We study the two-dimensional superconductor-insulator transition (SIT) in thin films of tantalum nitride. At zero magnetic field, films can be disorder-tuned across the SIT by adjusting thickness and film stoichiometry; insulating films exhibit classical hopping transport. Superconducting films exhibit a magnetic-field-tuned SIT, whose insulating ground state at high field appears to be a quantum-corrected metal. Scaling behavior at the field-tuned SIT shows classical percolation critical exponents $z\nu \approx 1.3$, with a corresponding critical field $H_c\ll H_{c2}$, the upper critical field. The Hall effect exhibits a crossing point near $H_c$, but with a nonuniversal critical value ${\rho }_{xy}^c$ comparable to the normal-state Hall resistivity. We propose that high-carrier-density metals will always exhibit this pattern of behavior at the boundary between superconducting and (trivially) insulating ground states.

The ease with which domain walls (DWs) in ferroelectric materials can be written and erased provides a versatile way to dynamically modulate heat fluxes. In this Rapid Communication we evaluate the thermal boundary resistance (TBR) of ${180}^{\circ }$ DWs in the prototype ferroelectric perovskite ${\mathrm{PbTiO}}_3$ within the numerical formalisms of nonequilibrium molecular dynamics and nonequilibrium Green's functions. An excellent agreement is obtained for the TBR of an isolated DW derived from both approaches, which reveals the harmonic character of the phonon-DW scattering mechanism. The thermal resistance of the ferroelectric material is shown to increase up to around 20%, in the system sizes considered here, due to the presence of a single DW, and larger resistances can be attained by the incorporation of more DWs along the path of thermal flux. These results, obtained at device operation temperatures, prove the viability of an electrically actuated phononic switch based on ferroelectric DWs.

Perovskite oxide heterostructures offer an important path forward for stabilizing and controlling low-dimensional magnetism. One of the guiding design principles for these materials systems is octahedral connectivity. In superlattices composed of perovskites with different crystal symmetries, variation of the relative ratio of the constituent layers and the individual layer thicknesses gives rise to nonequilibrium crystal symmetries that, in turn, lead to unprecedented control of interfacial ferromagnetism. We have found that in superlattices of ${\mathrm{CaMnO}}_3$ (CMO) and ${\mathrm{LaNiO}}_3$ (LNO), interfacial ferromagnetism can be modulated by a factor of 3 depending on LNO and CMO layer thicknesses as well as their relative ratio. Such an effect is only possible due to the nonequilibrium crystal symmetries at the interfaces and can be understood in terms of the anisotropy of the exchange interactions and modifications in the interfacial Ni-O-Mn and Mn-O-Mn bond angles and lengths with increasing LNO layer thickness. These results demonstrate the potential of engineering nonequilibrium crystal symmetries in designing ferromagnetism.

We predict a new ternary hydride ${\mathrm{MgSiH}}_6$ under high pressures, which is a metal with an ionic feature and takes on a simple cubic structure with space group $Pm\text{−}3$ above 250 GPa. Our first-principles calculations show that the cubic ${\mathrm{MgSiH}}_6$ is a potential high-temperature superconductor with a superconducting transition temperature $T_{\mathrm{c}}$ of $\sim 63$ K at 250 GPa. Further analysis suggests that phonon softening along mainly $\mathrm{\Gamma }\text{−}X$ and $\mathrm{\Gamma }\text{−}M$ directions induced by Fermi surface nesting plays a crucial role in the high-temperature superconductivity. Herein we propose the “triangle straight-line method” which provides a clear guide to determine the specific A + B → D type formation routes for ternary hydrides of the Mg-Si-H system and it effectively reveals two divergent paths to obtain ${\mathrm{MgSiH}}_6$ under high pressures: ${\mathrm{MgH}}_2+{\mathrm{SiH}}_4\to {\mathrm{MgSiH}}_6$ and MgSi + $3{\mathrm{H}}_2\to {\mathrm{MgSiH}}_6$. This method might be applicable to all ternary compounds, which will be very significant for further experimental synthesis.

