The sudden decrease in molar volume exhibited by most lanthanides under high pressure is often attributed to changes in the degree of localization of their 4$f$ electrons. We give evidence, based on electrical resistivity measurements of dilute Y(Gd) and Y(Tb) alloys to 120 GPa, that the volume collapse transitions in Gd and Tb metals have different origins, despite their being neighbors in the periodic table. Remarkably, the change under pressure in the magnetic state of isolated Pr or Tb impurity ions in the nonmagnetic Y host appears to closely mirror corresponding changes in pure Pr or Tb metals. The collapse in Tb appears to be driven by an enhanced negative exchange interaction between 4$f$ and conduction electrons under pressure (Kondo resonance) which, in the case of Y(Tb), dramatically alters the superconducting properties of the Y host, much like previously found for Y(Pr). In Gd, our resistivity measurements suggest that a Kondo resonance is not the main driver for its volume collapse. X-ray absorption and emission spectroscopies clearly show that 4$f$ local moments remain largely intact across both volume collapse transitions ruling out 4$f$ band formation (delocalization) and valence transition models as possible drivers. The results highlight the richness of behavior behind the volume collapse transition in lanthanides and demonstrate the stability of the 4$f$ level against band formation to extreme pressure.

Diamond, as a consequence of its superlative intrinsic physical properties, is an attractive material for a wide range of applications. Recent developments have greatly enhanced the quality of gas-phase grown diamonds. It has been observed that some defects are grown into diamond preferentially aligned to specific orientations with respect to growth surface. Of particular note, the nitrogen-vacancy center is polarized in (110)-grown material. Preferential alignment of these defects in diamond may enhance their use in applications such as quantum information and encryption, and in diamond-based magnetometers. The origin of the preferential orientation with respect to the growth surface is not completely understood, and a mechanistic model is highly desirable in order that one might both optimize defect incorporation and better understand the growth of diamond in a wider sense. We present the results of quantum-chemical simulations that provide insight into the preferential alignment of nitrogen-related defects grown into different diamond surface orientations, showing that the sequence of structure surfaces required to produce polarization aligns with their energies.

We study the relaxation dynamics of a nonequilibrium Tomonaga-Luttinger model after a sudden interaction switch-on (“quench”), focusing on a double-step initial momentum distribution function. In the framework of the nonequilibrium bosonization, the results are obtained in terms of singular Fredholm determinants that are evaluated numerically and whose asymptotics are found analytically. While the quasiparticle weights decay exponentially with time after the quench, this is not a relaxation into a thermal state, in view of the integrability of the model. The steady-state distribution emerging at infinite times retains two edges which support Luttinger-liquid-like power-law singularities smeared by dephasing. The obtained critical exponents and the dephasing length are found to depend on the initial nonequilibrium state.

An interatomic potential for the W-C system has been derived based on the second-nearest-neighbor modified embedded-atom method scheme. The potential parameters were constructed by fitting to the experimental information on the formation energy of interstitial carbon atoms, the migration energy of carbon atoms in body-centered-cubic (bcc) tungsten and lattice parameters, bulk modulus, and cohesive energy of $B$${}_h$ W${}_2$C carbide. We demonstrate that such potential can not only reproduce the behavior of carbon atoms in bcc tungsten, but also be employed for modeling energetics and structural properties of tungsten carbides. Moreover, this potential is used to study the stability of multicarbon-multivacancy ($C_n{V}_m$) clusters in bcc tungsten. The binding energy and configurations of $C_n{V}_m$ clusters that were not attainable with existing potentials or identified previously via $ab$ $initio$ methods are proposed. Results show that carbon atoms can be strongly trapped by vacancies and interstitials in bcc W. Such bonding between carbon and vacancies lead to the formation of $C_n{V}_m$ complexes and influence the physical properties of metallic tungsten. Besides, a positive binding energy of C-C cluster and C-SIA clusters was found, and both clusters form a hexagonal WC-like local structure. This result provides a plausible explanation for the experimentally observed formation of hexagonal WC (not W${}_2$C) in bcc W.

