Topological transition-metal dichalcogenides have been the center of research interests in materials science, recent days, due to their potential applications in spintronics, optoelectronics, and quantum computations. In this paper, using angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, we systematically studied the low-energy electronic structure of bulk ${\mathrm{ZrTe}}_2$. ARPES studies on ${\mathrm{ZrTe}}_2$ demonstrate free charge carriers at the Fermi level, which is further confirmed by the DFT calculations. An equal hole and electron carrier density estimated from the ARPES data points to ${\mathrm{ZrTe}}_2$ being a semimetal. The DFT calculations further suggest a band inversion between Te $p$ and Zr $d$ states at the $\mathrm{\Gamma }$ point, hinting at the nontrivial band topology in ${\mathrm{ZrTe}}_2$. Thus our studies suggest that ${\mathrm{ZrTe}}_2$ is a topological semimetal. Also, a comparative band structure study is done on ${\mathrm{ZrSe}}_2$, which shows a semiconducting nature of the electronic structure with an indirect band gap of 0.9 eV between $\mathrm{\Gamma }\left(A\right)$ and $M\left(L\right)$ high-symmetry points. Below we show that the metal-chalcogen bond length plays a critical role in the electronic phase transition from a semiconductor to a topological semimetal ingoing from ${\mathrm{ZrSe}}_2$ to ${\mathrm{ZrTe}}_2$.

With an auxiliary weak external magnetic field, we reexamine the fundamental thermodynamic function, Gibbs free energy $G(T,h)$, to study phase transitions in classical spin lattice models. A cross derivative, i.e., the second-order partial derivative of $G(T,h)$ with respect to both temperature and field, is calculated to precisely locate the critical temperature, which also reveals the nature of a transition. The strategy is efficient and universal, as exemplified by the five-state clock model, two-dimensional (2D) and 3D Ising models, and the $XY$ model, no matter if a transition is trivial or exotic with complex excitations. More importantly, other conjugate pairs could also be integrated into a similar cross derivative if necessary, which would greatly enrich our vision and means to investigate phase transitions both theoretically and experimentally.

We have measured the momentum distribution and renormalization factor $Z_{k_F}$ in liquid and solid lithium by high-resolution Compton scattering. High-resolution data over a wide momentum range exhibit a clear feature of the renormalization and a sharp drop of momentum densities at the Fermi momentum $k_F$. These results are compared with those computed by quantum Monte Carlo simulation performed both on a disordered crystal and a liquid exhibiting very good agreement. Asymptotic behavior of the experimental and theoretical momentum distributions are examined to estimate $Z_{k_F}$. The experimentally obtained $Z_{k_F}=0.{43}_{-0.01}^{+0.11}$ for liquid Li and $0.{54}_{-0.02}^{+0.11}$ for solid Li are in good agreement with theoretical results of $0.54\pm 0.01$ and $0.64\pm 0.01$, respectively.

We computed the Compton profile of solid and liquid lithium using quantum Monte Carlo (QMC) and compared it with recent experimental measurements, obtaining good agreement. Importantly, we find it crucial to account for proper core-valence orthogonalization and to address density differences when comparing with experiment. To account for disorder effects, we sampled finite-temperature configurations using molecular dynamics, then performed diffusion Monte Carlo (DMC) simulations on each configuration. We used Slater-Jastrow wave functions and grand-canonical twist-averaged boundary conditions. A QMC pseudopotential correction, derived from an all-electron DMC simulation of the perfect crystal, was also used. Our calculations provide an all-electron QMC benchmark for the Compton profile of lithium crystal and pseudopotential-corrected QMC Compton profiles for both the liquid and solid.

In this paper, the chiral Hall effect of strained Weyl semimetals without any external magnetic field is proposed. Electron-phonon coupling emerges in the low-energy fermionic sector through a pseudogauge potential. We show that, by using chiral kinetic theory, the chiral Hall effect appears as a response to a real time-varying electric field in the presence of structural distortion and it causes spatial chirality and charges separation in a Weyl system. We also show that the coupling of the electrons to acoustic phonons as a gapless excitation leads to emerging an optical absorption peak at $\omega ={\omega }_{el}$, where ${\omega }_{el}$ is defined as a characteristic frequency associated with the pseudomagnetic field. We also propose the strain-induced planar Hall effect as another transport signature of the chiral-anomaly equation.