We investigate the quasiparticle and condensate dynamics in response to the terahertz optical pulses in the weak spin-orbit-coupled $s$-wave superconducting semiconductor quantum wells by using the gauge-invariant optical Bloch equations in the quasiparticle approximation. Specifically, in the Bloch equations, not only can the microscopic description for the quasiparticle dynamics be realized, but also the dynamics of the condensate is included, with the superfluid velocity and the effective chemical potential naturally incorporated. We reveal that the superfluid velocity itself can contribute to the pump of quasiparticles (pump effect), with its rate of change acting as the drive field to drive the quasiparticles (drive effect). We find that the oscillations of the Higgs mode with twice the frequency of the optical field are contributed dominantly by the drive effect but not the pump effect as long as the driven superconducting momentum is less than the Fermi momentum. This is in contrast to the conclusion from the Liouville or Bloch equations in the literature, in which the drive effect on the anomalous correlation is overlooked with only the pump effect considered. Furthermore, in the gauge-invariant optical Bloch equations, the charge neutrality condition is consistently considered based on the two-component model for the charge, in which the charge imbalance of quasiparticles can cause the fluctuation of the effective chemical potential for the condensate. It is predicted that during the optical process, the quasiparticle charge imbalance can be induced by both the pump and drive effects, leading to the fluctuation of the chemical potential. This fluctuation of the chemical potential is further demonstrated to directly lead to a relaxation channel for the charge imbalance even with the elastic scattering due to impurities. This is very different from the previous understanding that in the isotropic $s$-wave superconductivity, the impurity scattering cannot cause any charge-imbalance relaxation.

We investigate the quasiparticle and condensate dynamics in response to the terahertz (THz) optical pulses in the strong spin-orbit-coupled ($s+p$)-wave superconducting semiconductor quantum wells by using the gauge-invariant optical Bloch equations in the quasiparticle approximation. Both the dynamics of triplet and singlet superconductivity are studied in response to the THz optical pulses. Specifically, for the triplet superconductivity, we predict that in the ($s+p$)-wave superconducting (100) quantum wells, with the vector potential parallel to the quantum wells, the optical field can cause the total spin polarization of Cooper pairs, oscillating with the frequency of the optical field. The direction of the total Cooper-pair spin polarization is shown to be parallel to the vector potential. For the singlet superconductivity, we show that due to the large spin-orbit coupling in InSb (100) quantum wells, there exist two Fermi surfaces including the inner and outer ones. In this specific configuration, the superconducting momentum can be tuned to be larger than the inner Fermi momentum but smaller than the outer one. We find that in this regime, the dynamics of the Higgs mode and charge imbalance shows different features in comparison with the conventional $s$-wave case.

We analyze a generalized Dirac system, where the dispersion along the $k_x$ and $k_y$ axes is $N\mathrm{th}$ power and linear along the $k_z$ axis. In the presence of the Zeeman field, there emerge $N$ monopole-antimonopole pairs by controlling it beyond a certain critical one. As the direction of the Zeeman field is rotated toward the $z$ axis, monopoles move to the north pole while antimonopoles move to the south pole. When the Zeeman field becomes parallel to the $z$ axis, they merge into one monopole or one antimonopole whose monopole charge is $\pm N$. The resultant system is a multiple-Weyl semimetal. Characteristic properties of such a system are that the anomalous Hall effect and the chiral anomaly are enhanced by $N$ times and that $N$ Fermi arcs appear. These phenomena will be observed experimentally in the cubic-Dirac and triple-Weyl fermion systems $\left(N=3\right)$.

Based on the extended variable range hopping mechanism in a disordered energy landscape with a Gaussian density of states, we determine the dependence of the magnetoresistivity on temperature, carrier density, magnetic field, and electric field. Experimental electric-field and temperature characteristics in device based on organic semiconductors are excellently reproduced with this unified description of the magnetoresistivity. We further show that the spin transport indeed can be explained by the fact that magnetic field will decrease the density of transport states and similar to the decrease of carrier density without it. We finally demonstrate that magnetoresistivity is strongly dependent on the carrier density; by changing the carrier density in the hopping system, the magnetoresistivity can increase more than 500 times.