We demonstrate here that ultrathin ferromagnetic Pt/Co/Pt films with perpendicular magnetic anisotropy exhibit a sizable Dzyaloshinskii-Moriya interaction (DMI) effect. Such a DMI effect modifies the domain-wall (DW) energy density and consequently, results in an asymmetric DW expansion driven by an out-of-plane magnetic field under an in-plane magnetic field bias. From an analysis of the asymmetry, the DMI effect is estimated to be strong enough for the DW to remain in the Néel-type configuration in contrast to the general expectations of these materials. Our findings emphasize the critical role of the DMI effect on the DW dynamics as the underlying physics of the asymmetries that are often observed in spin-transfer-related phenomena.

Bulk, shear, and compressional aggregate sound velocities of hydrogen and helium in the close-packed hexagonal structure are calculated over a wide pressure range using two complementary approaches: semiempirical lattice dynamics based on the many-body intermolecular potentials and density-functional theory in the generalized gradient approximation. The sound velocities are used to calculate the pressure dependence of the Debye temperature. The comparisons between experiment and first-principles and semiempirical calculations provide constraints on the density dependence of intermolecular interactions in the zero-temperature limit.

Using a numerical tight-binding approach based on the Chebyshev–Bogoliubov–de Gennes method we describe Josephson junctions made of multilayer graphene contacted by top superconducting gates. Both Bernal (ABA) and rhombohedral (ABC) stacking are considered and we find that the type of stacking has a strong effect on the proximity effect and the supercurrent flow. For both cases the pair amplitude shows a polarization between dimer and nondimer atoms, being more pronounced for rhombohedral stacking. Even though the proximity effect in nondimer sites is enhanced when compared to single-layer graphene, we find that the supercurrent is suppressed. The spatial distribution of the supercurrent shows that for Bernal stacking the current flows only in the topmost layers while for rhombohedral stacking the current flows throughout the whole structure.

Recent advances in the fabrication and characterization of anisotropic silica aerogels with exceptional homogeneity provide new insight into the nature of unconventional pairing in disordered anisotropic media. I report theoretical analysis and predictions for the equilibrium phases of superfluid ${}^3$He infused into a low-density, homogeneous uniaxial aerogel. Ginzburg-Landau (GL) theory for a class of equal-spin-pairing (ESP) states in a medium with uniaxial anisotropy is developed and used to analyze recent experiments on uniaxially strained aerogels. For ${}^3$He in an axially “stretched” aerogel, GL theory predicts a transition from normal liquid into a chiral Anderson-Morel phase at $T_{c_1}$ in which the chirality axis $\widehat{\mathbf{l}}$ is aligned along the strain axis. This orbitally aligned state is protected from random fluctuations in the anisotropy direction, has a positive nuclear magnetic resonance (NMR) frequency shift, a sharp NMR resonance line, and is identified with the high-temperature ESP-1 phase of superfluid ${}^3$He in axially stretched aerogel. A second transition into a biaxial phase is predicted to onset at a slightly lower temperature $T_{c_2}<T_{c_1}$. This phase is an ESP state, breaks time-reversal symmetry, and is defined by an orbital order parameter that spontaneously breaks axial rotation symmetry. This transition is driven by the coupling of an axially aligned one-dimensional “polar” order parameter to the two time-reversed two-dimensional axial Anderson-Brinkman-Morel states. The biaxial phase has a continuous degeneracy associated with the projection of its chiral axis in the plane normal to the anisotropy axis. Theoretical predictions for the NMR frequency shifts of the biaxial phase provide an identification of the ESP-2 as the biaxial state, partially disordered by random anisotropy (Larkin-Imry-Ma effect). The “width” of the jump in the NMR frequency shift at $T_{c_2}$ provides an estimate of the orbital domain size ${\xi }_{\mathrm{LIM}}\simeq 5\mu \text{m}$ at $18\text{bar}$. I show that the random anisotropy results from mesoscopic structures in silica aerogels. This model for the random anisotropy field is coarse grained on the atomic scale, and is formulated in terms of local anisotropy in the scattering of quasiparticles in an aerogel with orientational correlations. Long-range order of locally anisotropic scattering centers is related to the splitting of the two ESP phases.

