The high-temperature and high-pressure response of magnesium has been investigated through shock-release experiments performed up to shock melting. The longitudinal and bulk sound speeds of Mg are reported along the Hugoniot from 25 to 56 GPa and used to determine the elastic properties and Grüneisen parameter. The hexagonal close-packed (hcp)–body-centered cubic (bcc) phase transition is marked by a reduction in the determined shear wave speed. Thermal softening is observed to begin between 40 and 44 GPa, with incipient melt at 55.5 GPa, in close agreement with previous diffraction measurements under laser shock loading. Examination of the release profiles showed that two different responses were observed, depending on the peak stress. When plotted in a stress-energy phase diagram, the two responses are observed to form separate lines that intersect the static hcp-bcc phase transition and incipient melt. The results indicate that the hcp-bcc transition occurs on the Hugoniot at 28.4 GPa and place the hcp-bcc-liquid triple point ∼20 GPa. A multiphase equation of state is developed which places the melt boundary below the previously reported static measurements.

We derive the charge density up to second order in spatial gradient in inhomogeneous crystals using the semiclassical coarse graining procedure based on the wave packet method. It can be recast as divergence of polarization, whose first-order contribution consists of three parts, a perturbative correction to the original Berry connection expression, a topological part that can be written as an integral of the Chern-Simons 3-form, and a previously-unknown, quadrupolelike contribution. The topological part can be related to the quantized fractional charge carried by a vortex in two-dimensional systems. We then generalize our results to the multiband case and show that the quadrupolelike contribution plays an important role, as it makes the total polarization gauge-independent. Finally, we verify our theory in several model systems.

We report on the development and application of a parametrized crystal-field model for both ${\mathrm{C}}_1$ symmetry centers in trivalent erbium-doped ${\mathrm{Y}}_2{\mathrm{SiO}}_5$. High-resolution Zeeman and temperature dependent absorption spectroscopy was performed to acquire the necessary experimental data. The obtained data, in addition to the ground $({}^4\mathrm{I}_{15/2}{\mathrm{Z}}_1)$ state and exited $\left({}^4\mathrm{I}_{13/2}{\mathrm{Y}}_1\right)$ state Zeeman and hyperfine structure, were simultaneously fitted in order to refine an existing crystal-field interpretation of the ${\mathrm{Er}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ system. We demonstrate that it is possible to account for the electronic, magnetic, and hyperfine structure of the full $4f^{11}$ configuration of ${\mathrm{Er}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ and further, that it is possible to predict both optical polarization behavior and high magnetic field hyperfine structure of transitions in the $1.5\text{−}\mu \mathrm{m}$ telecommunications band.

$\mathrm{Ta}{\mathrm{Se}}_3$ is a layered van der Waals semimetal with several inverted band gaps throughout the entire Brillouin zone and nontrivial $Z_2$ topological indices, which place it at the boundary between a strong and a weak topological phase. Our transport experiments reveal a quadratic nonsaturating magnetoresistance (MR) with values reaching ${10}^4$% at 1.8 K and 14 T, whose origins have to be searched in the material's band structure. Here we combine angle-resolved photoelectron spectroscopy experiments, also with spin resolution, with ab initio calculations based on density functional theory in order to draw a connection between the Fermi surface topology and the measured transport properties. Simulations based on the calculated Fermi surface clarify that electron-hole compensation plays an important role for the observed MR in the bulk material. At the surface, the position of Fermi level differs, and it can be controlled by alkali metal deposition which accounts not only for the energy shift of the bands but it slightly modifies the dispersion of the valence and conduction bands. We propose that the observed band-gap renormalization might offer a route for engineering the topological phase in $\mathrm{Ta}{\mathrm{Se}}_3$, alternative to strain.

We report on the electronic and thermodynamic properties of the antiferromagnetic metal uranium mononitride with a Néel temperature $T_N\approx 53\mathrm{K}$. The fabrication of microstructures from single crystals enables us to study the low-temperature metamagnetic transition at approximately $58\mathrm{T}$ by high-precision magnetotransport, Hall-effect, and magnetic-torque measurements. We confirm the evolution of the high-field transition from a broad and complex behavior to a sharp first-order-like step, associated with a spin flop at low temperature. In the high-field state, the magnetic contribution to the temperature dependence of the resistivity is suppressed completely. It evolves into an almost quadratic dependence at low temperatures indicative of a metallic character. Our detailed investigation of the Hall effect provides evidence for a prominent Fermi-surface reconstruction as the system is pushed into the high-field state.