Ferroelectricity has been believed unable to coexist with metallicity since the free carriers can screen the internal Coulomb interactions of dipoles. Very recently, one kind of material called ferroelectric metal was reexamined. Here, we report the coexistence of metallicity and polar distortion in a candidate for ferroelectric metal $\mathrm{PbT}{\mathrm{i}}_{1\text{−}x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_3$ via doping engineering. The ferroelectriclike polar distortion in all the doped $\mathrm{PbT}{\mathrm{i}}_{1\text{−}x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_3$, with $x$ ranging from 0.04 to 0.12, was observed by the piezoresponse force microscopy and the scanning transmission electron microscopy measurements. $\mathrm{PbT}{\mathrm{i}}_{1\text{−}x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_3$ films become more conductive with more doping density, and a metallic behavior emerges when $x$ reaches 0.12. Our first-principles calculations further revealed that the doped Nb ions in the films can only provide free electrons, but are not able to damage the dipoles in unit cells even with the heaviest doping density of 0.12. We believe that these results confirm a feasibility of realizing the coexistence of metallicity and polar distortion for other ferroelectrics in a common way, and motivate the synthesis of some new materials with artificially designed properties even incompatible in nature.

We study the charge transfer dynamics following the formation of a phase or voltage biased superconducting nanojunction using a full counting statistics analysis. We demonstrate that the evolution of the zeros of the generating function allows one to identify the population of different many body states much in the same way as the accumulation of Yang-Lee zeros of the partition function in equilibrium statistical mechanics is connected to phase transitions. We give an exact expression connecting the dynamical zeros to the charge transfer cumulants and discuss when an approximation based on “dominant” zeros is valid. We show that, for generic values of the parameters, the system gets trapped into a metastable state characterized by a nonequilibrium population of the many body states which is dependent on the initial conditions. We study in particular the effect of the switching rates in the dynamics showing that, in contrast to intuition, the deviation from thermal equilibrium increases for the slower rates. In the voltage biased case the steady state is reached independent of the initial conditions. Our method allows us to obtain accurate results for the steady state current and noise in quantitative agreement with steady state methods developed to describe the multiple Andreev reflections regime. Finally, we discuss the system dynamics after a sudden voltage drop showing the possibility of tuning the many body states population by an appropriate choice of the initial voltage, providing a feasible experimental way to access the quench dynamics and control the state of the system.

Water ice is a molecular solid whose behavior under compression reveals the interplay of covalent bonding in molecules and forces acting between them. This interplay determines high-pressure phase transitions, the elastic and plastic behavior of ${\mathrm{H}}_2\mathrm{O}$ ice, which are the properties needed for modeling the convection and internal structure of the giant planets and moons of the solar system as well as ${\mathrm{H}}_2\mathrm{O}$-rich exoplanets. We investigated experimentally and theoretically elastic properties and phase transitions of cubic ${\mathrm{H}}_2\mathrm{O}$ ice at room temperature and high pressures between 10 and 82 GPa. The time-domain Brillouin scattering (TDBS) technique was used to measure longitudinal sound velocities ($V_L$) in polycrystalline ice samples compressed in a diamond anvil cell. The high spatial resolution of the TDBS technique revealed variations of $V_L$ caused by elastic anisotropy, allowing us to reliably determine the fastest and the slowest sound velocity in a single crystal of cubic ${\mathrm{H}}_2\mathrm{O}$ ice and thus to evaluate existing equations of state. Pressure dependencies of the single-crystal elastic moduli $C_{ij}\left(P\right)$ of cubic ${\mathrm{H}}_2\mathrm{O}$ ice to 82 GPa have been obtained which indicate its hardness and brittleness. These results were compared with ab initio calculations. It is suggested that the transition from molecular ice VII to ionic ice X occurs at much higher pressures than proposed earlier, probably above 80 GPa.

The charge-transfer insulating perovskite oxides currently used as fuel cell electrolytes undergo, at high temperature, an oxidation reaction $\frac{1}{2}{\mathrm{O}}_2\left(\mathrm{g}\right)+{\mathrm{V}}_{\mathrm{O}}^{••}\to {\mathrm{O}}_{\mathrm{O}}^{\mathrm{X}}+2{\mathrm{h}}^{•}$, that produces oxygen-type holes. Understanding the nature and mobility of these oxygen-type holes is an important step to improve the performance of devices, but presents a theoretical challenge since, in their localized form, they cannot be captured by standard density functional theory. Here, we employ the DFT+$U$ formalism with a Hubbard correction on the $p$ orbitals of oxygen to investigate several properties of these holes, in the particular case of ${\mathrm{BaSnO}}_3$. We describe the small oxygen-type hole polarons, the self-trapping at their origin, and their trapping by trivalent dopants (Ga, Sc, In, Lu, Y, Gd, La). Strong similarities with protonic defects are observed concerning the evolution of the trapping energy with ionic radius of the dopant. Moreover, we show that long-range diffusion of holes is a complex phenomenon, that proceeds by a succession of several mechanisms. However, the standard implementation of DFT+$U$ within the projector augmented-wave (PAW) formalism leads to use very large, unphysical values of $U$ for the O-$p$ orbital. We propose here a slightly modified DFT+$U$ scheme, that takes into account the fact that the O-$p$ is truncated in usual DFT+$U$ implementation in PAW. This scheme yields more physical values of $U$ than the ones traditionally used in the literature, and describes well the properties of the hole polaron.

