We report a combined experimental and theoretical study of the lattice dynamics of the type IX clathrate ${\mathrm{Ba}}_{24}{\mathrm{Ge}}_{100}$ by inelastic neutron scattering (INS) experiments and density functional theory (DFT) calculations. We observe low-energy optical modes at $\sim 2–3\mathrm{meV}$ due to the motion of the heavy Ba atoms along the high-symmetry axis and the largest dimension of the open Ge@20 cages present in the compound. Even though the phonon participation ratio indicates that these low-energy modes are localized, their $Q$ dependence shows that the dynamics of the Ba guests are correlated. We observe a strong change in the spectral weight of these modes when the compound undergoes a temperature-induced structural transformation in the temperature range 190–230 K. In the high-temperature phase, the low-energy optical modes show high intensities in the INS data and frequencies rather insensitive to temperature changes up to ∼550 K. In the low-temperature structural modification, the low-energy mode intensities are strongly depleted and apparently shifted to higher energies; this behavior is in line with an off-centering of the Ba atoms at low temperatures. Our DFT calculations successfully approximate the essential features in the dynamics of the high-temperature ${\mathrm{Ba}}_{24}{\mathrm{Ge}}_{100}$ structure.

Although the nature of the superconducting state has been intensively studied in FeSe, many fundamental issues including the pairing symmetry and superconducting fluctuations are still highly controversial. Here, we report a revised ${}^{77}\mathrm{Se}$-nuclear magnetic resonance (NMR) study on the superconducting state of ${}^{77}\mathrm{Se}$-enriched bulk FeSe. Below the superconducting temperature ($T_c$), by carefully avoiding the rf heating effect, a remarkable linewidth broadening and obvious reduction of the Knight shift are observed under external magnetic field along both in-plane and out-of-plane directions, suggesting an intrinsic superconducting nature. These exotic results unambiguously rule out the possibility of chiral $p$-wave pairing, and favor a pairing scenario with the mixing of $s^{\pm }$ and $d$ wave. A slight decrease of the Knight shift well above $T_c$ is also revealed under a moderated external magnetic field, suggesting exotic superconducting fluctuations beyond phase fluctuations in the two-dimensional limit. These renewed NMR results provide valuable constraints for the theoretical models on the exotic superconductivity in FeSe.

In this work we investigate local tuning of the iDMI and perpendicular magnetic anisotropy (PMA) using focused ${\mathrm{Ga}}^{+}$ ion beam irradiation, in an $\mathrm{Ir}\left|\mathrm{Co}\right|\mathrm{Pt}$ multilayer system. We show that the magnitude of the interface contribution to both effects can be significantly reduced by the irradiation with ${\mathrm{Ga}}^{+}$ ions. This leads to a reduction by a factor of 2 of the domain wall energy density, while still preserving the Néel character of the domain walls. Hence, we postulate that ${\mathrm{Ga}}^{+}$ ion irradiation is an effective way to locally reduce the energy barrier for skyrmion nucleation, providing a novel pathway for targeted skyrmion nucleation in racetrack type devices.

We consider the effects of weak quenched fermionic disorder on the quantum-phase transition between the Dirac semimetal and charge density wave (CDW) insulator in two spatial dimensions. The symmetry breaking transition is described by the Gross-Neveu-Yukawa (GNY) theory of Dirac fermions coupled to an Ising order parameter field. Treating the disorder using the replica method, we consider chemical potential, vector potential (gauge), and random mass disorders, which all arise from nonmagnetic charged impurities. We self-consistently account for the Landau damping of long-wavelength order-parameter fluctuations by using the nonperturbative RPA resummation of fermion loops and compute the renormalization-group (RG) flow to leading order in the disorder strength and $1/N$ ($N$ the number of Dirac fermion flavors). We find two fixed points, the clean GNY critical point, which is stable against weak disorder, and a dirty GNY multicritical point, at which the chemical potential disorder is finite and the other forms of disorder are irrelevant. We investigate the scaling of physical observables at this finite-disorder multicritical point which breaks Lorentz invariance and gives rise to distinct non-Fermi liquid behavior.