Atomic-sized contacts of iron, created in scanning tunneling microscope break junctions, present unusually high values of conductance compared to other metals. This result is counterintuitive since, at the nanoscale, body-centered-cubic metals are expected to exhibit lower coordination than face-centered-cubic metals. In this work we first perform classical molecular dynamics simulations of the contact rupture, using two different interatomic potentials. The first potential is isotropic, and produces mostly single-atom prerupture contacts. The second potential accounts for the directional bonding in the materials, and produces mostly highly coordinated prerupture structures, generally consisting of more than one atom in contact. To compare the two different types of structures with experiments, we use them as input to density functional theory electronic transport calculations of the conductance. We find that the highly coordinated structures, obtained from the anisotropic potential, yield higher conductances which are statistically in better agreement with those measured for body-centered-cubic iron. We thus conclude that the directional bonding plays an important role in body-centered-cubic metals.

Despite various novel broken symmetry states have been revealed in bilayer graphene (BLG) experimentally, the atomic-scale spectroscopic investigation has been greatly limited. Here we study high-resolution spectroscopic characteristics of high-quality BLG and observe rich broken-symmetry-induced Landau level (LL) splittings, including valley, spin, and orbit, by using ultralow-temperature and high-magnetic-field scanning tunneling microscopy and spectroscopy (STM and STS). Our experiment demonstrates that both the spin and orbital splittings of the lowest $n=\left(0,1\right)$ LL depend sensitively on its filling and exhibit an obvious enhancement at partial-filling states. More unexpectedly, the splitting of a fully filled and valley-polarized LL is also enhanced by partial filling of the LL with the opposite valley. These results reveal significant many-body effects in this system. At half-filling of the $n=\left(0,1\right)$ LL (filling factor $v=0$), a single-particle intermediate $v=0$ phase, which is the transition state between canted antiferromagnetic and layer-polarized states in the BLG, is measured and directly visualized at the atomic scale. Our atomic-scale STS measurement gives direct evidence that this intermediate $v=0$ state is the predicted orbital-polarized phase.

Theory of neutron diffraction on traveling and standing surface acoustic waves (SAWs) is considered. Results of experiment on observation of neutron diffraction by a SAW traveling both along and against the direction of a neutron wave, as well as a standing SAW, are presented. The experimental results are mostly consistent with theoretical predictions.

Majorana zero modes (MZMs)—bearing potential applications for topological quantum computing—are verified in quasi-one-dimensional (1D) fermion systems, including semiconductor nanowires, magnetic atomic chains, and planar Josephson junctions. However, the existence of multibands in these systems makes the MZMs fragile to the influence of disorder. Moreover, in practical perspective, the proximity induced superconductivity may be difficult and restricted for 1D systems. Here, we propose a flexible route to realize MZMs through 1D topological kink states by engineering a 2D electron gas with antidot lattices in which both the aforementioned issues can be avoided owing to the robustness of kink states and the intrinsically attainable superconductivity in high-dimensional systems. The MZMs are verified to be quite robust against disorders and the bending of kink states and can be conveniently tuned by varying the Rashba spin-orbit coupling strength. Our proposal provides an experimental feasible platform for MZMs with systematic manipulability and assembleability based on the present techniques in a 2D electron gas system.

Search for stable high-pressure compounds in the Ti-H system reveals the existence of titanium hydrides with new stoichiometries, including $Ibam\text{−}{\mathrm{Ti}}_2{\mathrm{H}}_5$, $I4/m\text{−}{\mathrm{Ti}}_5{\mathrm{H}}_{13}$, $I\overline{4}\text{−}{\mathrm{Ti}}_5{\mathrm{H}}_{14}$, $Fddd\text{−}{\mathrm{TiH}}_4$, $Immm\text{−}{\mathrm{Ti}}_2{\mathrm{H}}_{13}$, $P\overline{1}\text{−}{\mathrm{TiH}}_{12}$, and $C2/m\text{−}{\mathrm{TiH}}_{22}$. Our calculations predict $I4/mmm\to R\overline{3}m$ and $I4/mmm\to$ $Cmma$ transitions in TiH and ${\mathrm{TiH}}_2$, respectively. Phonons and the electron-phonon coupling in all searched titanium hydrides are analyzed at high pressure. It is found that $Immm\text{−}{\mathrm{Ti}}_2{\mathrm{H}}_{13}$, rather than the hydrogen-richest $C2/m\text{−}{\mathrm{TiH}}_{22}$, exhibits the highest superconducting critical temperature $T_c$. The estimated $T_c$ of $Immm\text{−}{\mathrm{Ti}}_2{\mathrm{H}}_{13}$ and $C2/m\text{−}{\mathrm{TiH}}_{22}$ are, respectively, 127.4–149.4 K (${\mu }^{*}=0.1$–0.15) at 350 GPa and 91.3–110.2 K at 250 GPa, as found by numerically solving the Eliashberg equations.

