By using optical absorption and Raman spectroscopy, we have investigated the effects of pressure in the structural, electronic, and vibrational properties of ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ in the 0–20 GPa range. Our study has provided a complete characterization of the electronic and vibrational structures of ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ in the low-pressure Pnma phase as a function of cell volume and the Cu-Cl bond length, $R_{\text{Cu-Cl}}$. We have focused on the electronic structure in the Pnma phase, which is mainly related to the tetrahedral ${\mathrm{CuCl}}_4^{2-}$ units distorted by the Jahn-Teller effect. The strong piezochromism of ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ is caused by the opposite shift of the charge-transfer band gap and the ${\mathrm{Cu}}^{2+}$ d-d crystal-field transitions with pressure. The high-pressure structure of ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ above 4.9 GPa yields structural transformations that we ascribe to a change of coordination around ${\mathrm{Cu}}^{2+}$. The high-pressure phase appears largely amorphized, therefore any structural information from x-ray diffraction is ruled out. Here we use electronic and vibrational probes to get inside the structural information.

We find the charge and heat currents caused by a temperature difference applied to a superconducting point contact or to a quantum point contact between a superconducting and normal conductors. The results are formulated in terms of the properties of the electron scattering matrix of the quantum point contact in its normal state, and are valid at any transmission coefficient. In the low-transmission limit, the theory provides reliable results, setting the limits for the use of the popular method of the tunneling Hamiltonian.

The effects of current induced Néel spin-orbit torques on the antiferromagnetic domain structure of epitaxial ${\mathrm{Mn}}_2\mathrm{Au}$ thin films were investigated by x-ray magnetic linear dichroism–photoemission electron microscopy. We observed current induced switching of antiferromagnetic domains essentially corresponding to morphological features of the samples. Reversible as well as irreversible Néel vector reorientation was obtained in different parts of the samples and the switching of up to 30% of all domains in the field of view of 10 $\mu \mathrm{m}$ is demonstrated. Our direct microscopical observations are compared to and fully consistent with anisotropic magnetoresistance effects previously attributed to current induced Néel vector switching in ${\mathrm{Mn}}_2\mathrm{Au}$.

Hydrodynamic flow in two-dimensional electron systems has so far been probed only by dc transport and scanning gate microscopy measurements. In this work we discuss theoretically signatures of the hydrodynamic regime in near-field optical microscopy. We analyze the dispersion of acoustic plasmon modes in two-dimensional electron liquids using a nonlocal conductivity that takes into account the effects of (momentum-conserving) electron-electron collisions, (momentum-relaxing) electron-phonon and electron-impurity collisions, and many-body interactions beyond the celebrated random phase approximation. We derive the dispersion and, most importantly, the damping of acoustic plasmon modes and their coupling to a near-field probe, identifying key experimental signatures of the crossover between collisionless and hydrodynamic regimes.

Superfluidity has recently been reported in double electron-hole bilayer graphene. The multiband nature of the bilayers is important because of the very small band gaps between conduction and valence bands. The long-range nature of the superfluid pairing interaction means that screening must be fully taken into account. We have carried out a systematic mean-field investigation that includes (i) contributions to screening from both intraband and interband excitations, (ii) the low-energy band structure of bilayer graphene with its small band gap and flattened Mexican-hat-like low-energy bands, (iii) the large density of states at the bottom of the bands, (iv) electron-hole pairing in the multibands, and (v) electron-hole pair transfers between the conduction and valence band condensates. We find that the superfluidity strongly modifies the intraband contributions to the screening, but that the interband contributions are unaffected. Unexpectedly, a net effect of the screening is to suppress Josephson-like pair transfers and to confine the superfluid pairing entirely to the conduction-band condensate even for very small band gaps, making the system behave similarly to a one-band superfluid.

Matrix product states (MPSs), a tensor network designed for one-dimensional quantum systems, were recently proposed for generative modeling of natural data (such as images) in terms of the “Born machine.” However, the exponential decay of correlation in MPSs restricts its representation power heavily for modeling complex data such as natural images. In this work, we push forward the effort of applying tensor networks to machine learning by employing the tree tensor network (TTN), which exhibits balanced performance in expressibility and efficient training and sampling. We design the tree tensor network to utilize the two-dimensional prior of the natural images and develop sweeping learning and sampling algorithms which can be efficiently implemented utilizing graphical processing units. We apply our model to random binary patterns and the binary MNIST data sets of handwritten digits. We show that the TTN is superior to MPSs for generative modeling in keeping the correlation of pixels in natural images, as well as giving better log-likelihood scores in standard data sets of handwritten digits. We also compare its performance with state-of-the-art generative models such as variational autoencoders, restricted Boltzmann machines, and PixelCNN. Finally, we discuss the future development of tensor network states in machine learning problems.

We report systematic temperature- and doping-dependent measurements of the $\mathrm{Fe}3s$ core-level photoemission spectra in the normal state of superconducting $\mathrm{Sr}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$. The analysis of the $\mathrm{Fe}3s$ spectrum provides an element-specific determination of the mean value of the magnitude of the Fe spin moment measured on the fast $({10}^{-16}-{10}^{-15}\mathrm{s})$ timescale of the photoemission process. The data reveal the ubiquitous presence in the normal state of Fe spin moments with magnitude fluctuating on short timescales. The data reveal a significant reduction of the magnitude of the effective Fe spin moment on going from the parent to the optimal doped compound. The doping dependence of the magnitude of the spin moment at higher doping level is less clear, being either constant, or even nonmonotonic, depending on temperature. This phenomenology indicates the importance of the interaction between spin and itinerant degrees of freedom in shaping the properties of the normal state. These findings reaffirm the complexity of the normal state of 122 Fe-pnictides, which are typically viewed as the least correlated of the high-temperature unconventional superconductors.