Using ultrafast resonant soft x-ray diffraction, we demonstrate photoinduced melting of antiferromagnetic order in the correlated electron insulator NdNiO${}_3$. Time-dependent analysis of the resonant diffraction spectra allows us to follow the temporal evolution of the charge imbalance between adjacent Ni sites. A direct correlation between the melting of magnetic order and charge rebalancing is found. Furthermore, we demonstrate that the magnetic ordering on the Ni and Nd sites, which are locked together in equilibrium, become decoupled during this nonthermal process.

We report here numerical results of the low-temperature behavior of a dipolar spin ice in a magnetic field along the [100] direction. Tuning the magnetic field, the system exhibits a half-magnetization plateau at low temperature. This half-polarized phase should correspond to a quantum solid phase in an effective 2D quantum boson model, and the transition from the Coulomb phase with a power-law correlation to this state can be regarded as a superfluid to a quantum solid transition. We discuss possible experimental signatures of this half-polarized state.

A pair of recent Monte Carlo studies have reported evidence for and against a crossover from weak- to strong-disorder criticality in the one-dimensional dirty boson problem. The Monte Carlo analyses rely on measurement of two observables: the effective Luttinger parameter $K_{\mathrm{eff}}$ and the superfluid susceptibility $\chi$. The former quantity was previously calculated analytically, using the strong-disorder renormalization group (SDRG), by Altman, Kafri, Polkovnikov, and Refael. Here, we use an extension of the SDRG framework to find a nonuniversal anomalous dimension ${\eta }_{\mathrm{sd}}$ characterizing the divergence of the susceptibility with system size, $\chi \sim L^{2-{\eta }_{\mathrm{sd}}}$. We show that ${\eta }_{\mathrm{sd}}$ obeys the hyperscaling relation ${\eta }_{\mathrm{sd}}=1/2K_{\mathrm{eff}}$. We also identify an important obstacle to measuring this exponent on finite-size systems and comment on the implications for numerics and experiments.

The pursuit for Majorana fermions is one of the top priorities in condensed matter physics at the moment. In this work, we propose a new method of fabricating Majorana Josephson devices in systems with a weak or no spin-orbit coupling and without external magnetic fields. Our proposal is based on curved semiconductor wires in the proximity of superconducting elements and a small number of nanomagnets. With this method it is possible to fabricate devices that are not feasible by employing straight topological wire segments. The proposed method is naturally scalable and opens up a possibility for a systematic fabrication of arrays of Majorana states where a pair of Majorana states is obtained from a single magnet.

We theoretically study the magnetic proximity effect in the three-dimensional (3D) topological insulator/ferromagnetic insulator (TI/FMI) structures in the context of possibility to manage the Dirac helical state in TI. Within a continual approach based on the $\mathbf{k}·\mathbf{p}$ Hamiltonian, we predict that, when a 3D TI is brought into contact with a 3D FMI, the ordinary bound state arising at the TI/FMI interface becomes spin polarized due to the orbital mixing at the boundary. Whereas the wave function of FMI decays into the TI bulk on the atomic scale, the induced exchange field, which is proportional to the FMI magnetization, builds up at the scale of the penetration depth of the ordinary interface state. Such an exchange field opens the gap at the Dirac point in the energy spectrum of the topological bound state existing on the TI side of the interface. We estimate the dependence of the gap size on the material parameters of the TI/FMI contact.