We study systems with energy bands in two dimensions, hosting higher order Van Hove singularities (HOVHS) in the presence of disorder, using standard diagrammatic techniques for impurity averaging. In the clean limit, such singularities cause power-law divergence in the density of states (DOS), and this is expected to strongly affect electronic correlation. In order to analyze the signatures of these singularities in disordered systems, we employ various Born approximations, culminating in the self-consistent (non) Born approximation. Although the divergence of the DOS is smeared, we find that the shape of the DOS, as characterized by the power-law tail and the universal ratio of prefactors, is retained slightly away from the singularity. This could help us to understand current and future experiments on materials that can be tuned to host HOVHS. The impurity-induced smearing is calculated and analyzed for several test cases of singularities. We also study the effects of impurities on electrical conductivity and determine the regimes where the quantitative features of the power-law DOS manifest in the conductivity.

SnTe materials are one of the most flexible material platforms for exploring the interplay of topology and different types of symmetry breaking. We study symmetry-protected topological states in SnTe nanowires in the presence of various combinations of Zeeman field, $s$-wave superconductivity and inversion-symmetry-breaking field. We uncover the origin of robust corner states and hinge states in the normal state. In the presence of superconductivity, we find inversion-symmetry-protected gapless bulk Majorana modes, which give rise to quantized thermal conductance in ballistic wires. By introducing an inversion-symmetry-breaking field, the bulk Majorana modes become gapped and topologically protected localized Majorana zero modes appear at the ends of the wire.

In the multiferroic tunnel junction (MFTJ) composed of ferromagnetic and ferroelectric materials, the tunneling electroresistance (TER) coexists with the tunneling magnetoresistance (TMR), making it an ideal platform for designing multifunctional electronic devices. Recently, the rapid development of van der Waals (vdW) materials opened up a new avenue of MFTJ due to their atomic thickness and significance in miniaturizing device sizes. Here by employing the nonequilibrium Green's function combined with density-functional theory, we systemically study the spin-dependent electronic transport properties of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ (FGT)/bilayer $\alpha -{\mathrm{In}}_2{\mathrm{Se}}_3$ (BIS)/FGT vdW MFTJs. We find that the MFTJ can form multiple nonvolatile resistance states by altering the polarization orientation of the ferroelectric barrier BIS and the magnetization alignment of the two ferromagnetic FGT electrodes, with a maximum TMR (TER) ratio up to $1.1\times {10}^7$% (744%). The TER ratio can be further increased to 1868% by using left and right symmetrical copper electrodes. More interestingly, the perfect spin filtering effect can be realized in our MFTJs and the spin current can be controlled by the sign of bias voltages, suggesting a promising route for spin valves that can flexibly manipulate spin currents. Our results demonstrate that giant TMR, large TER, as well as a tunable spin filter can coexist in one system, and that the feasible tunability of such kind of vdW MFTJs is beneficial in designing next-generation logic and memory devices.

We study the electronic phase diagram of fully relaxed moiré superlattices in the monolayer graphene on hexagonal boron nitrogen (MLG/BN) with varying twist angle ($\theta$) and high pressure. In systems with small $\theta$, the secondary gap (${\mathrm{\Delta }}_S^{\left(e\right)}$) on the electron side and the tertiary gap (${\mathrm{\Delta }}_T^{\left(h\right)}$) on the hole side are opened by the high pressure up to 8 GPa, and the pressure also greatly enhances the primary band gap (${\mathrm{\Delta }}_0$) at the charge neutrality point and the secondary gap (${\mathrm{\Delta }}_S^{\left(h\right)}$) on the hole side. Under a given high pressure, the gaps ${\mathrm{\Delta }}_T^{\left(h\right)}, {\mathrm{\Delta }}_S^{\left(e\right)}$, and ${\mathrm{\Delta }}_S^{\left(h\right)}$ become closed in succession with varying $\theta$ from $0^{\circ }$ to $2^{\circ }$. In contrast to the direct ${\mathrm{\Delta }}_0$ and ${\mathrm{\Delta }}_S^{\left(h\right)}$ at small $\theta , {\mathrm{\Delta }}_S^{\left(e\right)}$ only becomes direct for a range of medium pressure, and ${\mathrm{\Delta }}_T^{\left(h\right)}$ remains indirect. Compared with the local continuum Hamiltonian (${\widehat{H}}_c$), the effects of nonlocal terms and those beyond the first harmonics in the full Hamiltonian become important for the quantitative values of the gaps under high pressure, while the qualitative behavior of the gaps with increasing pressure can still be understood by the variations of parameters in ${\widehat{H}}_c$. The band isolation due to the pressure induced gaps leads to emergence or evolution of the van Hove singularities, the ballistic resistance peaks, and the peaks of the density of orbital magnetic moments, which may be observed experimentally.