The single mode relaxation time approximation has been demonstrated to greatly underestimate the lattice thermal conductivity of two-dimensional materials due to the collective effect of phonon normal scattering. Callaway's dual relaxation model represents a good approximation to the otherwise ab initio solution of the phonon Boltzmann equation. In this work we develop a discrete-ordinate-method (DOM) scheme for the numerical solution of the phonon Boltzmann equation under Callaway's model. Heat transport in a graphene ribbon with different geometries is modeled by our scheme, which produces results quite consistent with the available molecular dynamics, Monte Carlo simulations, and experimental measurements. Callaway's lattice thermal conductivity model with empirical boundary scattering rates is examined and shown to overestimate or underestimate the direct DOM solution. The length convergence of the lattice thermal conductivity of a rectangular graphene ribbon is explored and found to depend appreciably on the ribbon width, with a semiquantitative correlation provided between the convergence length and the width. Finally, we predict the existence of a phonon Knudsen minimum in a graphene ribbon only at a low system temperature and isotope concentration so that the average normal scattering rate is two orders of magnitude stronger than the intrinsic resistive one. The present work will promote not only the methodology for the solution of the phonon Boltzmann equation but also the theoretical modeling and experimental detection of hydrodynamic phonon transport in two-dimensional materials.

Interacting many-body systems that are driven far away from equilibrium can exhibit phase transitions between dynamically emerging quantum phases, which manifest as singularities in the Loschmidt echo. Whether and under which conditions such dynamical transitions occur in higher-dimensional systems with spontaneously broken continuous symmetries is largely elusive thus far. Here, we study the dynamics of the Loschmidt echo in the three-dimensional O($N$) model following a quantum quench from a symmetry-breaking initial state. The O($N$) model exhibits a dynamical transition in the asymptotic steady state, separating two phases with a finite and vanishing order parameter, that is associated with the broken symmetry. We analytically calculate the rate function of the Loschmidt echo and find that it exhibits periodic kink singularities when this dynamical steady-state transition is crossed. The singularities arise exactly at the zero crossings of the oscillating order parameter. As a consequence, the appearance of the kink singularities in the transient dynamics is directly linked to a dynamical transition in the order parameter. Furthermore, we argue, that our results for dynamical quantum phase transitions in the O($N$) model are general and apply to generic systems with continuous symmetry breaking.

The response of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}\text{/}\mathrm{Cu}\text{/}{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ trilayer structures to excitation by a femtosecond laser pulse has been studied in optical pump-probe experiments and compared with the response of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG) and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ reference samples. The optical pump induces a partial demagnetization of the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, a large thermal gradient within the YIG, and temperature differences across the interfaces within the sample stack. When a moderate magnetic field is applied close to normal to the sample plane, so as to quasialign the YIG magnetization with the field and cant the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ magnetization from the plane, ultrafast demagnetization initiates precession of the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ magnetization. The transient temperature profile within the samples has been modeled using a one-dimensional finite-element computational model of heat conduction, while the magnetization dynamics are well described by a macrospin solution of the Landau-Lifshitz-Gilbert equation. The precessional response of the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ layers within the trilayers and the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ reference sample are very similar for pump fluences of up to $1.5\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$, beyond which irreversible changes to the magnetic properties of the films are observed. These results suggest that the spin Seebeck effect is ineffective in modifying the precessional dynamics of the present $\mathrm{YIG}\text{/}\mathrm{Cu}\text{/}{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ samples when subject to ultrafast optical excitation.

