Recent experiments showed that Co undergoes a phase transition from the ferromagnetic hcp phase to the nonmagnetic fcc one around 100 GPa. Since the transition is of first order, a certain region of coexistence of the two phases is present. By means of ab initio calculations, we found that the hcp phase itself undergoes a series of electronic topological transitions (ETTs), which affects both elastic and magnetic properties of the material. Most importantly, we propose that the sequence of ETTs lead to the stabilization of a noncollinear spin arrangement in highly compressed hcp Co. Details of this noncollinear magnetic state and the interatomic exchange parameters that are connected to it are presented here.

We have discovered stoichiometric LiB with hexagonal boron layers by compressing known ${\mathrm{LiB}}_{0.9}$ with linear boron chains. The $sp$ to $s{p}^2$ rebonding occurred at room temperature and pressures above 21 GPa. The study was motivated by a long-standing prediction that LiB in the stable layered configuration could be a close analog to the ${\mathrm{MgB}}_2$ superconductor. Apparent stacking disorder in LiB and a stoichiometry shift in ${\mathrm{LiB}}_y$ (down to $y\approx 0.75$) made material characterization a challenge. Ab initio modeling allowed us to establish the pressure-dependent composition of ${\mathrm{LiB}}_y$ and predict related stable structures overlooked in previous studies. Synchrotron powder diffraction data on quenched samples in the diamond anvil cell indicate that LiB remains metastable at ambient pressure.

In material sciences, spectroscopic approaches combining ab initio calculations with experiments are commonly used to accurately analyze the experimental spectral data. Most state-of-the-art first-principles calculations are usually performed assuming an equilibrium static lattice. Yet, nuclear motion affects spectra even when reduced to the zero-point motion at 0 K. We propose a framework based on density-functional theory that includes quantum thermal fluctuations in theoretical x-ray absorption near-edge structure (XANES) and solid-state nuclear magnetic resonance (NMR) spectroscopies and allows to well describe temperature effects observed experimentally. Within the Born-Oppenheimer and quasiharmonic approximations, we incorporate the nuclear motion by generating several nonequilibrium configurations from the dynamical matrix. The averaged calculated XANES and NMR spectral data have been compared to experiments in MgO. The good agreement obtained between experiments and calculations validates the developed approach, which suggests that calculating the XANES spectra at finite temperature by averaging individual nonequilibrium configurations is a suitable approximation. This study highlights the relevance of phonon renormalization and the relative contributions of thermal expansion and nuclear dynamics on NMR and XANES spectra on a wide range of temperatures.

The effects of pressure on the ferroelectric properties of multiferroic ${\mathrm{GdMn}}_2{\mathrm{O}}_5$ is studied up to 18.2 kbar. Above a critical pressure of $p_c\approx 10$ kbar, the ferroelectric transition splits into two, with the first transition shifted to higher temperature. The pressure-temperature phase diagram is derived. The results indicate a pressure-induced decoupling of the Gd moments from the Mn spin system. The conclusion is supported by thermal expansion data which show a large increase of the $c$ axis at the ambient-pressure ferroelectric transition. The pressure-induced contraction of the $c$ lattice parameter is considered to be the origin of the decoupling of both magnetic subsystems above $p_c$.

We report nuclear magnetic resonance (NMR) data for both the In(1) and In(2) sites in the heavy-fermion material ${\mathrm{CeRhIn}}_5$ under hydrostatic pressure. The Knight shift data reveal a suppression of the hyperfine coupling to the In(1) site as a function of pressure, and the electric field gradient ${\nu }_{\alpha \alpha }$ at the In(2) site exhibits a change of slope $d{\nu }_{\alpha \alpha }/dP$ at $P_{c1}=1.75$ GPa. These changes to the coupling constants reflect alterations to the electronic structure at the quantum critical point.

The coexistence of ferromagnetic and antiferromagnetic phases and their role in the photoinduced insulator-to-metal transition in ${\mathrm{Pr}}_{1-x}\mathrm{Ca}{}_x{\mathrm{MnO}}_3$ are revealed via ultrafast resonant x-ray diffraction and broadband optical reflectivity measurements. The antiferromagnetic scattering signal and ferromagnetically sensitive reflectivity measurements show similar, strongly temperature dependent time scales. We attribute the common dynamics to an activation barrier between the equilibrium insulating phase and the photoinduced metallic phase related to interactions between the phase-separated ferromagnetic and antiferromagnetic insulating phases.

