Motivated by theory and experiments on strain-induced pseudo-Landau levels (LLs) of Dirac fermions in graphene and topological materials, we consider its extension for Bogoliubov quasiparticles (QPs) in a nodal superconductor (SC). We show, using an effective low-energy description and numerical lattice calculations for a $d$-wave SC, that a spatial variation of the electronic hopping amplitude or a spatially varying $s$-wave pairing component can act as a pseudomagnetic field for the Bogoliubov QPs, leading to the formation of pseudo-LLs. We propose realizations of this phenomenon in the cuprate SCs, via strain engineering in films or nanowires, or $s$-wave proximity coupling in the vicinity of a nematic instability, and discuss its signatures in tunneling experiments.

We have studied the C14 hexagonal Laves phase of ${\mathrm{YbAg}}_2$ at normal conditions and under external pressure up to 19 GPa by the time-differential perturbed angular $\gamma -\gamma$ correlation spectroscopy (TDPAC) using ${}^{111}\mathrm{Cd}$ probe nuclei. Under pressure the valence of Yb undergoes a two-stage transition from 2.8 to 3. The two-stage scenario is characterized by two distinct quadrupole frequencies of ${}^{111}\mathrm{Cd}$ probes in a silver sublattice, monotonically increasing with pressure and saturating at 8 and 16 GPa. Our experimental data are compared with the density functional studies of the electron band structure of ${\mathrm{YbAg}}_2$, whose results are used for discussion and interpretation of these experiments. We have found that there are two different electric field gradients at inequivalent silver sites and that $4d$ states of silver participate in metal bonding, allowing for the formation of the hexagonal Laves phase.

The initialization, control, and readout of a single-hole spin qubit is precisely simulated by accurately solving the extended Anderson impurity model in the real-time domain with the hierarchical equations of motion approach. The initialization is realized by ionization of an exciton with high fidelity. Then, a SU(2) control is achieved via the combination of Larmor precession of the hole spin in Voigt geometry magnetic field and rotation about the optical axis with a geometric phase. Finally, the readout of the qubit is implemented through photocurrent recording. Our theory not only reproduces the recent experimental results with one set of internal parameters but also predicts a maximal fidelity by adjusting the dot-electrode coupling strength.

A patchwork method is used to study the dynamics of loss and recovery of an initial configuration in spin glass models in dimensions $d=1$ and $d=2$. The patchwork heuristic is used to accelerate the dynamics to investigate how models might reproduce the remarkable memory effects seen in experiment. Starting from a ground-state configuration computed for one choice of nearest-neighbor spin couplings, the sample is aged up to a given scale under new random couplings, leading to the partial erasure of the original ground state. The couplings are then restored to the original choice and patchwork coarsening is again applied, in order to assess the recovery of the original state. Eventual recovery of the original ground state upon coarsening is seen in two-dimensional Ising spin glasses and one-dimensional clock models, while one-dimensional Ising spin systems neither lose nor gain overlap with the ground state during the recovery stage. The recovery for the two-dimensional Ising spin glasses suggests scaling relations that lead to a recovery length scale that grows as a power of the aging length scale.

Double-stripe magnetism $[\mathbf{Q}=(\pi /2,\pi /2\left)\right]$ has been proposed as the magnetic ground state for both the iron-telluride and ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$ families of superconductors. Double-stripe order is captured within a $J_1\text{−}{J}_2\text{−}{J}_3$ Heisenberg model in the regime $J_3\gg J_2\gg J_1$. Intriguingly, besides breaking spin-rotational symmetry, the ground-state manifold has three additional Ising degrees of freedom associated with bond ordering. Via their coupling to the lattice, they give rise to an orthorhombic distortion and to two nonuniform lattice distortions with wave vector $(\pi ,\pi )$. Because the ground state is fourfold degenerate, modulo rotations in spin space, only two of these Ising bond order parameters are independent. Here, we introduce an effective field theory to treat all Ising order parameters, as well as magnetic order, and solve it within a large-$N$ limit. All three transitions, corresponding to the condensations of two Ising bond order parameters and one magnetic order parameter are simultaneous and first order in three dimensions, but lower dimensionality, or equivalently weaker interlayer coupling, and weaker magnetoelastic coupling can split the three transitions, and in some cases allows for two separate Ising phase transitions above the magnetic one.