The Fe $L$-edge resonant magneto-optical Kerr effect of a buried Fe nanofilm was investigated by rotating-analyzer ellipsometry and the results were compared with those from three theoretical simulations. The reversal of the Kerr rotation angle ${\theta }_{\mathrm{K}}$ between the $L_3$ and $L_2$ edges, observed in the experiment, was consistent with classical electromagnetic simulation using empirical optical constants. The spectral ${\theta }_{\mathrm{K}}$ feature was reproduced by the first-principles calculation of the KKR-Green's function method on the itinerant electronic system. The demonstration indicates that spectra of the $L$-edge resonant MOKE can be understood in terms of both the macroscopic and microscopic pictures.

Using high-resolution sound velocity measurements, we determined the temperature dependence of the principal elastic constants of the multiferroic compound ${\mathrm{Ba}}_3{\mathrm{NbFe}}_3{\mathrm{Si}}_2{\mathrm{O}}_{14}$ at different magnetic fields. All four elastic constants revealed an anomaly at $T_N=27.1\left(1\right)$ K, which coincides very well with the emergence of the magnetic chiral state. More importantly, we carried out velocity measurements as a function of a field rotating in the basal trigonal plane at different fields and temperatures. In particular, in the magnetically ordered state, we observed an angular hysteresis of $\mathrm{\Delta }{C}_{11}/C_{11}$, which might be associated with the field-induced ferroelectric polarization suddenly switching from the $b^{*}$ axis to the $a$ direction, due to the magnetoelectric effect previously observed in ${\mathrm{Ba}}_3{\mathrm{NbFe}}_3{\mathrm{Si}}_2{\mathrm{O}}_{14}$. The data analysis also leads to an estimate for the lattice distortion $\left(e_1-e_2\le {10}^{-5}\right)$ accounting for a symmetry reduction in the magnetic chiral state possibly associated with the induced polarization along the $a$ axis in zero field.

We report on the spontaneous appearance of antiferromagnetic order in a model gapped quantum paramagnet $\mathrm{Ni}{\left({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x\right)}_2·4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2$ induced by a change in bromine concentration $x$. This transition is qualitatively similar to a $z=1$ magnetic quantum critical point. However, the observed critical scaling of thermodynamic and magnetic properties has rather unusual critical exponents.

Hydrogen (H)-doped LaFeAsO is a prototypical iron-based superconductor. However, its phase diagram extends beyond the standard framework, where a superconducting (SC) phase follows an antiferromagnetic (AF) phase upon carrier doping; instead, the SC phase is sandwiched between two AF phases appearing in lightly and heavily H-doped regimes. We performed nuclear magnetic resonance (NMR) measurements under pressure, focusing on the second AF phase in the heavily H-doped regime. The second AF phase is strongly suppressed when a pressure of 3.0 GPa is applied, and apparently shifts to a highly H-doped regime, thereby a “bare” quantum critical point (QCP) emerges. A quantum critical regime emerges in a paramagnetic state near the QCP, however, the influence of the AF critical fluctuations to the SC phase is limited in the narrow doping regime near the QCP. The optimal SC condition ($T_c\sim 48$ K) is unaffected by AF fluctuations.

Monoclinic gallium oxide ($\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$) has important applications in power devices and deep UV optoelectronic devices because of such novel properties as a wide band gap, high breakdown electric field, and a wide range of $n$-type doping conductivity. However, the intrinsic failure mechanisms of $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ remain unknown, which limits the fabrication and packaging of $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$-based electronic devices. Here we used density-functional theory at the Perdew-Burke-Ernzerhof level to examine the shear-induced failure mechanisms of $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ along various plausible slip systems. We found that the $\left(001\right)/\langle 010\rangle$ slip system has the lowest ideal shear strength of 3.8 GPa among five plausible slip systems, suggesting that $\left(001\right)/\langle 010\rangle$ is the most plausible activated slip system. This slip leads to an intrinsic failure mechanism arising from breaking the longest Ga−O bond between octahedral Ga and fourfold-coordinated O. Then we identified the same failure mechanism of $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ under biaxial shear deformation that mimics indentation stress conditions. Finally, the general stacking fault energy (SFE) surface is calculated for the (001) surface from which we concluded that there is no intrinsic stacking fault structure for $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. The deformation modes and SFE calculations are essential to understand the intrinsic mechanical processes of this semiconductor material, which provides insightful guidance for designing high-performance semiconductor devices.