MXenes are a family two-dimensional transition-metal carbide and nitride materials, which often exhibit very good metallic conductivity and are thus of great interest for applications in, e.g., flexible electronics, electrocatalysis, and electromagnetic interference shielding. However, surprisingly little is known about the fermiology of MXenes, i.e., the shape and size of their Fermi surfaces, and its effect on the material properties. One reason for this may be that MXene surfaces are almost always covered by a mixture of functional groups, and studying Fermi surfaces of disordered systems is cumbersome. Here, we study fermiology of four common Ti-based MXenes as a function of the surface functional group composition. We first calculate the effective band structures of systems with explicit mixed surfaces and observe gradual evolution in the filling of the Ti-d band and resulting shift of Fermi level. We then demonstrate that these band structures can be closely approximated by using pseudohydrogenated surfaces, and also compare favorably to the experimental angle-resolved photoemission spectroscopy results. By modifying the pseudohydrogen charge we then proceed to plot Fermi surfaces for all systems and extract their properties, such as the Fermi-surface area and average Fermi velocity. These are in turn used to evaluate the electrical conductivity with the relaxation time fitted to experimentally measured conductivities.

Unconventional fermions, such as threefold, fourfold, sixfold, and eightfold fermions have attracted intense attention in recent years. However, the concrete materials hosting unconventional fermions are still in urgent scarcity. In this work, based on first-principle calculations and symmetry analysis, we reveal rich unconventional fermions in existing compound $R_2{\mathrm{C}}_3$ ($R=\mathrm{Y}$, La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm, Yb, Lu). We show that these compounds host quadratic dispersive threefold, linear dispersive fourfold, and sixfold points (SPs) near the Fermi level in their electronic band structures when spin-orbital coupling (SOC) is not included. We also find that a uniaxial strain can transform the unconventional fermions into other types of fermions, depending on the directions of strain. When SOC is considered, a SP transforms to an eightfold degenerate point and a fourfold degenerate point. Overall, our work provides a family of realistic materials to study the unconventional fermions.

We predicted one Dyakonov surface magnon (DSM) and two Dyakonov magnon polaritons (DSMPs) localized at the antiferromagnet (AF)-air interface. Their concise dispersion equations and necessary conditions of existence were obtained. The DSM is the magnetostatic limit of the first DSMP, which are situated inside the AF reststrahlen frequency band. The second DSMP has no magnetostatic limit and lies outside the frequency band. The specifical frequency ranges occupied by the DSM and DSMPs were analytically found. The DSMPs are hybrid-polarization surface polaritons and the DSM is a hybrid-polarization surface magnetostatic mode. The first DSMP possesses large attenuation constants so that it is highly localized at the surface. Compared with it, the second DSMP exhibits relatively small attenuation constants. According to the features of polarization obtained for either DSMP, we used the TE and TM radiations incident on the AF surface to calculate attenuated total reflection (ATR) spectra, respectively. These ATR spectra not only further prove the existence of the DSMPs but also demonstrate their observability in experiment. However, the DSM should be detected by means of a light-scattering technique.

We report a spin-wave study on multiferroic ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$ under magnetic fields up to 12 T using low-energy inelastic neutron scattering. In-plane transverse $\left(T_1\right)$ spin-wave modes are highly dispersive along $\left(h00\right)$ and rather flat but strong in intensity along $\left(30l\right)$. In addition, two dispersive electromagnon modes have been observed around 3.5 meV. Dispersion of the out-of-plane transverse modes $\left(T_2\right)$ under fields reveals that the single-ion anisotropy constant decreases with increasing magnetic field, which is consistent with the linear spin-wave theory. Our results imply that the field-dependent single-ion anisotropy plays a crucial role in determining the characteristics of $T_2$ and electromagnon modes in the three-dimensional anisotropic spin-wave spectrum.