Fermionic natural occupation numbers do not only obey Pauli's exclusion principle, but are even further restricted by so-called generalized Pauli constraints. Such restrictions are particularly relevant whenever they are saturated by given natural occupation numbers $\vec{\lambda }=\left({\lambda }_i\right)$. For few-site Hubbard models we explore the occurrence of this pinning effect. By varying the on-site interaction $U$ for the fermions we find sharp transitions from pinning of $\vec{\lambda }$ to the boundary of the allowed region to nonpinning. We analyze the origin of this phenomenon which turns out be either a crossing of natural occupation numbers ${\lambda }_i\left(U\right),{\lambda }_{i+1}\left(U\right)$ or a crossing of $N$-particle energies. Furthermore, we emphasize the relevance of symmetries for the occurrence of pinning. Based on recent progress in the field of ultracold atoms our findings suggest an experimental setup for the realization of the pinning effect.

The superposition coefficients of the coupled states of a photon and an exciton confined in a thin film are provided using Fano's method [U. Fano, Phys. Rev. 124, 1866 (1961)], in which the exciton interacts with continuum photon states. The coupled states are classified into leaky and surface modes, the latter of which are investigated with a focus on the dispersion relation, the exciton component, and their film-thickness dependence. The obtained coupled states enable us to study exciton-mediated optical properties that cannot be described by a semiclassical scheme, such as photoluminescence and the radiative decay of a biexciton. As applications of the coupled states, we calculate the population decay dynamics and photoluminescence spectra of an exciton in a thin film.

Topological aspects of superconductivity in quantum spin Hall systems (QSHSs) such as thin layers of three-dimensional topological insulators (TIs) or two-dimensional TIs are the focus of current research. Here, we describe a superconducting quantum spin Hall effect (quantum spin Hall system in proximity to an $s$-wave superconductor and in orbital in-plane magnetic fields), which is protected against elastic backscattering by combined time-reversal and particle-hole symmetry. This effect is characterized by spin-polarized edge states, which can be manipulated in weak magnetic fields due to a giant effective orbital $g$ factor, allowing the generation of spin currents. The phenomenon provides a solution to the outstanding challenge of detecting the spin polarization of the edge states. Here we propose the detection of the edge polarization in a three-terminal junction using unusual transport properties of the superconducting quantum Hall effect: a nonmonotonic excess current and a zero-bias conductance peak splitting.

We calculate expressions for the state-dependent quasiparticle lifetime ${\tau }_{\sigma }$, the thermal conductivity $\kappa$, the shear viscosity $\eta$, and discuss the spin diffusion coefficient $D$ for Fermi-liquid films in two dimensions. The expressions are valid for low temperatures and arbitrary polarization. In two dimensions, as in three dimensions, the integrals over the transition rates factor into energy and angular parts. However, the angular integrations contain a weak divergence. This problem is addressed using the method of K. Miyake and W. J. Mullin [Phys. Rev. Lett. 50, 197 (1983); J. Low Temp. Phys. 56, 499 (1984)]. The low-temperature expressions for the transport coefficients are essentially exact. We find that ${\kappa }^{-1}\sim TlnT$, and ${\eta }^{-1}\sim T^2$ for arbitrary polarizations $0\le \mathcal{P}\le 1$. These results are in agreement with earlier zero-polarization results of H. H. Fu and C. Ebner [Phys. Rev. A 10, 338 (1974).], but differ from the temperature dependence of the shear viscosity found by D. S. Novikov (arXiv:cond-mat/0603184). They also differ from the discontinuous change of temperature dependence in $D$ from zero to nonzero polarization that was discovered by Miyake and Mullin. We note that in two dimensions the shear viscosity requires a unique analysis. We obtain predictions for the density, temperature, and polarization dependence of $\kappa ,\eta$, and $D$ for second-layer ${}^3\mathrm{He}$ films on graphite, and thin ${}^3\mathrm{He}-{}^4\mathrm{He}$ superfluid mixtures. For ${}^3\mathrm{He}$ on graphite, we find roughly an order of magnitude increase in magnitude for $\kappa$ and $\eta$ as the polarization is increased from 0 to 1. For $D$ a similar large increase is predicted from zero polarization to the polarization where $D$ is a maximum $\left(\sim 0.74\right)$. We discuss the applicability of ${}^3\mathrm{He}$ thin films to the question of the existence of a universal lower bound for the ratio of the shear viscosity to the entropy density.

The topological response to external perturbations is an effective probe to characterize different topological phases of matter. Besides the Hall conductance, the Hall viscosity is another example of such a response that measures how electronic wave functions respond to changes in the underlying geometry. Topological (Chern) insulators are known to have a quantized Hall conductance. A natural question is how the Hall viscosity behaves for these materials. So far, most of studies on the Hall viscosity of Chern insulators have focused on the continuum limit. The presence of lattice breaks the continuous translational symmetry to a discrete group and this causes two complications: it introduces a new length scale associated with the lattice constant, and makes the momentum periodic. We develop two different methods of how to implement a lattice deformation: (1) a lattice distortion is encoded as a shift in the lattice momentum and (2) a lattice deformation is treated microscopically in the gradient expansion of the hopping matrix elements. After establishing the method of deformation, we can compute the Hall viscosity through a linear response (Kubo) formula. We examine these methods for three models: the Hofstadter model, the Chern insulator, and the surface of a 3D topological insulator. Our results in certain regimes of parameters, where the continuum limit is relevant, are in agreement with previous calculations. We also provide possible experimental signatures of the Hall viscosity by studying the phononic properties of a single-crystal 3D topological insulator.