The antiferromagnet and semimetal $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{As}}_2$ has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the $c$ axis in $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{As}}_2$. In the search for such phases, we investigate here the effects of hydrostatic pressure in $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{As}}_2$. For that, we present specific heat, transport, and $\mu \mathrm{SR}$ measurements under hydrostatic pressure up to $\sim 2.5\mathrm{GPa}$, combined with ab initio density functional theory (DFT) calculations. Experimentally, we establish that the ground state of $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{As}}_2$ changes from in-plane antiferromagnetic $\left({\mathrm{AFM}}_{ab}\right)$ to ferromagnetic at a critical pressure of $\approx 2$ GPa, which is likely characterized by the moments dominantly lying within the $ab$ plane $\left({\mathrm{FM}}_{ab}\right)$. The ${\mathrm{AFM}}_{ab}\text{−}{\mathrm{FM}}_{ab}$ transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our theoretical results indicate that $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{As}}_2$ moves closer to the sought-for ${\mathrm{FM}}_c$ state (moments $\parallel c$) with increasing pressure further. We predict that a pressure of $\approx 23$ GPa will stabilize the ${\mathrm{FM}}_c$ state if Eu remains in a $2+$ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{As}}_2$ and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.

We investigate the impact of geometric constriction on the viscous flow of electron liquid through quantum point contacts. We provide analysis on the electric potential distribution given the setup of a slit configuration and use the method of conformal mapping to obtain analytical results. The potential profile can be tested and contrasted experimentally with the scanning tunneling potentiometry technique. We discuss intricate physics that underlies the Gurzhi effect, i.e., the enhancement of conductivity in the viscous flow, and compare results for different boundary conditions. In addition, we calculate the temperature dependence of the momentum relaxation time as a result of impurity assisted quasiballistic interference effects and discuss various correlational corrections that lead to the violation of Matthiessen's rule in the hydrodynamic regime. We caution that spatially inhomogeneous profiles of current in the Gurzhi crossover between Ohmic and Stokes flows might also appear in the nonhydrodynamic limit where nonlocality plays an important role. This conclusion is corroborated by calculation of dispersive conductivity in the weakly impure limit.

We use an exact analytical technique [Phys. Rev. B 101, 115405 (2020), Phys. Rev. B 102, 165117 (2020)] to recover the surface Green's functions for Bernal (ABA) and rhombohedral (ABC) graphite. For rhombohedral graphite we recover the predicted surface flat bands. For Bernal graphite we find that the surface state spectral function is similar to the bilayer one, but the trigonal warping effects are enhanced, and the surface quasiparticles have a much shorter lifetime. We subsequently use the $T$-matrix formalism to study the quasiparticle interference patterns generated on the surface of semi-infinite ABA and ABC graphite in the presence of impurity scattering. We compare our predictions to experimental STM data of impurity-localized states on the surface of Bernal graphite which appear to be in a good agreement with our calculations.

We investigate the electronic structure of tungsten ditelluride (${\mathrm{WTe}}_2$) flakes with different thicknesses in magnetotransport studies. The temperature-dependent resistance and magnetoresistance (MR) measurements both confirm the breaking of carrier balance induced by thickness reduction, which suppresses the “turn-on” behavior and large positive MR. The Shubnikov-de-Haas oscillation studies further confirm the thickness-dependent change of electronic structure of ${\mathrm{WTe}}_2$ and reveal a possible temperature-sensitive electronic structure change. Finally, we report the thickness-dependent anisotropy of the Fermi surface, which reveals that multilayer ${\mathrm{WTe}}_2$ is an electronic 3D material and the anisotropy decreases as thickness decreases.