It was proposed by Chen et al., [Phys. Rev. Lett. 114, 145301 (2015)] that spin-2 chains display an extended critical phase with enhanced $\mathrm{SU}\left(3\right)$ symmetry. This hypothesis is highly unexpected for a spin-2 system and, as we argue, would imply an unconventional mechanism for symmetry emergence. Yet, the absence of convenient critical points for renormalization group perturbative expansions, allied with the usual difficulty in the convergence of numerical methods in critical or small-gapped phases, renders the verification of this hypothetical $\mathrm{SU}\left(3\right)$-symmetric phase a non-trivial matter. By tracing parallels with the well-understood phase diagram of spin-1 chains and searching for signatures robust against finite-size effects, we draw criticism on the existence of this phase. We perform non-Abelian density matrix renormalization group studies of the multipolar static correlation function, energy spectrum scaling, single-mode approximation, and entanglement spectrum to shed light on the problem. We determine that the hypothetical $\mathrm{SU}\left(3\right)$ spin-2 phase is, in fact, dominated by ferro-octupolar correlations and also observe a lack of Luttinger-liquid-like behavior in correlation functions that suggests that is perhaps not critical. We further construct an infinite family of spin-$S$ systems with similar ferro-octupolar-dominated quasi-$\mathrm{SU}\left(3\right)$-like phenomenology; curiously, we note that the spin-3 version of the problem is located in a subspace of exact ${\mathrm{G}}_2$ symmetry, making this a point of interest for search of Fibonacci topological properties in magnetic systems.

We study the equilibrium and nonequilibrium electronic transport properties of multiprobe topological systems using a combination of the Landauer-Büttiker approach and nonequilibrium Green's functions techniques. We obtain general expressions for both nonequilibrium and equilibrium local electronic currents that, by suitable projections, allow one to compute charge, spin, valley, and orbital currents. We show that external magnetic fields give rise to equilibrium charge currents in mesoscopic system and study the latter in the quantum Hall regime. Likewise, a spin-orbit interaction leads to local equilibrium spin currents, that we analyze in the quantum spin Hall regime. We show that an accurate theoretical assessment of the equilibrium currents is quite challenging and propose a two-measurement protocol that facilitates a comparison between experiment and theory.

The electronic state of ${\mathrm{WTe}}_2$, a candidate of type-II Weyl semimetal, is investigated by using laser-based spin- and angle-resolved photoemission spectroscopy. We prepare the pair of ${\mathrm{WTe}}_2$ samples, one with a (001) surface and the other with a ($00\overline{1}$) surface, by the “sandwich” method”, and we measure the band structures of each surface separately. The Fermi arcs are observed on both surfaces. We identify that the Fermi arcs on the two surfaces both originate from surface states. We further find a surface resonance band, which connects with the Fermi-arc band, forming a Dirac-cone-like band dispersion. Our results indicate that the bulk electron and hole bands are much closer in momentum space than band calculations.

Emergent $U\left(1\right)$ gauge theories and artificial photons in frustrated magnets are outstanding examples of many-body collective phenomena. The classical and quantum regimes of these systems provide platforms for classical and quantum spin liquids, and are the subject of current active theoretical and experimental investigations. Recently, realizations of rank-2 $U\left(1\right)$ (R2-U1) gauge theories in three-dimensional frustrated magnets have been proposed. Such systems in the quantum regime may lead to the so-called fracton ordered phases—a new class of topological order that has been associated with quantum stabilizer codes and holography. However, there exist few distinguishing characteristics of these states for their detection in real materials. Here we focus on the classical limit and present the dynamical spin structure factor for a R2-U1 spin liquid state on a breathing pyrochlore lattice. Remarkably, we find unique signatures of the R2-U1 state, and we contrast them with the results obtained from a more conventional $U\left(1\right)$ spin liquid. These results provide a path of investigation for future inelastic neutron scattering experiments on candidate materials.