The $A$-site cation-ordered spinel $\mathrm{LiFeC}{\mathrm{r}}_4{\mathrm{O}}_8$ is a unique system which manifests properties of the frustrated pyrochlore lattice, including the breathing distortion and antiferromagnetic coupling between $A$ and $B$ sites. This compound crystallizes in a cubic spinel structure with a noncentrosymmetric space group $\left(F\overline{4}3m\right)$, where $\mathrm{L}{\mathrm{i}}^{+}$ and $\mathrm{F}{\mathrm{e}}^{3+}$ ions are ordered in a zinc-blende-type manner at the $A$ site and $\mathrm{C}{\mathrm{r}}^{3+}$ ions at the $B$ site form a breathing pyrochlore lattice. It undergoes a ferrimagnetic transition at $T_{\mathrm{N}}\sim 94\mathrm{K}$, below which temperature-dependent magnetization data exhibit a maximum at $T_{\mathrm{SG}}\sim 60\mathrm{K}$, corresponding to a spin-gap arising from breathing distortion. At low temperature $(T_{\mathrm{MS}}\sim 23\mathrm{K})$, concurrent structural $\left(I\overline{4}m2\right)$ and magnetic phase transitions occur because of significant geometrical frustration of the pyrochlore lattice, giving rise to a complex conical magnetic order. Intriguingly, the observed dielectric anomalies at all three magnetic transition temperatures in zero applied magnetic field indicate the occurrence of magnetodielectric effects. Further, the temperature dependence of Raman modes shows three distinct anomalous behaviors that correlate with the dielectric anomalies at $T_{\mathrm{N}},T_{\mathrm{SG}}$, and $T_{\mathrm{MS}}$, which suggest the presence of spin-lattice coupling.

Fullerene condensed-matter solids can possess thermal conductivities below their minimum glassy limit while theorized to be stiffer than diamond when crystallized under pressure. These seemingly disparate extremes in thermal and mechanical properties raise questions into the pressure dependence on the thermal conductivity of ${\mathrm{C}}_{60}$ fullerite crystals, and how the spectral contributions to vibrational thermal conductivity changes under applied pressure. To answer these questions, we investigate the effect of strain on the thermal conductivity of ${\mathrm{C}}_{60}$ fullerite crystals via pressure-dependent molecular dynamics simulations under the Green-Kubo formalism. We show that the thermal conductivity increases rapidly with compressive strain, which demonstrates a power-law relationship similar to their stress-strain relationship for the ${\mathrm{C}}_{60}$ crystals. Calculations of the density of states for the crystals under compressive strains reveal that the librational modes characteristic in the unstrained case are diminished due to densification of the molecular crystal. Over a large compression range (0–20 GPa), the Leibfried-Schlömann equation is shown to adequately describe the pressure dependence of thermal conductivity, suggesting that low-frequency intermolecular vibrations dictate heat flow in the ${\mathrm{C}}_{60}$ crystals. A spectral decomposition of the thermal conductivity supports this hypothesis.

On the basis of a phenomenological Landau model combined with comprehensive experimental studies, the magnetostructural transition behavior and field induced caloric effects for NiMnGaCu Heusler alloys have been investigated. In ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{25-x}{\mathrm{Ga}}_{25}{\mathrm{Cu}}_x$ alloys with $x=5.5$, 6, and 6.5, both magnetocaloric entropy change ($\mathrm{\Delta }S$) and elastocaloric temperature change ($\mathrm{\Delta }T$) increase with the increment of Cu content. The maximum $\mathrm{\Delta }S$ of $1.01\mathrm{J}/\mathrm{mol}\mathrm{K}$ and $\mathrm{\Delta }T$ of 8.1 K are obtained for the alloy with $x=6.5$. In order to explore the physical origin behind the large caloric effect, here we quantitatively propose a crucial coefficient of magnetoelastic coupling $\tilde{\kappa }$ by utilizing a thermodynamic formalism within the framework of the Landau approach. It has been verified that the enhancement of the strength of magnetoelastic coupling between lattice and magnetic freedoms results in the increased caloric response for NiMnGaCu alloys. Thus, the strengthened coupling of the magnetoelastic effect can be considered as an effective way to improve the caloric performance for these alloys having the same sign of magnetic and elastic entropy changes contributed to the total caloric effect.