In the context of one-dimensional fermionic systems, helical Luttinger liquids are characterized not only by intriguing spin properties but also by the possibility of being manipulated by means of electrostatic gates, exploiting finite Rashba coupling. We use this property to show that a heterostructure composed of a helical Luttinger liquid, contacted to two metallic leads and supplemented by top gates, can be used as a tunable thermal valve. By relying on bosonization techniques and scattering of plasmonic modes, we investigate the performance of this valve with respect to electron-electron interactions, temperature, and properties of the gates. The maximal modulation of the thermal conductance that the proposed device can achieve is, for experimentally relevant parameters, around $7\%$. Such variation can be both positive or negative. Moreover, a modification in the geometry of the gate can lead to particular temperature dependencies related to interference effects. We also argue that the effects we predict can be used to establish the helical nature of the edge states in two-dimensional topological insulators.

Recently, we have introduced [A. M. Marques et al., Phys. Rev. B 103, 235425 (2021)] the concept of $2^n$-root topology and applied it to one-dimensional systems. These models require $n$ squaring operations to their Hamiltonians, intercalated with different constant energy downshifts at each level, in order to arrive at a decoupled block corresponding to a known topological insulator (TI) that acts as the source of the topological features of the starting $2^n\text{−}\mathrm{root}$ TI $\left(\sqrt[2^n]{\text{TI}}\right)$. In the process, $n$ nontopological residual models with degenerate spectra and in-gap impurity states appear, which dilute the topologically protected component of the starting edge states. Here, we generalize this method to several two-dimensional models, by finding the 4-root version of lattices hosting weak and higher-order boundary modes (both topological and nontopological) of a Chern insulator and of a topological semimetal. We further show that a starting model with a non-Hermitian region in parameter space and a complex energy spectrum can nevertheless display a purely real spectrum for all its successive squared versions, allowing for an exact mapping between certain non-Hermitian models and their Hermitian lower root-degree counterparts. A comment is made on the possible realization of these models in artificial lattices.

Transition metal dichalcogenides have a rich exciton landscape consisting of a variety of bright and dark excitonic states. We consider the lowest-energy dark states in ${\text{MoS}}_2$, which can be referred to as hybrid excitons, as they are formed by a Dirac electron and a Schrödinger hole. The chiral nature of the Dirac electron introduces asymmetry to the excited exciton state spectrum and couples the relative motion of the electron and hole with the center-of-mass motion. We demonstrate that this coupling generates an additional contribution to the Berry curvature of hybrid excitons. The additional contribution is substrate dependent and accounts for almost one quarter of the Berry curvature in suspended samples. The nontrivial geometry of hybrid excitons is manifested in the optical anomalous valley Hall effect, which can be observed via recently developed pump-probe photoemission spectroscopy. We argue that the Hall angle of hybrid excitons is approximately one half of that for bright excitons. Moreover, the anticipated long lifetime of hybrid excitons favors an extended propagation distance and allows the spatial separation of hybrid excitons with different valley indices.

An extremely large linear magnetoresistance (LMR) is a ubiquitous phenomenon emerging from topological Dirac and Weyl semimetals. However, the connection between an LMR and a nontrivial topology is under extensive debate. In this paper, by precisely controlling the thickness of ${\mathrm{SrNbO}}_3$ thin films grown on ${\mathrm{SrTiO}}_3$ substrates, we observe an LMR over a large carrier density range with a magnetoresistance as high as $150000\%$ at a carrier density $n\sim {10}^{21}{\mathrm{cm}}^{-3}$, far away from the quantum-limit regime. The temperature-, magnetic-field-, and carrier-density-dependent LMR in ${\mathrm{SrNbO}}_3/{\mathrm{SrTiO}}_3$ heterostructures provides compelling evidence of a mobility-driven LMR in coherent electronic systems. Our results uncover the general principle of an LMR and shed light on proper categorization of transport properties in topological and correlated materials.