The temperature induced neutral to ionic phase transition (TI-NIT) is a rare phenomenon occurring in mixed stack charge transfer (CT) crystals made up of alternating $\pi$-electron donor (D) and acceptor (A) molecules. We were able to grow crystals of tetrathiafulvalene-fluoranil (TTF-FA), and to show that it undergoes TI-NIT like the prototype CT crystal TTF-chloranil. We characterized both room and low-T phases through IR and Raman spectroscopy and x-ray diffraction, demonstrating that while TTF-FA is quasineutral at room temperature, its ionicity jumps from 0.15 to 0.7 at low T, therefore crossing the neutral-ionic borderline. The transition, occurring around 150 K, is first order, with large thermal hysteresis and accompanied by crystal cracking. In the high-T phase D and A molecules lie on inversion center, i.e., the stacks are regular, whereas the low-T phase is characterized by the loss of the inversion symmetry along the stack as the stacks are strongly dimerized and by the doubling of the unit cell.

In the ideal disorder-free situation, a two-dimensional band-gap insulator has an activation energy for conductivity equal to half the band gap $\mathrm{\Delta }$. But transport experiments usually exhibit a much smaller activation energy at low temperature, and the relation between this activation energy and $\mathrm{\Delta }$ is unclear. Here we consider the temperature-dependent conductivity of a two-dimensional insulator on a substrate containing Coulomb impurities, with random potential amplitude $\mathrm{\Gamma }\gg \mathrm{\Delta }$. We show that the conductivity generically exhibits three regimes of conductivity, and only the highest-temperature regime exhibits an activation energy that reflects the band gap. At lower temperatures, the conduction proceeds through activated hopping or Efros-Shklovskii variable-range hopping between electron and hole puddles created by the disorder. We show that the activation energy and characteristic temperature associated with these processes steeply collapse near a critical impurity concentration. Larger concentrations lead to an exponentially small activation energy and exponentially long localization length, which in mesoscopic samples can appear as a disorder-induced insulator-to-metal transition. We also arrive at a similar steep disorder driven insulator-metal transition in thin films of three-dimensional topological insulators with large dielectric constant, for which Coulomb impurities inside the film create a large disorder potential due to confinement of their electric field inside the film.

We employ singular value decomposition (SVD) to study the eigenvalue spectra of random spin systems. By SVD, eigenvalue spectrum is decomposed into orthonormal modes $W_k$ with weight ${\lambda }_k$. We show that the scree plot (${\lambda }_k$ with respect to $k$) in the ergodic phase contains two branches that both follow power law ${\lambda }_k\sim k^{-\alpha }$ but with different exponents $\alpha$. By evaluating $W_k$, we verified the part of ${\lambda }_k$ with $k>k_{\text{Th}}$ is universal that follows random matrix theory, where $k_{\text{Th}}$ is related to the Thouless energy. We further demonstrate that $\alpha$ corresponds only to the exponential part of the level spacing distribution while being insensitive to the level repulsion, or equivalently the system's symmetry. Consequently, $\alpha$ gives an underestimation for the many-body localization transition point, which suggests a nonergodic behavior that may be attributed to the Griffiths regime.

