as well as the one on many nonmagnetic metal substrates, including Ag(001). We have identified the mechanism of the itinerant ferromagnetism as the kinetic-energy gain due to the minority-spin hopping of $3d$ electrons in Mn atoms through B atoms. The second-order perturbative analysis treating hybridization as overlap integrals clarifies that the perpendicular magnetic anisotropy in freestanding MnB and MnB/Ag(001) is mainly attributed to virtual transition processes between up-spin occupied states and down-spin unoccupied states. In contrast, the in-plane anisotropy in MnB/Pd(001) comes from virtual transition processes among down-spin occupied states and up-spin unoccupied states.

We investigate lateral $p-n$ junctions, electrostatically defined in 14-nm-wide HgTe-based quantum wells (QWs) with inverted band structure. The $p-n$ junction resistances are close to $h/2e^2$, consistent with some previous experiments on $8–10\mathrm{nm}$ QWs, and the current-voltage characteristics are highly linear, indicating the transport via ballistic helical edge states. Shot noise measurements are performed in order to further verify the underlying transport mechanism. We discuss the role of unknown inelastic relaxation rates in the leads and in the edge channels for the correct interpretation of the noise data. Although the interpretation in favor of the helical edge states seems more consistent, a definite conclusion cannot be drawn based on the present experiment. Our approach looks promising for the study of short quasiballistic edges in topological insulators in suitable geometry.

A semiclassical approach that incorporates quantum mechanical behavior of heat capacity in direct caloric effect simulations is proposed. Application of this methodology to study the electrocaloric effect in prototypical ferroelectrics ${\mathrm{PbTiO}}_3$  and ${\mathrm{BaTiO}}_3$  reveals severe underestimation of electrocaloric response at lowest temperatures by classical simulations. The discrepancy between semiclassical and classical results is found to be largest in ferroics with Debye temperature exceeding the Curie point. A route to enhance the electrocaloric effect by tuning the Debye temperature in composite materials is proposed.

We measured the temperature dependence of the magnetic susceptibility $\chi \left(T\right)$ and the specific heat $C\left(T\right)$ and the magnetic-field dependence of the magnetization $M\left(H\right)$ of $A_2{\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}(A$ = Rb or K) powder. We consider that the probable spin model is an interacting spin-1 antiferromagnetic tetramer model. We evaluated values of the intratetramer interactions as $J_1=9$ K and $J_2=18$ K, and the effective intertetramer interaction as $J_{\mathrm{eff}}=4$ K for ${\mathrm{Rb}}_2{\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$. The susceptibility and magnetization at 1.3 K of ${\mathrm{K}}_2{\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ are very close to those of ${\mathrm{Rb}}_2{\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$. We observed a phase transition to a magnetically ordered state in $C\left(T\right)/T$ in magnetic fields above 3 T. The transition temperature increases with magnetic field. Probably, the ordered state appears around 1.8 K even in 0 T. The ordered state in 0 T, however, is not stable enough like an order in the vicinity of a quantum critical point. Longitudinal-mode magnetic excitations may be observable in single crystalline $A_2{\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}(A$ = Rb or K).

By using magnetization and small-angle neutron scattering measurements, we have investigated the magnetic behavior of the $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{I}{\mathrm{r}}_x\mathrm{Si}$ system to explore the effect of increased carrier density and spin-orbit interaction on the magnetic properties of MnSi. We determine estimates of the spin wave stiffness and the Dzyaloshinskii-Moriya (DM) interaction strength and compare with $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{C}{\mathrm{o}}_x\mathrm{Si}$ and $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{F}{\mathrm{e}}_x\mathrm{Si}$. Despite the large differences in atomic mass and size of the substituted elements, $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{C}{\mathrm{o}}_x\mathrm{Si}$ and $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{I}{\mathrm{r}}_x\mathrm{Si}$ show nearly identical variations in their magnetic properties with substitution. We find a systematic dependence of the transition temperature, the ordered moment, the helix period, and the DM interaction strength with electron count for $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{I}{\mathrm{r}}_x\mathrm{Si},\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{C}{\mathrm{o}}_x\mathrm{Si}$, and $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{F}{\mathrm{e}}_x\mathrm{Si}$, indicating that the magnetic behavior is primarily dependent upon the additional carrier density, rather than on the mass or size of the substituting species. This indicates that the variation in magnetic properties, including the DM interaction strength, is primarily controlled by the electronic structure, as Co and Ir are isovalent. Our work suggests that although the rigid band model of electronic structure, along with Moriya's model of weak itinerant magnetism, describes this system surprisingly well, phenomenological models for the DM interaction strength are not adequate to describe this system.

Magnetic skyrmions with a topological particle nature have recently attracted attention as a potential information carrier for novel magnetic storage devices. For single-phase bulk crystals, skyrmions usually appear for a very narrow temperature region just below the magnetic ordering temperature $T_c$, and the stabilization of skyrmions for a wider temperature range remains an important challenge. Here, by investigating the impact of uniaxial tensile stress for a chiral magnet ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, we demonstrate that only less than 0.2% of uniaxial elongation can dramatically stabilize skyrmions for an entire temperature range from $T_c$ to the lowest temperature. The stability of skyrmions essentially depends on the geometrical relationship among the directions of strain, magnetic field, and crystallographic axes, which is consistently explained in terms of the anisotropic modulation of the Dzyaloshinskii-Moriya interaction and magnetocrystalline anisotropy. Our finding may provide a good strategy for materials design to enhance the stability of skyrmions.