Ramp compression experiments characterize high-pressure states of matter at temperatures well below those present in shock compression. However, because temperature is typically not directly measured during ramp compression, it is uncertain how much heating occurs under these shock-free conditions. Here, we performed a series of ab initio simulations on carbon in order to match the density-stress measurements of Smith et al. [Smith et al., Nature (London) 511, 330 (2014)]. We considered isotropically as well as uniaxially compressed solid carbon in the diamond and BC8 phases, with and without defects, as well as liquid carbon. Our idealized model ascribes heating during ramp compression to an initially uniaxially compressed cell transforming isochorically into an isotropically (hydrostatic equivalent) compressed state having lower internal energy, hence higher temperature so as to conserve energy. Multiple such heating events can occur during a single ramp experiment, leading to higher temperatures than with isentropic compression. Comparison with experiments shows that heating alone does not explain the equation of state measurements on diamond, instead implying that a significant uniaxial stress component remains present at high compression. The temperature predictions of our ramp compression model remain to be verified with future laboratory measurements.

Active acoustic metamaterials consisting of paired sensor-driver unit cells offer a promising path towards the practical realization of exciting transformation acoustics devices. The design of these cells is founded in a microscopic acoustic model that describes materials as collections of subwavelength polarized sources which respond to the local conditions of pressure and particle velocity. The current ability to express the polarizabilities that characterize these sources in terms of the effective macroscopic acoustic properties is limited to only a few simple cases and is not applicable to inhomogeneous bulk media of arbitrary geometries. Here, we address this challenge and derive general closed-form expressions relating the bulk modulus to the monopole polarizability and the mass density tensor to the dipole polarizability. Furthermore, we use these expressions to adapt transformation acoustics to the microscopic model. We demonstrate the accuracy of our approach by comparing the fields scattered by several devices, including cylindrical cloaks with steep property gradients and anisotropy, with the fields scattered by the devices' realizations with polarized sources.

${\mathrm{Sc}}_3{\mathrm{Mn}}_3{\mathrm{Al}}_7{\mathrm{Si}}_5$ is a rare example of a correlated metal in which the Mn moments form a kagome lattice. The absence of magnetic ordering to the lowest temperatures suggests that geometrical frustration of magnetic interactions may lead to strong magnetic fluctuations. We have performed inelastic neutron scattering measurements on ${\mathrm{Sc}}_3{\mathrm{Mn}}_3{\mathrm{Al}}_7{\mathrm{Si}}_5$, finding that phonon scattering dominates for energies from ∼20–50 meV. These results are in good agreement with ab initio calculations of the phonon dispersions and densities of states, and as well reproduce the measured specific heat. A weak magnetic signal was detected at energies less than ∼10 meV, present only at the lowest temperatures. The magnetic signal is broad and quasielastic, as expected for metallic paramagnets.

We systematically investigated the manipulation of antiferromagnetic interfacial states through current-induced spin-orbit torques (SOT) in the $\mathrm{Pt}/\mathrm{Co}/{\mathrm{Ir}}_{25}{\mathrm{Mn}}_{75}\left(\mathrm{Ir}\text{−}\mathrm{Mn}\right)$ system with varying Ir-Mn or Co thickness. The high tunability of antiferromagnetic interfacial states, that the antiferromagnetic interfacial spins gradually switched from upward to downward or vice versa by SOT, was achieved for the samples with $t_{\mathrm{IrMn}}\ge 4\mathrm{nm}$, whereas the switching ability of antiferromagnetic interfacial spins via SOT under a perpendicular field or a longitudinal field was different, which was attributed to the influence from partial canted interfacial spins. Moreover, the interfacial spins of Ir-Mn layer can be also effectively tuned by SOT across Ru layer in the Pt/Co/Ru/Ir-Mn system, where the exchange coupling between Co and Ir-Mn decreased with increasing the thickness of the Ru layer. Our work provides a comprehensive understanding for manipulating antiferromagnetic interfacial states via SOT, which will promote innovative designs for antiferromagnetic spintronic devices.