Two-dimensional (2D) crystalline colloidal monolayers sliding over a laser-induced optical lattice providing the periodic “corrugation” potential recently emerged as a new tool for the study of friction between ideal crystal surfaces. Here, we focus in particular on static friction, the minimal sliding force necessary to depin one lattice from the other. If the colloid and the optical lattices are mutually commensurate, the colloid sliding is always pinned by static friction; however, when they are incommensurate, the presence or absence of pinning can be expected to depend upon the system parameters, like in one-dimensional (1D) systems. If a 2D analogy to the mathematically established Aubry transition of one-dimensional systems were to hold, an increasing periodic corrugation strength $U_0$ should turn an initially free-sliding, superlubric colloid into a pinned state, where the static friction force goes from zero to finite through a well-defined dynamical phase transition. We address this problem by the simulated sliding of a realistic model 2D colloidal lattice, confirming the existence of a clear and sharp superlubric-pinned transition for increasing corrugation strength. Unlike the 1D Aubry transition, which is continuous, the 2D transition exhibits a definite first-order character, with a jump of static friction. With no change of symmetry, the transition entails a structural character, with a sudden increase of the colloid-colloid interaction energy, accompanied by a compensating downward jump of the colloid-corrugation energy. The transition value for the corrugation amplitude $U_0$ depends upon the misalignment angle $\theta$ between the optical and the colloidal lattices, superlubricity surviving until larger corrugations for angles away from the energetically favored orientation, which is itself generally slightly misaligned, as shown in recent work. The observability of the superlubric-pinned colloid transition is proposed and discussed.

Extended quantum systems can be theoretically described in terms of the Schwinger-Keldysh functional integral formalism, whose action conveniently describes both dynamical and static properties. We show here that in thermal equilibrium, defined by the validity of fluctuation-dissipation relations, the action of a quantum system is invariant under a certain symmetry transformation, and thus it is distinguished from generic systems. In turn, the fluctuation-dissipation relations can be derived as the Ward-Takahashi identities associated with this symmetry. Accordingly, the latter provides an efficient test for the onset of thermodynamic equilibrium and it makes checking the validity of fluctuation-dissipation relations unnecessary. In the classical limit, this symmetry reduces to the well-known one that characterizes equilibrium in the stochastic dynamics of classical systems coupled to thermal baths, described by Langevin equations.

Motivated by the presence of competing magnetic interactions in the heavy fermion family $\mathrm{Ce}T{X}_2$ ($T = \text{transition}\text{metal}$, $X=\text{pnictogen}$), here we study the novel parent compound ${\mathrm{CeCd}}_{0.7}{\mathrm{Sb}}_2$ by combining magnetization, electrical resistivity, and heat-capacity measurements. Contrary to the antiferromagnetic (AFM) ground state observed in most members of this family, the magnetic properties of our ${\mathrm{CeCd}}_{0.7}{\mathrm{Sb}}_2$ single crystals revealed a ferromagnetic ordering at $T_{\mathrm{c}}=3\mathrm{K}$ with an unusual soft behavior. By using a mean field model including anisotropic nearest-neighbor interactions and the tetragonal crystalline electric field (CEF) Hamiltonian, a systematic analysis of our macroscopic data was obtained. Our fits allowed us to extract a simple but very distinct CEF scheme, as compared to the AFM counterparts. As in the previously studied ferromagnet ${\mathrm{CeAgSb}}_2$, a pure $|\pm 1/2\rangle$ ground state is realized, hinting at a general trend within the ferromagnetic members. More generally, we propose a scenario for the understanding of the magnetism in this family of compounds based on the subtle changes of dimensionality in the crystal structure.