Quasiparticle excitations in FeSe were studied by means of specific heat $\left(C\right)$ measurements on a high-quality single crystal under rotating magnetic fields. The field dependence of $C$ shows three-stage behavior with different slopes, indicating the existence of three gaps $({\mathrm{\Delta }}_1,{\mathrm{\Delta }}_2$, and ${\mathrm{\Delta }}_3)$. In the low-temperature and low-field region, the azimuthal angle $\left(\varphi \right)$ dependence of $C$ shows a fourfold symmetric oscillation with a sign change. On the other hand, the polar angle $\left(\theta \right)$ dependence manifests as an anisotropy-inverted twofold symmetry with unusual shoulder behavior. Combining the angle-resolved results and the theoretical calculation, the smaller gap ${\mathrm{\Delta }}_1$ is proved to have two vertical-line nodes or gap minima along the $k_z$ direction, and is determined to reside on the electron-type $\varepsilon$ band. ${\mathrm{\Delta }}_2$ is found to be related to the electron-type $\delta$ band, and is isotropic in the $ab$ plane but largely anisotropic out of the plane. ${\mathrm{\Delta }}_3$ residing on the hole-type $\alpha$ band shows a small out-of-plane anisotropy with a strong Pauli paramagnetic effect.

The interaction of point defects with an external stress field or with other structural defects is usually well described within continuum elasticity by the elastic dipole approximation. Extraction of the elastic dipoles from atomistic simulations is therefore a fundamental step to connect an atomistic description of the defect with continuum models. This can be done either by a fitting of the point-defect displacement field, by a summation of the Kanzaki forces, or by a linking equation to the residual stress. We perform here a detailed comparison of these different available methods to extract elastic dipoles, and show that they all lead to the same values when the supercell of the atomistic simulations is large enough and when the anharmonic region around the point defect is correctly handled. But, for small simulation cells compatible with ab initio calculations, only the definition through the residual stress appears tractable. The approach is illustrated by considering various point defects (vacancy, self-interstitial, and hydrogen solute atom) in zirconium, using both empirical potentials and ab initio calculations.

${}^{75}\mathrm{As}$ NMR measurements were performed on a polycrystalline sample of spin-$\frac{1}{2}$ alternating spin chain Heisenberg antiferromagnet ${\mathrm{AgVOAsO}}_4$. The temperature-dependent NMR shift $K\left(T\right)$, which is a direct measure of the intrinsic spin susceptibility, agrees very well with the spin-$\frac{1}{2}$ alternating-chain model, justifying the assignment of the spin lattice. From the analysis of $K\left(T\right)$, magnetic exchange parameters were estimated as follows: the leading exchange $J/k_{\mathrm{B}}\simeq 38.4$ K and the alternation ratio $\alpha =J^{\prime }/J\simeq 0.69$. The transferred hyperfine coupling between the ${}^{75}\mathrm{As}$ nucleus and ${\mathrm{V}}^{4+}$ spins obtained by comparing the NMR shift with the bulk susceptibility amounts to $A_{\mathrm{hf}}\simeq 3.3\text{T}{\mu }_{\mathrm{B}}$. The effect of interchain couplings on the low-temperature activated behavior of $K\left(T\right)$ and the spin-lattice relaxation rate $1/T_1$ is identified.

Using the transfer-matrix approach and solving time-domain differential equations, we analyze the loss compensation mechanism in multilayer systems composed of an absorbing transparent conductive oxide and dielectric doped with an active material. We reveal also another regime with the possibility of enhanced transmission with suppressed reflection originating from the resonant properties of the multilayers. For obliquely incident and evanescent waves, such enhanced transmission under suppressed reflection turns into the reflectionless regime, which is similar to that observed in the $\mathcal{PT}$-symmetric structures, but does not require $\mathcal{PT}$ symmetry. We infer that the reflectionless transmission is due to the full loss compensation at the resonant wavelengths of the multilayers.

We show that in conventional one-dimensional insulators excess charges created close to the boundaries of the system can be expressed in terms of the Berry phases associated with the electronic Bloch wave functions. Using this correspondence, we uncover a link between excess charges and the topological invariants of the recently classified one-dimensional topological phases protected by spatial symmetries. Excess charges can be thus used as a probe of crystalline topologies.