We present a first-principles equation-of-state (EOS) table of deuterium aimed at improving the previously established first-principles equation-of-state table (FPEOS) [S. X. Hu et al., Phys. Rev. B 84, 224109 (2011); S. X. Hu et al., Phys. Plasmas 22, 056304 (2015)]. The EOS table presented here, referred to as iFPEOS, introduces (1) a universal density functional theory (DFT) treatment of all density and temperature conditions, (2) a fully consistent treatment of exchange-correlation (XC) thermal effects across the entire range of temperatures covered, and (3) quantum treatment of ions. Based on ab initio molecular dynamics driven by thermal density functional theory, iFPEOS includes density points in the range $1\times {10}^{-3}\le \rho \le 1.6\times {10}^3$ $\mathrm{g}/{\mathrm{cm}}^3$ and temperature points in the range 800 K $\le T\le 256$ MK, thus covering the challenging warm dense matter (WDM) regime. For an improved description of the electronic structure, iFPEOS employs an advanced free-energy XC density functional with explicit temperature dependence, which is at the metageneralized gradient approximation level of DFT. We use the latest orbital-free free-energy density functional for the high-temperature regime where it shows excellent agreement with standard Mermin-Kohn-Sham DFT. For quantum treatment of ions we use path-integral molecular dynamics in order to take into account nuclear quantum effects. Results are compared to other EOS models and most recent experimental measurements of deuterium properties such as the molecular-to-atomic fluid transition, the principal and reshock Hugoniot, and sound speed. We find that iFPEOS provides an improved agreement with experimental data compared to other first-principles EOS models in the WDM regime for pressures up to 200 GPa and temperatures up to $60000$ K. For higher pressures and temperatures, however, iFPEOS is in agreement with other models in predicting lower compressibility and higher sound speed along the Hugoniot, compared to experiment.

The combination of reduced dimensionality and tunable structural distortions in layered perovskite oxides makes these materials ideal platforms for designing novel properties and functionalities. One example is hybrid improper ferroelectricity in $n=2$ Ruddlesden-Popper oxides, where the combination of a layered crystal structure and rotations of the metal-oxide octahedra break symmetry and induce a polarization. Precisely controlling the octahedral rotation distortions, for example by the application of hydrostatic pressure, provides a pathway to tune and optimize the properties of these materials. We combine group theoretic methods, density functional theory calculations, and Landau theory analysis to investigate how octahedral rotations respond to pressure in the hybrid improper ferroelectrics ${\mathrm{Sr}}_3{\mathrm{Zr}}_2{\mathrm{O}}_7, {\mathrm{Ca}}_3{\mathrm{Ti}}_2{\mathrm{O}}_7$, and ${\mathrm{Sr}}_3{\mathrm{Sn}}_2{\mathrm{O}}_7$. We find that factors that are known to control the pressure response of $\mathrm{A}\mathrm{B}{\mathrm{O}}_3$ perovskites—the formal charge of the $A$- and $B$-site cations, tolerance factor, and $B$-site chemistry—also impact the pressure response of these layered perovskites. We also show that coupling between the octahedral rotation and strain order parameters plays a key role in determining the overall pressure response. Despite some similarities, we find that these layered perovskites display a distinct pressure response compared to their $\mathrm{A}\mathrm{B}{\mathrm{O}}_3$ perovskite analogs. By identifying trends and underlying mechanisms that control octahedral rotations in Ruddlesden-Popper oxides under pressure, this work lays the foundation for tailoring the structure and properties of these materials.

Magnetic skyrmions have attracted attention as particlelike swirling spin textures with nontrivial topology, and their self-assembled periodic order i.e., the skyrmion crystal (SkX) is anticipated to host unique magnonic properties. In this paper, we investigate magnetic resonance in the quenched SkX state, which is obtained by the rapid cooling of the high-temperature equilibrium SkX phase in the chiral magnetic insulator ${\mathrm{Cu}}_2\mathrm{O}\mathrm{Se}{\mathrm{O}}_3$. At low temperatures, sextupole and octupole excitation modes of skyrmions are identified, which are usually inactive for oscillating magnetic fields $B^{\nu }$ with GHz-range frequency $\nu$ but turn out to be detectable through the hybridization with the $B^{\nu }$-active counterclockwise and breathing modes, respectively. The observed magnetic excitation spectra are well reproduced by theoretical calculations, which demonstrates that the effective magnetic anisotropy enhanced at low temperatures is the key for the observed hybridization between the $B^{\nu }$-active and $B^{\nu }$-inactive modes.