The magnetic and magnetotransport properties of metallic $1T\text{−}{\mathrm{VTe}}_2$ single crystals were investigated at temperatures from 1.3 to 300 K and in magnetic fields up to 35 T. Upon applying a high magnetic field, it is found that the electrical resistivity displays a crossover from the logarithmic divergence of the single-impurity Kondo effect to the Fermi liquid behavior at low temperatures. The Brillouin scale of the negative magnetoresistivity above the Kondo temperature $T_{\mathrm{K}}$ = 12 K indicates that the Kondo features originate from intercalated V ions, with $S$ = 1/2. Both magnetic susceptibility and Hall effect show an anomaly around $T_{\mathrm{K}}$. By using the modified Hamann expression we successfully describe the temperature-dependent resistivity under various magnetic fields, which shows the characteristic peak below $T_{\mathrm{K}}$ due to the splitting of the Kondo resonance.

Dynamical phase transition (DPT) characterizes the abrupt change in dynamical properties in nonequilibrium quantum many-body systems. It has been demonstrated that extra fluctuating modes besides the conventional order parameter field can bring about the Landau-forbidden quantum critical points by changing the order of the equilibrium phase transition. However, the counterpart phenomena in DPTs have rarely been explored. Here, we study the DPT in the Dirac system after a sudden quench and find that the fermion fluctuations can round a putative first-order DPT into a dynamical critical point, which is referred to as a fermion-induced dynamical critical point (FIDCP). This FIDCP gives the universal short-time scaling behavior controlled by the dynamical chiral fixed point despite the system going through a first-order transition after thermalization. In the novel scenario of FIDCP, the quantum Yukawa coupling $g_q$ is indispensable for inducing the FIDCP even though it is irrelevant in the infrared scale. We call these variables dynamical dangerously irrelevant scaling variable. Moreover, a dynamical tricritical point which separates the first-order DPT and the FIDCP is discovered by tuning this dynamical dangerously irrelevant scaling variable. We further mention possible experimental realizations.

Tensor networks, a model that originated from quantum physics, has been gradually generalized as efficient models in machine learning in recent years. However, in order to achieve exact contraction, only treelike tensor networks such as the matrix product states and tree tensor networks have been considered, even for modeling two-dimensional data such as images. In this work, we construct supervised learning models for images using the projected entangled pair states (PEPS), a two-dimensional tensor network having a similar structure prior to natural images. Our approach first performs a feature map, which transforms the image data to a product state on a grid, then contracts the product state to a PEPS with trainable parameters to predict image labels. The tensor elements of PEPS are trained by minimizing differences between training labels and predicted labels. The proposed model is evaluated on image classifications using the Modified National Institute of Standards and Technology database (MNIST) and the Fashion-MNIST datasets. We show that our model is significantly superior to existing models using treelike tensor networks. Moreover, using the same input features, our method performs as well as the multilayer perceptron classifier, but with much fewer parameters and is more stable. Our results shed light on potential applications of two-dimensional tensor network models in machine learning.

The theoretical understanding of plasmon behavior is crucial for an accurate interpretation of inelastic scattering diagnostics in many experiments. We highlight the utility of linear response time-dependent density functional theory (LR-TDDFT) as a first-principles framework for consistently modeling plasmon properties. We provide a comprehensive analysis of plasmons in aluminum from ambient to warm dense matter conditions and assess typical properties such as the dynamical structure factor, the plasmon dispersion, and the plasmon lifetime. We compare our results with scattering measurements and with other TDDFT results as well as models such as the random phase approximation, the Mermin approach, and the dielectric function obtained using static local field corrections of the uniform electron gas parametrized from path-integral Monte Carlo simulations. We conclude that results for the plasmon dispersion and lifetime are inconsistent between experiment and theories and that the common practice of extracting and studying plasmon dispersion relations is an insufficient procedure to capture the complicated physics contained in the dynamic structure factor in its full breadth.

The layered cobaltite ${\left[{\mathrm{Ca}}_2{\mathrm{CoO}}_3\right]}_{0.62}\left[{\mathrm{CoO}}_2\right]$, often expressed as the approximate formula ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$, is a promising candidate for efficient oxide thermoelectrics, but an origin of its unusual thermoelectric transport is still in debate. Here we investigate in-plane anisotropy of the transport properties in a broad temperature range to examine the detailed conduction mechanism. The in-plane anisotropy between $a$ and $b$ axes is clearly observed both in the resistivity and the thermopower, which is qualitatively understood with a simple band structure of the triangular lattice of Co ions derived from the angle-resolved photoemission spectroscopy experiments. On the other hand, at high temperatures the anisotropy becomes smaller and the resistivity shows a temperature-independent behavior, both of which indicate a hopping conduction of localized carriers. Thus, the present observations reveal a crossover from low-temperature itinerant to high-temperature localized states, signifying both characters for the enhanced thermopower.