Polarized absorption spectra of the ${}^5{I}_8\to {}^5{F}_5$ transition of the $\mathrm{H}{\mathrm{o}}^{3+}$ ion in the $\mathrm{HoF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ single crystal were measured at $T=2\mathrm{K}$ as a function of magnetic field 0–60 kOe directed along the $a$, $b$, and $c$ axes of the crystal. The spin-reorientation transitions were observed in the field $H\parallel c$ at 5.35 kOe, in the field $H\parallel a$ at 10.25 kOe, and in the field $H\parallel b$ at 9.1 kOe. We also observed a small hysteresis of the transitions. We found that the optical properties of the crystal change as a function of the magnetic field orientation in the basal plane in spite of the nominally axial symmetry of the crystal. We found a number of specific local properties in the $\mathrm{H}{\mathrm{o}}^{3+}$ excited states. Nonequivalent positions of the $\mathrm{H}{\mathrm{o}}^{3+}$ ion appear in some excited states. It was shown that during one of the electron transitions the local symmetry changes not only in the excited state but in the ground state as well. We observed local metamagnetic transitions in some $\mathrm{H}{\mathrm{o}}^{3+}$ excited states. In some excited states, the $\mathrm{H}{\mathrm{o}}^{3+}$ ion is in the easy-plane state while the total crystal is in the easy-axis state and vice versa. This means that the magnetic moments of the $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{H}{\mathrm{o}}^{3+}$ ions in these excited states of the $\mathrm{H}{\mathrm{o}}^{3+}$ ions are mutually perpendicular. Such phenomenon testifies to the existence of the antisymmetric Fe-Ho exchange interaction in the $\mathrm{H}{\mathrm{o}}^{3+}$ excited states.

The precise manipulation of transverse magnetic domain walls in finite/infinite nanowires with artificial defects under the influence of very short spin-polarized current pulses is investigated. We show that for a classical $3d$ ferromagnet material like nickel, the exact positioning of the domain walls at room temperature is possible only for pulses with very short rise and fall time that move the domain wall reliably to nearest neighboring pinning position. The influence of the shape of the current pulse and of the transient effects on the phase diagram current-pulse length are discussed. We show that large transient effects appear even when $\alpha =\beta$, below a critical value, due to the domain wall distortion caused by the current pulse shape and the presence of the notches. The transient effects can oppose or amplify the spin-transfer torque (STT), depending on the ratio $\beta /\alpha$. This enlarges the physical comprehension of the domain wall (DW) motion under STT and opens the route to the DW displacement in both directions with unipolar currents.

Here we report the in situ high-pressure (up to $\sim 50$-GPa) Hall-effect measurements on single-crystal black phosphorus. We find a strong correlation between the sign of the Hall coefficient, an indicator of the dominant carrier type, and the superconducting transition temperature ($T_C$). Importantly, we find a change from electron-dominant to hole-dominant carriers in the simple cubic phase of phosphorus at a pressure of $\sim 17.2$ GPa, providing an explanation for the puzzling valley it displays in its superconducting $T_C$ vs pressure phase diagram. Our results reveal that hole carriers play an important role in developing superconductivity in elemental phosphorus and the valley in $T_C$ at 18.8 GPa is associated with a Lifshitz transition.

We theoretically study the Josephson current in a superconductor/quantum anomalous Hall insulator/superconductor junction by using the lattice Green function technique. When an in-plane external Zeeman field is applied to the quantum anomalous Hall insulator, the Josephson current $J$ flows without a phase difference across the junction $\theta$. The phase shift $\phi$ appearing in the current-phase relationship $J\propto sin(\theta -\phi$) is proportional to the amplitude of Zeeman fields and depends on the direction of Zeeman fields. A phenomenological analysis of the Andreev reflection processes explains the physical origin of $\phi$. In a quantum anomalous Hall insulator, time-reversal symmetry and mirror-reflection symmetry are broken simultaneously. However, magnetic mirror-reflection symmetry is preserved. Such characteristic symmetry properties enable us to have a tunable $\phi$ junction with a quantum Hall insulator.

We present a theory for charge and heat transport parallel to the interfaces of a multilayer (ML) of the ABA type, where A and B are materials with strongly correlated electrons. When separated, both materials are half-filled Mott-Hubbard insulators with large gaps in their excitation spectrum. In a ML, the renormalization of the energy bands gives rise to a charge reconstruction which breaks the charge neutrality of the planes next to the interface. The ensuing electrical field couples self-consistently to the itinerant electrons, so that the properties of the ML crucially depend on an interplay between the on-site Coulomb forces and the long range electrostatic forces. Using the Falicov-Kimball model, we compute the Green's function and the local charge on each plane of the ML by inhomogeneous DMFT and find the corresponding electrical potential from Poisson's equation. The self-consistent solution is obtained by an iterative procedure, which yields the reconstructed charge profile, the electrical potential, the planar density of states, the transport function, and the transport coefficients of the device. For the right choice of parameters, we find that a heterostructure built of two Mott-Hubbard insulators exhibits, in a large temperature interval, a linear conductivity and a large temperature-independent thermopower. The charge and energy currents are confined to the central part of the ML. Our results indicate that correlated multilayers have the potential for applications; by tuning the band shift and the Coulomb correlation on the central planes, we can bring the chemical potential in the immediate proximity of the Mott-Hubbard gap edge and optimize the transport properties of the device. In such a heterostructure, a small gate voltage can easily induce a MI transition. Furthermore, the right combination of strongly correlated materials with small ZT can produce, theoretically at least, a heterostructure with a large ZT.