Recent in-situ x-ray diffraction (XRD) measurements on laser-shock compressed Ag foils demonstrated a face-centered-cubic to body-centered-cubic transformation at ∼150 GPa and melting between 172 and 197 GPa [Phys. Rev. Lett. 124, 235701 (2020)]. As a complement to the XRD work, we conducted plate impact experiments to obtain shock velocity and wave profile measurements on thicker Ag samples shock-compressed to peak stresses between 30 and 300 GPa. The shock velocity–particle velocity results were fitted very well by a linear relation over the entire stress range, providing an accurate determination of the Ag Hugoniot (locus of the stress-volume states achieved under shock compression). For peak stresses below 187 GPa and above 210 GPa—corresponding to the solid and liquid phases, respectively—the wave profiles show clean single waves. No wave profile features related to the fcc-bcc transformation at ∼150 GPa were observed, implying minimal volume change for the transformation. For stresses between 187 and 210 GPa, an initial jump was followed by a time-dependent increase in the particle velocity (20–80 ns risetime) to the peak state—corresponding to the solid-liquid mixed phase response. Unlike the solid and liquid response, the mixed-phase response cannot be readily analyzed analytically. Instead, numerical simulations incorporating an accurate multiphase equation of state for Ag—not currently available—are required to analyze the wave profiles measured at 187–210 GPa stresses. The present work shows the potential for using wave profile measurements to examine the melting transition under shock compression.

${\mathrm{Cu}}_2{\mathrm{IrO}}_3$ is among the newest layered honeycomb iridates and a promising candidate to harbor a Kitaev quantum spin liquid state. Here, we investigate the pressure and temperature dependence of its structure through a combination of powder x-ray diffraction and x-ray absorption fine structure measurements, as well as ab initio evolutionary structure search. At ambient pressure, we revise the previously proposed $C2/c$ solution with a related but notably more stable $P{2}_1/c$ structure. Pressures below 8 GPa drive the formation of Ir-Ir dimers at both ambient and low temperatures, similar to the case of ${\mathrm{Li}}_2{\mathrm{IrO}}_3$. At higher pressures, the structural evolution dramatically depends on temperature. A large discontinuous reduction of the Ir honeycomb interplanar distance is observed around 15 GPa at room temperature, likely driven by a collapse of the O-Cu-O dumbbell bonds. At 15 K, pressures beyond 20 GPa first lead to an intermediate phase featuring a continuous reduction of the interplanar distance, which then collapses at 30 GPa across yet another phase transition. However, the resulting structure around 40 GPa is not the same at room and low temperatures. Remarkably, the reduction in interplanar distance leads to an apparent healing of the stacking faults at room temperature, but not at 15 K. Possible implications on the evolution of electronic structure of ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$ with pressure are discussed.

Ammann-Beenker lattice is a two-dimensional quasicrystal with eightfold symmetry, which can be described as a projection of a cut from a four-dimensional simple cubic lattice. We consider the vertex tight-binding model on this lattice and investigate the strictly localized states at the center of the spectrum. We use a numerical method based on the generation of finite lattices around a given perpendicular space point and QR decomposition of the Hamiltonian to count the strictly localized states. We apply this method to count the frequency of localized states in lattices of up to 100 000 sites. We obtain an orthogonal set of compact localized states by diagonalizing the position operator projected onto the manifold spanned by the zero-energy states. We identify 20 localized state types and calculate their exact frequencies through their perpendicular space images. Unlike the Penrose lattice, all the localized state types are eightfold symmetric around an eight edge vertex, and all vertex types can support localized states. The total frequency of these 20 types gives a lower bound of $f_{\mathrm{LS}}=30796-21776\sqrt{2}\simeq 0.08547$ for the fraction of strictly localized states in the spectrum. This value is in agreement with the numerical calculation and very close to the recently conjectured exact fraction of localized states $f_{\mathrm{Ex}}=3/2-\sqrt{2}\simeq 0.08579$.

In gratings synthetically moving at nearly the velocity of light a symmetry breaking transition is observed between free-flowing fluidlike Bloch waves observed at lower grating velocities and, at luminal velocities, condensed, localized states of light captured in each period of the grating and locked to its velocity. We introduce a technique for calculating in this regime and use it to study the transition in detail shedding light on the critical exponents and the periodic oscillations in transmitted intensity seen in the pretransition regime.