We study the transport properties of a Wigner crystal in one- and two-dimensional asymmetric periodic potentials. We show that the Aubry transition takes place above a certain critical amplitude of potential with the sliding and pinned phase below and above the transition. Due to the asymmetry the Aubry pinned phase is characterized by the diode charge transport of the Wigner crystal. We argue that the recent experimental observations of Aubry transition with cold ions and colloidal monolayers can be extended to asymmetric potentials making it possible to observe a Wigner crystal diode with these physical systems and electrons on liquid helium.

Motivated by recent transport measurements in high-$T_c$ cuprate superconductors in a magnetic field, we study the thermal Hall conductivity in materials with topological order, focusing on the contribution from neutral spinons. Specifically, different Schwinger boson mean-field Ansätze for the Heisenberg antiferromagnet on the square lattice are analyzed. We allow for both Dzyaloshinskii-Moriya interactions, and additional terms associated with scalar spin chiralities that break time-reversal and reflection symmetries, but preserve their product. It is shown that these scalar spin chiralities, which can either arise spontaneously or are induced by the orbital coupling of the magnetic field, can lead to spinon bands with nontrivial Chern numbers and significantly enhanced thermal Hall conductivity. Associated states with zero-temperature magnetic order, which is thermally fluctuating at any $T>0$, also show a similarly enhanced thermal Hall conductivity.

Using a combination of density functional theory calculations and molecular dynamics simulations we show that the strength and toughness of hexagonal boron nitride (hBN) containing isolated vacancy defects are strongly anisotropic, regardless of the size of the defect core. The degree of anisotropy is preserved for a number of defect structures including monovacancy, tetravacancy, tridecavacancy, triheptacontavacancy, or heptatriacontavacancy defects. The chirality-dependent effects are strongly nonlinear and well characterized by close-form mathematical equations indicating pronounced strength and toughness along the zigzag direction compared to the strength and toughness along the armchair direction. Also, the size dependence of the strength and toughness of the defective lattice shows an inverse relationship with the effective diameter of the defect core. An atomistic analysis of the deformation fields reveals that nonuniformity in bond length, bond strain, and force distribution in the nonlinear regime of mechanical deformation surrounding the defect cores forms the physical basis for the observed anisotropy. The anisotropic character of the lattice is governed primarily by the nearest-neighbor covalent interactions (dominated by the first-nearest neighbors). Consequently, as soon as a set of bonds rupture at the defect core, the entire lattice undergoes catastrophic failure underscoring the brittle nature of the fracture state in hBN. Results also suggest that chirality-dependent elastic effects are dominated by the third-order elastic modulus which stiffens the lattice at higher chiral angles, whereas the second- and fourth-order elastic moduli soften the lattice affecting the strength and toughness of the lattice in an intricate manner.

We study the uniform time-reversal-invariant $U\left(1\right)$ quantum spin liquid (QSL) with low-energy fermionic quasiparticles for rare-earth pyrochlore magnets and explore its magnetic instability, employing an augmented fermionic parton mean field theory approach. Self-consistent calculations stabilize a uniform $U\left(1\right)$ QSL with both gapped and gapless parton excitations as well as fractionalized magnetically ordered phases in an experimentally relevant part of the phase diagram near the classical phase boundaries of the magnetically ordered phases. The gapped QSL has a band structure with a nonzero $Z_2$ topological invariant. The fractionalized magnetically ordered phases bear signatures of both QSL through fermionic excitations as well as magnetic order. Thus, this provides a possible way to understand the unconventional diffuse neutron scattering in rare-earth pyrochlores such as ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, ${\mathrm{Er}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, and ${\mathrm{Er}}_2{\mathrm{Pt}}_2{\mathrm{O}}_7$ at low or zero external magnetic fields. We calculate the dynamic spin structure factor to understand the nature of the diffuse two-particle continuum.

Recent experiments on twisted bilayer graphene (tBG) close to magic angle show that a small relative rotation in a van der Waals heterostructure greatly alters its electronic properties. We consider various scattering mechanisms and show that the carrier transport in tBG is dominated by a combination of charged impurities and acoustic gauge phonons. Charged impurities still dominate at low temperature and densities because of the inability of Dirac fermions to screen long-range Coulomb potentials at charge neutrality; however, the gauge phonons dominate for most of the experimental regime because, although they couple to current, they do not induce charge and are therefore unscreened by the large density of states close to magic angle. We show that the resistivity has a strong monotonically decreasing carrier density dependence at low temperature due to charged impurity scattering, and weak density dependence at high temperature due to gauge phonons. Away from charge neutrality, the resistivity increases with temperature, while it does the opposite close to the Dirac point. A nonmonotonic temperature dependence observed only at low temperature and carrier density is a signature of our theory that can be tested in experimentally available samples.

A fully dynamic theoretical approach to layered optomagnonic structures, based on a time Floquet scattering-matrix method, is developed. Its applicability is demonstrated on a simple design of a dual photonic-magnonic cavity, formed by sandwiching a magnetic garnet thin film between two dielectric Bragg mirrors, subject to continuous excitation of a perpendicular standing spin wave. Some remarkable phenomena, including nonlinear photon-magnon interaction effects and enhanced inelastic light scattering in the strong-coupling regime, fulfilling a triple-resonance condition, are analyzed and the limitations of the quasistatic adiabatic approximation are established.