We examined the electrical resistivity ($\rho$) of antiferromagnetic (AFM) Kondo semiconductors $\mathrm{Ce}({\mathrm{Ru}}_{1-x}{\mathrm{Rh}}_x$)${}_2{\mathrm{Al}}_{10}$ ($x=0$ and 0.1) under pressure in order to obtain information on the electronic states under pressure, especially near the critical pressure ($P_{\mathrm{c}}$) from the AFM ordered state to the paramagnetic one, where the $\mathrm{Ce}\text{−}4f$ electron character is a more localized state in $x=0.1$ than in $x=0$. From the results, nearly the same $P_{\mathrm{c}}$ was obtained; $P_{\mathrm{c}}\sim 4.7$ and 4.5 GPa in $x=0$ and 0.1, respectively. In both samples, the Kondo semiconducting increase of $\rho$ is observed up to $P\sim 3$ GPa, above which, however, the increase disappears and a broad maximum appears at high temperatures. Below the maximum, $\rho$ exhibits a metallic decrease with decreasing temperature down to the AFM transition temperature $T_0$, suggesting that the $c\text{−}f$ hybridization gap could be not necessary to form the unusual AFM order. We also examined pressure effects on the magnetic susceptibility $\chi$ of both samples up to $P\sim 2$ GPa, and found that $\chi$ along the easy axis is strongly suppressed by pressure in both samples. In $x=0$, the broad maximum just above $T_0$ shifts to high temperatures with increasing pressure. On the other hand, for $x=0.1$, a clear cusp at $T_0$ remains sharp and no broad peak appears at least up to 2 GPa. Such a difference in the pressure response of $\chi$ could originate from the difference in the electronic state between $x=0$ and 0.1.

The influence of excitonic and local-field effects on the second-harmonic-generation spectrum of zinc-blende ZnS, ZnSe, and ZnTe is studied from first principles using the Bethe-Salpeter equation. The calculations are based on the theory of R. Leitsmann et al. [Phys. Rev. B 71, 195209 (2005)], which we extended to the low-frequency range. The dielectric function and the second-harmonic-generation spectrum have been obtained within the independent (quasi)particle approximation and within the Bethe-Salpeter approach. The calculations demonstrate that the linear and the nonlinear optical properties are similarly affected by excitonic and local-field effects. The computed spectra are furthermore compared with measurements and calculations within the time-dependent density-polarization functional theory (TD-DPFT) in the real-time framework. The present approach most commonly suggests stronger excitonic effects than observed in the real-time TD-DPFT calculations, while local-field corrections are comparably described. Although agreement is found between the Bethe-Salpeter spectra and measurements for the dielectric function, deviations from the experimental data are observed for second-harmonic generation.

A key challenge in manipulating the magnetization in heavy-metal/ferromagnetic bilayers via the spin-orbit torque is to identify materials that exhibit an efficient charge-to-spin current conversion. Ab initio electronic structure calculations reveal that the intrinsic spin Hall conductivity (SHC) for pristine $\beta$-W is about 60% larger than that of $\alpha$-W. More importantly, we demonstrate that the SHC of $\beta$-W can be enhanced via Ta alloying. This is corroborated by spin Berry curvature calculations of ${\mathrm{W}}_{1-x}{\mathrm{Ta}}_x$ ($x\sim 12.5\%$) alloys which show a giant enhancement of the spin Hall angle of up to $\approx -0.5$. The underlying mechanism is the synergistic behavior of the SHC and longitudinal conductivity with the Fermi level position. These findings not only pave the way for enhancing the intrinsic spin Hall effect in $\beta$-W, but also provide guidelines to exploit substitutional alloying to tailor the spin Hall effect in various materials.

While recent advances in band theory and sample growth have expanded the series of extremely large magnetoresistance (XMR) semimetals in transition-metal dipnictides $TmP{n}_2$ ($Tm=\mathrm{Ta}$, Nb; $Pn=\mathrm{P}$, As, Sb), the experimental study on their electronic structure and the origin of XMR is still absent. Here, using angle-resolved photoemission spectroscopy combined with first-principles calculations and magnetotransport measurements, we performed a comprehensive investigation on ${\mathrm{MoAs}}_2$, which is isostructural to the $TmP{n}_2$ family and also exhibits quadratic XMR. We resolve a clear band structure well agreeing with the predictions. Intriguingly, the unambiguously observed Fermi surfaces (FSs) are dominated by an open-orbit topology extending along both the [100] and [001] directions in the three-dimensional Brillouin zone. We further reveal the trivial topological nature of ${\mathrm{MoAs}}_2$ by bulk parity analysis. Based on these results, we examine the proposed XMR mechanisms in other semimetals, and conclusively ascribe the origin of quadratic XMR in ${\mathrm{MoAs}}_2$ to the carriers motion on the FSs with dominant open-orbit topology, innovating in the understanding of quadratic XMR in semimetals.

The electric field gradient (EFG) tensor at the ${}^{75}\mathrm{As}$ site couples to the orbital occupations of the As $p$ orbitals and is a sensitive probe of local nematicity in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. We use nuclear magnetic resonance to measure the nuclear quadrupolar splittings and find that the EFG asymmetry responds linearly to the presence of a strain field in the paramagnetic phase. We extract the nematic susceptibility from the slope of this linear response as a function of temperature and find that it diverges near the structural transition, in agreement with other measures of the bulk nematic susceptibility. Our work establishes an alternative method to extract the nematic susceptibility which, in contrast to transport methods, can be extended inside the superconducting state.