It is well known that second harmonic generation (SHG) from dipoles within the bulk vanishes in inversion-symmetric semiconductor materials as a consequence of parity symmetry. Hence SHG is then ascribed in the form of either surface dipole effects or bulk-related electric quadrupole or magnetic dipole effects. By incorporating the redefined spatial dispersion into the simplified bond-hyperpolarizability model, we show that the SHG spatial dispersion contribution for Si(001) and Si(111) facet orientations can be reformulated by a third-rank tensor containing one independent parameter, namely, the complex SHG spatial dispersion hyperpolarizability. Our results show that certain unexplained rotational anisotropy SHG intensity features for different incoming light wavelengths or optical penetration depths can be well reproduced only if the contribution from spatial dispersion is incorporated.

The relative stiffnesses and strengths of hexagonal diamond and cubic diamond constitute long-standing unresolved issues, because relevant experimental results are lacking. Laser interferometry was used to experimentally determine the longitudinal sound speeds and moduli in hexagonal diamond and cubic diamond formed during the shock compression of graphite. The hexagonal diamond longitudinal moduli are significantly larger than the cubic diamond longitudinal moduli, and even exceed averaged cubic diamond single crystal values. The measured hexagonal diamond longitudinal moduli, combined with high-pressure bulk moduli for cubic diamond single crystals, show that shock-formed hexagonal diamond shear moduli are larger than the shear moduli for cubic diamond single crystals.

Time crystalline structures are characterized by regularity that single-particle or many-body systems manifest in the time domain, closely resembling the spatial regularity of ordinary space crystals. Here we show that time and space crystalline structures can be combined together and even six-dimensional time-space lattices can be realized. As an example, we demonstrate that such time-space crystalline structures can reveal the six-dimensional quantum Hall effect quantified by the third Chern number.

We present a microscopic study of a doped quantum spin liquid candidate, the hyperkagome ${\mathrm{Na}}_3{\mathrm{Ir}}_3{\mathrm{O}}_8$ compound, by using ${}^{23}\mathrm{Na}$ NMR. We determine the intrinsic behavior of the uniform q = 0 susceptibility via shift measurements and the dynamical response by probing the spin-lattice relaxation rate. Throughout the studied temperature range, the susceptibility is consistent with a semimetal behavior, though with electronic bands substantially modified by correlations. Remarkably, the antiferromagnetic fluctuations present in the insulating parent compound ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$ survive in the studied compound. The spin dynamics are consistent with ${120}^{\circ }$ excitation modes displaying short-range correlations.

Alkaline earth metal peroxides are typical examples of ionic compounds containing polyanions. We herein report a stable $\mathrm{Ba}{\mathrm{O}}_2$ phase at high pressure up to 130 GPa found via a first-principles computational structure search and high-pressure experimental investigations. The identified monoclinic structure (space group $C2/m$) can be derived by sublattice distortions of Ba atoms and peroxide groups associated with the phonon mode softening of the lower-pressure $Cmmm$ structure. Contrary to the previous expectation of polymerization of the peroxide group at elevated pressure, this phase retains the peroxide group and, interestingly, exhibits an insulating behavior demonstrating an increase of the band gap under compression. Our synchrotron x-ray diffraction (XRD) measurements could not distinguish between $Cmmm$ and $C2/m\mathrm{Ba}{\mathrm{O}}_2$ definitively because the difference in XRD patterns is very subtle. However, our data do not show any sign of polymerization transition up to 120 GPa. Raman spectra of the O-O peroxide vibration show a small anomaly in frequency at 110 GPa, which is qualitatively like that predicted theoretically due to the $Cmmm$ to $C2/m$ phase transition, thus supporting the predicted transformation.

We present a Green's function approach to calculate the Dzyaloshinskii-Moriya interactions (DMIs) from first-principles electronic structure calculations that is computationally more efficient and accurate than the most commonly employed supercell and generalized Bloch-based approaches. The method is applied to the (111) Co/Pt bilayer where the Co and/or Pt thickness dependence of the DMI coefficients is calculated. Overall, the calculated DMIs are in relatively good agreement with the corresponding values reported experimentally. Furthermore, we investigate the effect of strain in the DMI tensor elements and show that the isotropic Néel DMI can be significantly modulated by the normal strains ${\epsilon }_{xx}$ and ${\epsilon }_{yy}$ and is relatively insensitive to the shear strain ${\epsilon }_{xy}$. Moreover, we show that anisotropic strains, $({\epsilon }_{xx}-{\epsilon }_{yy})$ and ${\epsilon }_{xy}$, result in the emergence of anisotropic Néel- and Bloch-type DMIs, respectively.