$\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type dioxides are significant both for geoscience and condensed-matter physics. For example, $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type $\mathrm{Si}{\mathrm{O}}_2$ has been proposed to be one of the dominant components in the mantles of super-Earths and $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type $\mathrm{Ti}{\mathrm{O}}_2$ has been shown to have a large visible absorbance. Here we report the discovery of an $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type phase in a typical transition-metal dichalcogenide (TMD), $\mathrm{TiT}{\mathrm{e}}_2$, using crystal structure prediction and first-principles calculations. Ambient layered $\mathrm{TiT}{\mathrm{e}}_2$ will first transform to a monoclinic $C2/m$ phase and then finally to the hexagonal $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type phase above 33 GPa. $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type $\mathrm{TiT}{\mathrm{e}}_2$ is predicted to be metallic, contrasting with the semiconductivity of $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type $\mathrm{Ti}{\mathrm{O}}_2$. The same high-pressure phase ($\mathrm{F}{\mathrm{e}}_2\mathrm{P}$ type) appears both in transition-metal dioxides and TMDs, indicating that the stacking patterns of anions and cations play an increasingly important role in determining the high-pressure phase.

Two-dimensional intrinsic ferromagnetic semiconductors are expected to stand out in the spintronic field. Recently, monolayer ${\mathrm{VI}}_3$ was experimentally synthesized, but the weak ferromagnetism and low Curie temperature $T_C$ limit its potential application. Here we report that the Janus structure can elevate $T_C$ to 240 K. And we discuss the reason for high $T_C$ in the Janus structure, which is the lower virtual exchange gap between $t_{2g}$ and $e_g$ states of nearest-neighbor V atoms. In addition, $T_C$ can be further substantially enhanced by tensile strain due to the increasing ferromagnetism driven by rapidly quenched direct exchange interaction. Our work supports a feasible approach to enhance Curie temperature of monolayer ${\mathrm{VI}}_3$ and unveils stable intrinsic FM semiconductors for realistic applications in spintronics.

Antiferromagnetic (AFM) spintronics exploits the Néel vector as a state variable for novel electronic devices. Recent studies have demonstrated that the Néel vector can be switched by a spin-orbit torque. These studies however are largely limited to collinear antiferromagnets of proper magnetic space-group symmetry. There is, however, a large group of high-temperature noncollinear antiferromagnets, which are suitable for such switching. Here, we predict that spin torque can be efficiently used to switch a noncollinear AFM order in antiperovskite materials. Based on first-principles calculations and atomistic spin-dynamics modeling, we show that in antiperovskites $A\mathrm{NM}{\mathrm{n}}_3$ ($A=\mathrm{Ga}$, Ni, etc.) with the AFM ${\mathrm{\Gamma }}_{4\mathrm{g}}$ ground state, the AFM order can be switched on the picosecond timescale using a spin torque generated by a spin current. The threshold switching current density can be tuned by the $A\mathrm{NM}{\mathrm{n}}_3$ stoichiometry engineering, changing the magnetocrystalline anisotropy. The ${\mathrm{\Gamma }}_{4\mathrm{g}}$ AFM phase supports a sizable anomalous Hall effect, which can be used to detect the spin-torque switching of the AFM order. The predicted ultrafast switching dynamics and the efficient detection of the AFM order state make noncollinear magnetic antiperovskites a promising material platform for AFM spintronics.

In contrast to the conventional von Neumann bipartite entanglement entropy (bEE), we show that a more appropriate description of the one-dimensional doped Mott insulator is a new kind of mutual entanglement entropy (mEE) between the charge and spin degrees of freedom. Such a charge-spin mEE can clearly distinguish the important and distinct features between the $t\text{−}J$ model and the so-called $\sigma ·t\text{−}J$ model. In the latter, the phase string sign structure is switched off such that a single doped hole always behaves like a Bloch wave in the whole regime of $J/t$, whereas in the former it exhibits a series of level crossing with the total momentum jumps in the single-hole ground state from spin-charge separation at $J/t\to 0$ to spin-charge recombination at large $J/t$, which are failed to be detected by bEE. We further show that the distinctions between the two models persist to finite energy density, which can be similarly well characterized by mEE but not by bEE. By studying the dynamic time evolution of the states set out of equilibrium at the beginning, we show that mEE indeed always increases with the time, satisfying the common characteristic of entropy.