We propose an efficient algorithm to numerically solve Anderson impurity problems using matrix product states. By introducing a modified chain mapping we obtain significantly lower entanglement, as compared to all previous attempts, while keeping the short-range nature of the couplings. Employing a thermofield transformation, our approach naturally extends to finite temperatures, with applications to dynamical mean field theory, nonequilibrium dynamics, and quantum transport.

We derive boundary conditions at the interfaces of magnetoelastic heterostructures under ferromagnetic resonance for arbitrary magnetization directions and interface shapes. We apply our formalism to $\mathrm{magnet}|\mathrm{nonmagnet}$ bilayers and magnetic grains embedded in a nonmagnetic thin film, revealing a nontrivial magnetization angle dependence of acoustic phonon pumping.

The magnetization of graphene flakes as a function of size, shape, boundary type, and edge defects is computed in a tight-binding model. Flakes with Klein boundaries exhibit a smaller orbital magnetization than flakes with other boundaries. The difference in magnitude is significant for flakes with a few hundred to a few thousand atoms. One can tune the magnetization of a zigzag or Klein flake quasicontinuously by adding atoms, one by one, to the edges of the zigzag flake, or removing atoms from a Klein flake. Flakes with an odd number of atoms show a paramagnetic spin response due to particle-hole symmetry. The addition of a next-nearest neighbor term to the Hamiltonian, which breaks particle-hole symmetry, does not destroy this effect as long as there is approximate particle-hole symmetry. Other defects that affect the chemical potential, such as edge atoms with an onsite potential, or doping, can affect the paramagnetism but preserve the difference in magnetization between Klein and non-Klein flakes. These results are consistent with experiments which see paramagnetic response at low magnetic fields crossing over to diamagnetic response at higher fields.

The domain wall dynamics driven by an out-of-plane magnetic field was measured for a series of magnetic trilayers with different strengths of the interfacial Dzyaloshinskii-Moriya interaction (DMI). The features of the field-driven domain wall velocity curves strongly depend on the ratio of the field $H_D$ stabilizing chiral Néel walls to the demagnetizing field within the domain wall, $H_{\mathrm{DW}}$. The measured Walker velocity, which in systems with large DMI is maintained after the Walker field, giving rise to a velocity plateau up to the Slonczewski field $H_S$, can be related to the DMI strength. Yet, when $H_D$ and $H_{\mathrm{DW}}$ have comparable values, a careful analysis needs to be done in order to evaluate the impact of the DMI on the domain wall velocity. By means of a one-dimensional model and two-dimensional simulations, we extend this method and we clarify the interpretation of the experimental curves measured for samples where $H_D$ and $H_{\mathrm{DW}}$ are comparable.

Kibble-Zurek theory (KZ) stands out as the most robust theory of defect generation in the dynamics of phase transitions. KZ utilizes the structure of equilibrium states away from the transition point to estimate the excitations due to the transition using adiabatic and impulse approximations. Here we show, the actual nonequilibrium dynamics lead to a qualitatively different scenario from KZ, as far correlations between the defects (rather than their densities) are concerned. For a quantum Ising chain, we show, this gives rise to a Gaussian spatial decay in the domain wall (kinks) correlations, while KZ would predict an exponential fall. We propose a simple but general framework on top of KZ, based on the “quantum coarsening” dynamics of local correlators in the supposed impulse regime. We outline how our picture extends to generic interacting situations.

A theory is developed for the ultrafast low-to-high spin transition in divalent iron. Following the photoinduced metal-to-ligand charge transfer (MLCT), the ${}^{1}A_1$ MLCT state decays on a sub-100-fs timescale into the ${}^{5}T_2$ MLCT using a ${}^{3}T_1$ MLCT state as intermediary. Damping of nuclear motion is crucial in obtaining a full singlet-to-quintet transition mediated by the spin-orbit interaction. Destructive interference suppresses transitions to metal-centered (MC) states. The relaxation to the MC ${}^{5}T_2$ is a result of Coulomb scattering with the surroundings.