The magnetic structure is usually determined by the neutron diffraction measurements. However, in the case of complex multisublattice magnetics, this method fails to give an unambiguous result. Here, on the example of multiferroic ${\mathrm{HoFe}}_3{\left({\mathrm{BO}}_3\right)}_4$, we show that in the case of rare-earth (RE) compounds the right magnetic structure can be determined by additionally using optical spectroscopy and a theoretical analysis based on spectroscopic data. ${\mathrm{HoFe}}_3{\left({\mathrm{BO}}_3\right)}_4$ demonstrates a series of phase transitions and interesting magnetic and magnetoelectric properties. The available information on the magnetic structure of the compound, necessary for understanding and utilizing these properties, is contradictory. To resolve the existing ambiguities, we apply a combined approach. The high-resolution spectroscopy data deliver a set of the ${\mathrm{Ho}}^{3+}$ crystal-field (CF) levels in the paramagnetic and both easy-plane and easy-axis magnetic phases. These data are used to determine CF and ${\mathrm{Ho}}^{3+}\text{−}{\mathrm{Fe}}^{3+}$ exchange parameters and, then, to calculate the temperature dependencies of the magnetic susceptibility tensor of ${\mathrm{HoFe}}_3{\left({\mathrm{BO}}_3\right)}_4$. Based on these calculations, we suggest an easy-plane antiferromagnetic structure with a collinear arrangement of the Fe spins along the $a$ axis and induced noncolinear moments of magnetically nonequivalent Ho ions. The suggested structure is further confirmed by single-crystal elastic neutron scattering experiments. We argue that specific features of the magnetic properties of RE iron borates isostructural to ${\mathrm{HoFe}}_3{\left({\mathrm{BO}}_3\right)}_4$ are governed by the energy patterns and the symmetry properties of the wave functions of the lower CF levels of the RE ground multiplet in the crystal field of the $C_2$ symmetry.

We provide and analyze a periodic Anderson model for studying magnetism and superconductivity in $\mathrm{U}{\mathrm{Te}}_2$, a recently discovered candidate for a topological spin-triplet superconductor. The 24-band tight-binding model reproduces the band structure obtained from a $\mathrm{D}\mathrm{F}\mathrm{T}+U$ calculation consistent with an angle-resolved photoemission spectroscopy. The Coulomb interaction of $f$-electrons enhances Ising ferromagnetic fluctuation along the $a$-axis and stabilizes spin-triplet superconductivity of either $B_{3u}$ or $A_u$ symmetry. When effects of pressure are taken into account in hopping integrals, the magnetic fluctuation changes to an antiferromagnetic one, and accordingly spin-singlet superconductivity of $A_g$ symmetry is stabilized. Based on the results, we propose pressure-temperature and magnetic field-temperature phase diagrams revealing multiple superconducting phases as well as an antiferromagnetic phase. In particular, a mixed-parity superconducting state with spontaneous inversion symmetry breaking is predicted.

Different from spin-$\frac{1}{2}$ Weyl points which are robust due to the protection of topology, the unconventional chiral quasiparticles usually require extra crystalline symmetries for their existence, indicating that such quasiparticles are sensitive to perturbation. Herein, we present that the spin-1 Weyl can transform into quadratic Weyl phonons depending on symmetry variation. Specifically, the spin-1 Weyl nodes arisen from three-dimensional (3D) irreducible representations (IRs) of chiral point groups, $O$(432) or $T$(23), are verified to split into quadratic Weyl points if symmetry breaking decomposes 3D IRs into two-dimensional IRs. Symmetry analysis and low-energy effective models are performed to identify the splitting mechanisms. The evolution of Berry curvature and surface states driven by symmetry breaking is obtained in real materials. Our work not only builds the connection between double Weyl phonons but also offers guidance for exploring the transition among unconventional quasiparticles.

$\mathrm{U}{\mathrm{Te}}_2$ is a likely spin-triplet superconductor that also exhibits evidence for chiral Majorana edge states. A characteristic structural feature of $\mathrm{U}{\mathrm{Te}}_2$ is inversion-symmetry related pairs of U atoms, forming rungs of ladders. Here we show how each rung's two sublattice degrees of freedom play a key role in understanding the electronic structure and the origin of superconductivity. In particular, $\mathrm{DFT}+U$ calculations generically reveal a topological band near the chemical potential originating from a band inversion associated with $5f$ electrons residing on these rungs, necessitating a microscopic description that includes these rung degrees of freedom. Furthermore, we show that a previously identified strong ferromagnetic interaction within a U-U rung leads to a pseudospin-triplet superconducting state that accounts for a nonzero polar Kerr angle, the observed magnetic field-temperature phase diagrams, and nodal Weyl fermions. Our analysis may also be relevant for other U-based superconductors.