With the emergence of graphene and other two-dimensional (2D) materials, transition-metal dichalcogenides have been investigated intensely as potential 2D materials using experimental and theoretical methods. ${\mathrm{VSe}}_2$ is an especially interesting material since its bulk modification exhibits a charge-density wave (CDW), the CDW is retained even for few-layer nanosheets, and monolayers of ${\mathrm{VSe}}_2$ are predicted to be ferromagnetic. In this work, we show that electron correlation has a profound effect on the magnetic properties and dynamic stability of ${\mathrm{VSe}}_2$ monolayers and bilayers. Including a Hubbard-$U$ term in the density-functional-theory calculations strongly affects the magnetocrystalline anisotropy in the $1T-{\mathrm{VSe}}_2$ structure while leaving the $2H$-polytype virtually unchanged. This demonstrates the importance of electronic correlations for the electrical and magnetic properties of $1T-{\mathrm{VSe}}_2$. The Hubbard-$U$ term changes the dynamic stability and the presence of imaginary modes of ferromagnetic $1T-{\mathrm{VSe}}_2$ while affecting only the amplitudes in the nonmagnetic phase. The Fermi surface of nonmagnetic $1T-{\mathrm{VSe}}_2$ allows for nesting along the CDW vector, but it plays no role in ferromagnetic $1T-{\mathrm{VSe}}_2$. Following the eigenvectors of the soft modes in nonmagnetic $1T-{\mathrm{VSe}}_2$ monolayers yields a CDW structure with a $4\times 4$ supercell and Peierls-type distortion in the atomic positions and electronic structure. The magnetic order indicates the potential for spin-density-wave structures.

Both longitudinal and anomalous Hall conductivities are computed in the model of two-dimensional Dirac fermions with a mass in the presence of weak Gaussian spin-independent disorder with an arbitrary correlation function. The anomalous Hall conductivity is shown to be highly sensitive to the correlation properties of the random potential, such as the correlation length, while it remains independent of the integral disorder strength. This property extends beyond the Dirac model making the anomalous Hall effect an interesting tool to probe disorder correlations.

We investigate the robustness of Majorana edge modes under disorder and interactions. We exploit a recently found mapping of the interacting Kitaev chain in the symmetric region ($\mu =0, t=\mathrm{\Delta }$) to free fermions. Extending the exact solution to the disordered case allows us to calculate analytically the topological phase boundary for all interaction and disorder strengths, which has been thought to be only accessible numerically. We discover a regime in which moderate disorder in the interaction matrix elements enhances topological order well into the strongly interacting regime $U>t$. We also derive the explicit form of the many-body Majorana edge wave function, revealing how it is dressed by many-particle fluctuations from interactions. The qualitative features of our analytical results are valid beyond the fine-tuned integrable point, as expected from the robustness of topological order and as corroborated here by an exact diagonalization study of small systems.

We use angle-resolved photoemission spectroscopy with a circularly polarized 7-eV laser, and find that the angle-resolved pattern of circular dichroism (CD) for the topological surface state of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is dramatically changed with surface aging. Shortly after sample cleaving, the CD pattern exhibits a $\sin (\varphi$)-like modulation ($\varphi$ is the Fermi surface angle) reminiscent of the helical spin texture. Intriguingly, this pattern gradually changes with time, and eventually turns into a $\sin \left(2\varphi \right)$-like modulation on the aged surface. These two patterns are also revealed in quantum well states with different quantum numbers, indicating that the CD pattern is a valid measure to identify the surface confinement of electrons. Our experiments thus demonstrate the time evolution of electron localization in the topological surface state.

We study Dirac fermions in two spatial dimensions (2D) coupled to strongly fluctuating $U\left(1\right)$ gauge fields in the presence of quenched disorder. Such systems are dual to theories of free Dirac fermions, which are vortices of the original theory. In analogy to superconductivity, when these fermionic vortices localize, the original system becomes a perfect conductor, and when the vortices possess a finite conductivity, the original fermions do as well. We provide several realizations of this principle and thereby introduce examples of strongly interacting 2D metals that evade Anderson localization.

Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.