We report the observation of the Rashba-Edelstein magnetoresistance (REMR) at room temperature in two two-dimensional (2D) materials, single layer graphene (SLG) and 2D semiconductor $\mathrm{Mo}{\mathrm{S}}_2$. The measurements were performed in heterostructures with yttrium iron garnet (YIG), namely, YIG/SLG and ${\text{YIG/MoS}}_2$, by means of the modulated magnetoresistance technique. We show that the action of the direct and inverse Rashba-Edelstein effects (REEs) in a YIG/2DM structure couples current-induced spin accumulation to the electric resistance, producing the REMR. The resistance change is similar to the one observed in spin Hall magnetoresistance experiments, but the origin of the change here lies in the 2D Rashba-Edelstein effects and not in the 3D spin Hall effects. The measured REE lengths for the two materials are in good agreement with the values obtained with the electric spin pumping technique.

TiNi-based shape-memory alloys are known to exhibit a strain glass state under certain conditions, generally in the presence of high-density defects such as excess solute atoms or alloying elements, dislocations, and nanoprecipitates. In this paper, we report a strain glass transition in a nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy. The nanocrystalline strain glass state is achieved by a combined effect of high-density grain boundaries and high concentration doping of Pt atoms in the B2 matrix. The nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ strain glass alloy showed a large near-complete progressive superelasticity with a recovery strain of about 6% and a low apparent Young's modulus of about 30 GPa in a wide temperature range of over 200 °C. In situ synchrotron x-ray diffraction measurement showed that the strain glass B2 [B2(SG)] phase experienced B2(SG)→R→B19 transformation upon loading and B19→B2(SG) upon unloading. The findings of this study provide insight for the development of nanocrystalline strain glass shape-memory alloys.

A theory of temperature dynamics in many-body systems driven by time-dependent external sources is introduced. The formalism is based on the combination of perturbation theory and the fluctuational-electrodynamics approach in many-body systems. By using response theory, an explicit formula for temperature and phase shifts is derived and expressed in terms of the amplitude and phase of external power sources. Although the proposed method is highly efficient because it can skip the transient response, it is valid when external powers are weak. As an illustration of this theoretical framework, we have shown the dynamics of temperatures in one, two, and three degrees of freedom systems driven by sine wave input powers. Finally, we highlight some emergent phenomena arising from purely dynamical many-body effects, including amplification, attenuation, delaying, or accelerating temperature responses. This work could find important applications in the domain of dynamical thermal management at the nanoscale.

This study aims at assessing the thermal nuclei motion effects on the multipole transition channels involved in two core-level spectroscopies, x-ray absorption spectroscopy (XAS) and x-ray Raman scattering (XRS). Temperature effects on the $1s\to s$ monopole, $1s\to p$ dipole, and $1s\to d$ quadrupole transitions are investigated using two reference systems for which we present original experimental data: $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ at the Al $K$ edge probed by XRS at room temperature and rutile $\mathrm{Ti}{\mathrm{O}}_2$ at the Ti $K$ pre-edge probed by XAS at temperatures ranging from 6 to 700 K. Through the latter, this work enlightens the part of the pre-edge peak enhancement due to temperature in the $K$ pre-edge region of $3d$ transition metal, which is known to be routinely used to determine the concentration, valence or symmetry of the probed element in a given sample. Nuclear thermal fluctuations are taken into account using a method based on density functional theory that consists in averaging spectra over atomic configurations, generated within the harmonic approximation and obeying quantum statistics at finite temperature. Since only a finite number of such configurations are used, the numerically averaged spectra generally lose the symmetry of the equilibrium crystal positions. In this paper, we demonstrate that the physical average has to be symmetric and propose a method to restore the physical angular dependence of the spectra. The approach is successfully applied to investigate the angular dependent XAS spectra in rutile as a function of temperature. The two systems under study allow to draw general conclusions regarding the effect of nuclear quantum fluctuations on the different transition channels available to both core-level spectroscopies.