Magnetization dynamics in $\mathrm{F}{\mathrm{e}}_{60}\mathrm{A}{\mathrm{l}}_{40}$ thin films possessing depth-varying saturation magnetization ($M_{\mathrm{S}}$) have been studied experimentally and theoretically. Variation in $M_{\mathrm{S}}$ was achieved by irradiation of 40 nm thick, chemically ordered (B2 phase) $\mathrm{F}{\mathrm{e}}_{60}\mathrm{A}{\mathrm{l}}_{40}$ films with $\mathrm{N}{\mathrm{e}}^{+}$ ions with energies between 0–30 keV. The initial B2 phase is paramagnetic, and as the penetrating ions cause chemical disordering, the ion-affected region transforms to the ferromagnetic A2 phase. The effective ferromagnetic thickness and the depth of the A2/B2 phase boundary depend on the ion energy ($E$); the effective thicknesses are 8.5 and 40 nm, respectively, for $E=2.5$ and 30 keV. Thermally excited spin waves in films with varying effective ferromagnetic thicknesses were analyzed by employing Brillouin light scattering and vector network analyzer ferromagnetic resonance spectroscopy. The analytical calculations are in good agreement with the experimental values and show that the observed spin-wave modes are directly related to the effective ferromagnetic thickness; films irradiated with $E<15\mathrm{keV}$ only show the Damon-Eshbach mode, whereas for $15\le E<20\mathrm{keV}$, an additional lower frequency standing spin-wave mode is observed. In films irradiated with $E \ge 20\mathrm{keV}$, the Damon-Eshbach mode is observed to lie between two standing spin-wave modes. Furthermore, the A2/B2 phase boundary can be shown to act as an asymmetric pinning site. Controlling the depth of the phase boundary by varying the ion energy can be a path to manipulate spin-wave propagation in materials displaying the phenomenon of disorder induced ferromagnetism.

We demonstrate here the phase control of the neutral exciton quantum beats in InGaAs/GaAs quantum dots. A longitudinal magnetic field is used as a tuning parameter to change the phase of the oscillations in a deterministic way. This effect arises from the competition between the Zeeman splitting and the electron/hole exchange interaction on the exciton dipole symmetry. To explore this mechanism, we have developed a pump-probe setup based on the optical heterodyne detection of the quantum dots reflectivity allowing one to measure the exciton dynamics from a small quantum dots ensemble ($\sim 300$). Particular attention is paid to give a detailed theoretical analysis of the measurements. The experimental results are in excellent agreement with the model.

In the topological phase of $p$-wave superconductors, zero-energy Majorana quasiparticle excitations can be well defined in the presence of local density-density interactions. Here, we examine this phenomenon from the perspective of matrix representations of the commutator $\mathcal{H}=[H,•]$, with the aim of characterizing the multiparticle content of the many-body Majorana mode. To do this we show that, for quadratic fermionic systems, $\mathcal{H}$ can always be decomposed into subblocks that act as multiparticle generalizations of the Bogoliubov–de Gennes/Majorana forms that encode single-particle excitations. In this picture, density-density-like interactions will break this exact excitation-number symmetry, coupling different subblocks and lifting degeneracies so that the eigenoperators of the commutator $\mathcal{H}$ take the form of individual eigenstate transitions $|n\rangle \langle m|$. However, the Majorana mode is special in that zero-energy transitions are not destroyed by local interactions and it becomes possible to define many-body Majoranas as the odd-parity zero-energy solutions of $\mathcal{H}$ that minimize their excitation number. This idea forms the basis for an algorithm which is used to characterize the multiparticle excitation content of the Majorana zero modes of the one-dimensional $p$-wave lattice model. We find that the multiparticle content of the Majorana zero-mode operators is significant even at modest interaction strengths. This has important consequences for the stability of Majorana-based qubits when they are coupled to a heat bath. We will also discuss how these findings differ from previous work regarding the structure of the many-body Majorana operators and point out that this should affect how certain experimental features are interpreted.

By combining first-principles calculations with Wannier-based tight-binding modeling, we demonstrate that a ${\text{TiO}}_2/{\text{VO}}_2$ heterostructure that was previously proposed as a prototypical semi-Dirac system becomes a Chern insulator (quantum anomalous Hall insulator) in the presence of spin-orbit coupling. We show that this occurs only when the semi-Dirac structure is of a special type that can be formed by the merging of three conventional Dirac points. Our results reveal how the nontrivial topology with a nonzero Chern number emerges naturally from this kind of semi-Dirac structure, establishing a general scenario that provides a different route to the formation of Chern-insulating states in practical materials systems.

The entanglement of coherently split electron-hole pairs in an electronic conductor is typically not considered accessible due to particle number conservation and fermionic superselection rules. We demonstrate here that current cross-correlation measurements at the outputs of an electronic Mach-Zehnder interferometer can nevertheless provide a robust witness of electron-hole entanglement. Specifically, we consider neutral excitations generated by modulating the transmission of an unbiased quantum point contact periodically in time. For an optimized modulation profile, an entangled state with one positively-charged leviton (a hole) and one negatively-charged leviton (an electron) gets delocalized over the two paths of the interferometer and is detected at the output arms. We evaluate the influence of finite electronic temperatures and dephasing corresponding to recent experiments.