The unusual electronic and optical properties of armchair and zigzag graphene nanoribbons (GNRs) subject to in-plane transverse electric and perpendicular magnetic fields have been systematically investigated. Our calculations were carried out within the generalized multiorbital tight-binding model based on a Hamiltonian which takes into account hopping integrals among the ($s,p_x,p_y$, and $p_z$) atomic orbitals as well as the external electric and magnetic fields. The electronic structure consists of $\pi$ bands arising from the $p_z$ orbital and $\sigma$ bands originating from the ($s,p_x$, and $p_y$) orbitals. The energy bands and optical spectra are diversified by both the nature of the edge of the nanoribbon and the strength of the external fields. Armchair GNRs display a width-dependent energy gap in addition to low-energy $\sigma$ bands whereas the zigzag system has the unfilled flatband with $\pi$ edge states at zero energy and partially filled wide-range $\sigma$ bands. An applied in-plane electric field leads to the splitting of energy bands and shifted Fermi level thereby enriching the interband and intraband optical conductivities. The interplay between an external magnetic field and the edge geometry gives rise to extraordinary quantized Landau levels and special optical spectra.

We investigate the optical property of monolayer and layered $\mathrm{Bi}{\mathrm{I}}_3$ and reveal the presence of excitons only in the monolayer crystal. We evaluate the energy spectrum of a dielectric function by using time-dependent density functional theory. Bulk crystal of $\mathrm{Bi}{\mathrm{I}}_3$ is an atomic layered semiconductor with the band gap corresponding to the frequency of visible light. The numerical result for the bulk crystal is confirmed to be consistent with the previous experimental results and does not depend on the number of layers except the monolayer. We reveal the excitons appearing below the resonant peak associated with the interband excitation in the monolayer crystal. The unique optical property can be directly observed in the optical absorption or differential reflectance spectrum and distinguish the monolayer crystal from the stacked $\mathrm{Bi}{\mathrm{I}}_3$.

Transition metal dichalcogenides (TMDs) combine interesting optical and spintronic properties in an atomically thin material, where the light polarization can be used to control the spin and valley degrees of freedom for the development of novel optospintronic devices. These promising properties emerge due to their large spin-orbit coupling in combination with their crystal symmetries. Here, we provide simple symmetry arguments in a group-theory approach to unveil the symmetry-allowed spin-scattering mechanisms, and indicate how one can use these concepts towards an external control of the spin lifetime. We perform this analysis for both monolayer (inversion asymmetric) and bilayer (inversion symmetric) crystals, indicating the different mechanisms that play a role in these systems. We show that in monolayer TMDs, electrons and holes transform fundamentally differently––leading to distinct spin-scattering processes. We find that one of the electronic states in the conduction band is partially protected by time-reversal symmetry, indicating a longer spin lifetime for that state. In bilayer and bulk TMDs, a hidden spin polarization can exist within each layer despite the presence of global inversion symmetry. We show that this feature enables control of the interlayer spin-flipping scattering processes via an out-of-plane electric field, providing a mechanism for electrical control of the spin lifetime.

Zero-bias charge conductance peak (ZBCCP) is a significant symbol of Majorana zero modes (MZMs). The proximity effect of $s$-wave superconductor is usually demanded in the fabrication of MZMs. So in transport experiments, the system is inevitably coupled to the $s$-wave superconductor. Here we study how the ZBCCP is affected by coupling of the $s$-wave superconductor. The results show that the ZBCCP could be changed into a zero-bias valley due to the coupling of $s$-wave superconductor, although the conductance at the zero bias still keeps a quantized value $2e^2/h$. So it does not mean no MZM exists when no ZBCCP is experimentally observed. In addition, the spin transport is investigated. Four reflection processes (the normal reflection, spin-flip reflection, normal Andreev reflection, and equal-spin Andreev reflection) usually occur. The reflection coefficients are strongly dependent on the spin direction of the incident electron, and they may be symmetrical or Fano resonance shapes. But the spin conductance always shows a zero-bias peak with the height $e/2\pi$ regardless of the direction of the spin bias and the coupling strength of the $s$-wave superconductor. So measuring spin transport properties could be a more reliable method to judge the existence of MZMs.