Controllable, partially isolated few-level systems in semiconductors have recently gained multidisciplinary attention due to their widespread nanoscale sensing and quantum technology applications. Quantitative simulation of the dynamics and related applications of such systems is a challenging theoretical task that requires faithful description not only of the few-level systems but also their local environments. Here, we develop a method that can describe relevant relaxation processes induced by a dilute bath of nuclear and electron spins. The method utilizes an extended Lindblad equation in the framework of cluster approximation of a central spin system. We demonstrate that the proposed method can accurately describe ${\mathrm{T}}_1$ time of an exemplary solid-state point-defect qubit system, in particular, the nitrogen-vacancy (NV) center in diamond, at various magnetic fields and strain.

Nanodiamond is poised to become an attractive material for hyperpolarized ${}^{13}\mathrm{C}$ magnetic resonance imaging if large nuclear polarizations can be achieved without the accompanying rapid spin-relaxation driven by paramagnetic species. Here we report enhanced and long-lived ${}^{13}\mathrm{C}$ polarization in synthetic nanodiamonds tailored by acid-cleaning and air-oxidation protocols. Our results separate the contributions of different paramagnetic species on the polarization behavior, identifying the importance of substitutional nitrogen defect centers in the nanodiamond core. These results are likely of use in the development of nanodiamond-based imaging agents with size distributions of relevance for examining biological processes.

Based on probing electronic transport properties, we propose an experimental test for the recently discovered rich topological phase diagram of one-dimensional Floquet topological insulators with Rashba spin-orbit interaction [Kennes et al., Phys. Rev. B 100, 041103(R) (2019)]. Using the Keldysh-Floquet formalism, we compute electronic transport properties of these nanowires, where we propose to couple the leads in such a way, as to primarily address electronic states with a large weight at one edge of the system. By tuning the Fermi energy of the leads to the center of the topological gap, we are able to directly address the topological edge states, granting experimental access to the topological phase diagram. Surprisingly, when tuning the lead Fermi energy to special values in the bulk which coincide with extremal points of the dispersion relation, we find additional peaks of similar magnitude to those caused by the topological edge states. These peaks reveal the presence of continua of states centered around aforementioned extremal points whose wave functions are linear combinations of delocalized bulk states and exponentially localized edge states, where the ratio of edge- to bulk-state amplitude is maximal at the extremal point of the dispersion. We discuss the transport properties of these nontopological edge states, explain their emergence in terms of an intuitive yet quantitative physical picture and discuss their relationship with Van Hove singularities in the bulk of the system. The mechanism giving rise to these states is not specific to the model we consider here, suggesting that they may be present in a wide class of one-dimensional systems.

We propose a mechanism of angular momentum conversion from optical transverse spin in surface plasmon polaritons (SPPs) to conduction electron spin. Free electrons in the metal follow the transversally spinning electric field of the SPP, and the resulting orbital motions create inhomogeneous static magnetization in the metal. By solving the spin diffusion equation in the SPP, we find that the magnetization field generates an electron spin current. We show that there exists a resonant condition where the spin current is resonantly enhanced, and the polarization of the spin current is flipped. Our theory reveals an alternative functionality of SPPs as a spin current source.

Sensitive ac-susceptibility measurements of a topological insulator, $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ single crystal, using mutual two coil inductance technique shows coupling and decoupling of high conducting surface states. The coupling of the surface states exists upto thickness of 70 microns, which is much larger than the direct coupling limit of thickness ∼5 to 10 nanometers found in thin films. The high conducting topological surface states are coupled through the crystal via high electrically conducting channels, generated by Selenium vacancies. These conducting channels through the bulk disintegrate beyond 70 μm thickness and at high temperatures, thereby leading to decoupling of the topological surface states. We show the decoupled surface states persist upto room temperature in the topological insulator. Analysis of Nyquist plot of ac-susceptibility response of the TI using a resistor (R)–inductor (L) model shows an inductive nature of the coupling between surface states found in these $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ crystals.

The modern theory of orbital magnetization addresses crystalline materials at the noninteracting level: Therein the observable is the $\mathbf{k}$-space integral of a geometrical integrand. Alternatively, magnetization admits a local representation in $\mathbf{r}$ space, i.e., a “density,” which may address noncrystalline and/or inhomogeneous materials as well; the Chern number admits an analogous density. Here, we provide the formulation for ribbons, polymers, nanowires, and single-layer materials, where both $\mathbf{k}$-space and $\mathbf{r}$-space integrations enter the definition of the two observables.