In this paper, we investigate by ab initio DFT how the O:H ratio influences the formation and lattice energy, metastability, and optical properties of Y and La anion-disordered $R{\mathrm{O}}_{\mathrm{x}}{\mathrm{H}}_{3-2\mathrm{x}}$ oxyhydrides. To achieve this, a set of special quasirandom structures (SQS) is introduced to model anion-disorder along the whole ${R\mathrm{H}}_3-R_2{\mathrm{O}}_3$ composition line. A comparison with an extensive set of anion-ordered polymorphs of the same composition shows the comparable energy of the anion-disordered phase, which, in particular, in the H-rich composition interval showed the lowest relative energy. In turn, the metastability of the anion-disordered phase depends on the cation size (Y versus La), which determines the maximum H content above which the ${\mathrm{CaF}}_2$-type structure itself becomes unstable. To overcome the accuracy limitations of classical DFT, the modified Becke-Johnson (mBJ) scheme is employed in the study of the electronic properties. We show that major differences occur between H-rich and O-rich R oxyhydrides, as the octahedral ${\mathrm{H}}^{-}$ present for $x<1$ form electronic states at the top of the valence band, which reduce the energy band gap and dominate the electronic transitions at lower energies, thus increasing the refractive index of the material in the VIS-nIR spectral range. Comparing the DFT results to experimental data on photochromic Y oxyhydride films reinforces the hypothesis of anion-disorder in the H-rich films ($x<1$), while it hints towards some degree of anion ordering in the O-rich ones ($x>1$). Our paper exemplifies a strategy to calculate ab initio the electronic/optical properties of a wide range of materials with occupational disorder.

Burton-Cabrera-Frank (BCF) theory has proven to be a versatile framework to relate surface morphology and dynamics during crystal growth to the underlying mechanisms of adatom diffusion and attachment at steps. For an important class of crystal surfaces, including the basal planes of hexagonal close-packed and related systems, the steps in a sequence on a vicinal surface can exhibit properties that alternate from step to step. Here we develop BCF theory for such surfaces, relating observables such as alternating terrace widths as a function of growth conditions to the kinetic coefficients for adatom attachment at steps. We include the effects of step transparency and step-step repulsion. A general solution is obtained for the dynamics of the terrace widths, assuming quasi-steady-state adatom distributions on the terraces. An explicit simplified analytical solution is obtained under widely applicable approximations. From this we obtain expressions for the full-steady-state terrace fraction as a function of growth rate. Fits of the theoretical predictions to recent experimental determinations of the steady state and dynamics of terrace fractions on GaN (0001) surfaces during organometallic vapor phase epitaxy give values of the kinetic coefficients for this system. In Appendixes, we also connect a model for diffusion between kinks on steps to the model for diffusion between steps on terraces, which quantitatively relates step transparency to the kinetics of atom attachment at kinks, and consider limiting cases of diffusion-limited, attachment-limited, and mixed kinetics.

Secondary β relaxation is considered a key issue in glassy materials, while its structural origin is still not well understood. In this paper, we report pronounced and unpronounced β-relaxation behaviors in ${\mathrm{Y}}_{60}{\mathrm{Ni}}_{16}{\mathrm{Al}}_{24}$ and ${\mathrm{Y}}_{60}{\mathrm{Fe}}_{16}{\mathrm{Al}}_{24}$ metallic glasses (MGs) obtained from dynamical mechanical analyses, respectively, despite the same composition and similar atomic sizes of Ni and Fe atoms. We elucidate that the β relaxation in ${\mathrm{Y}}_{60}{\mathrm{Ni}}_{16}{\mathrm{Al}}_{24}$ and ${\mathrm{Y}}_{60}{\mathrm{Fe}}_{16}{\mathrm{Al}}_{24}$ MGs mainly depends on the vibration of small Ni and Fe atoms using synchrotron x-ray techniques and theoretical calculations, respectively. From the x-ray absorption fine structure and Voronoi tessellation statistics, it is found that the dominant local structure of mobile atoms changes from $\langle 0,3,6,0\rangle$ centered by Ni atoms into $\langle 0,2,8,0\rangle$ centered by Fe atoms. More large Y atoms in shells promote the local vibration of center Ni atoms. In contrast, with more Y atoms replaced by Fe atoms, the vibration of center Fe atoms gets significantly slowed down, thus leading to an unpronounced β relaxation. Our results indicate that the atomic dynamics of mobile atoms in MGs are not only related to their surrounding local geometry but also chemical constitutions, i.e., the short-range order. In this paper, we provide an idea for understanding the structural origin of β-relaxation behaviors in MGs and other glassy materials from their local atomic environments and dynamics point of view.