We study the electron spin resonance in a dilute magnon gas that is realized in a ferromagnetic spin system at low temperature. A quantum cluster expansion is developed to show that the frequency shift of the single-magnon peak changes its sign and the linewidth reaches its maximum across a scattering resonance between magnons. Such characteristic behaviors are universal and can be used to experimentally locate the two-magnon resonance when an external parameter such as pressure is varied. Future achievement of the two-magnon resonance may have an impact comparable to the Feshbach resonance in ultracold atoms and will open up a rich variety of strongly correlated physics such as the recently proposed Efimov effect in quantum magnets. We also suggest how the emergence of an Efimov state of three magnons and its binding energy may be observed with the electron spin resonance.

We introduce an efficient Langevin method to study bilinear fermionic Hamiltonians interacting with classical fields. Our approach is orders of magnitude faster than previous methods when applied to very large systems with high accuracy requirements. To demonstrate the method, we study complex noncoplanar chiral spin textures on the triangular Kondo lattice model. We also explore nonequilibrium mesoscale physics such as chiral domain coarsening and ${\mathbb{Z}}_2$ vortex annihilation.

We consider a model of the double perovskite Ba${}_2$FeReO${}_6$, a room-temperature ferrimagnet with correlated and spin-orbit-coupled Re $t$${}_{2g}$ electrons moving in the background of Fe moments stabilized by Hund's coupling. We show that for such $3d/5d$ double perovskites, strong correlations on the $5d$ element (Re) are essential in driving a half-metallic ground state. Incorporating both strong spin-orbit coupling and the Hubbard repulsion on Re leads to a band structure consistent with ab initio calculations. Using our model, we find a large spin polarization at the Fermi level and obtain a semiquantitative understanding of the saturation magnetization of Ba${}_2$FeReO${}_6$, as well as x-ray magnetic circular dichroism data indicating a significant orbital magnetization. Based on the orbital populations obtained in our theory, we predict a specific doping dependence to the tetragonal distortion accompanying ferrimagnetic order. Finally, the combination of a net magnetization and spin-orbit interactions is shown to induce Weyl nodes in the band structure, and we predict a significant intrinsic anomalous Hall effect in hole-doped Ba${}_2$FeReO${}_6$. The uncovered interplay of strong correlations and spin-orbit coupling lends partial support to our previous work, which used a local moment description to capture the spin-wave dispersion found in neutron scattering measurements. Our work is of interest in the broader context of understanding metallic double perovskites which are of fundamental importance and of possible relevance to spintronic applications.

The structure of extrinsic defects in topologically ordered states of matter is host to a rich set of universal physics. Extrinsic defects in $2+1$-dimensional topological states include linelike defects, such as boundaries between topologically distinct states, and pointlike defects, such as junctions between different line defects. Gapped boundaries in particular can themselves be topologically distinct, and the junctions between them can localize topologically protected zero modes, giving rise to topological ground-state degeneracies and projective non-Abelian statistics. In this paper, we develop a general theory of point defects and gapped line defects in $2+1$-dimensional Abelian topological states. We derive a classification of topologically distinct gapped boundaries in terms of certain maximal subgroups of quasiparticles with mutually bosonic statistics, called Lagrangian subgroups. The junctions between different gapped boundaries provide a general classification of point defects in topological states, including as a special case the twist defects considered in previous works. We derive a general formula for the quantum dimension of these point defects and a general understanding of their localized “parafermion” zero modes and we define a notion of projective non-Abelian statistics for them. The critical phenomena between topologically distinct gapped boundaries can be understood in terms of a general class of quantum spin chains or, equivalently, “generalized parafermion” chains. This provides a way of realizing exotic $1+1\mathrm{D}$ generalized parafermion conformal field theories in condensed-matter systems.

The topological insulating behavior of β-GaS and GaS-II is investigated under uniaxial tensile strains, based on ab initio calculations with the Tran–Blaha modified Becke–Johnson potential. We found that the strain-induced topological phase transition only occurs in GaS-II, notwithstanding the same space group. The distinct topological insulating behavior of the two isomers are demonstrated to originate from the variation of the ${\Gamma }_3^{+}-{\Gamma }_2^{-}$ band gap, which is a direct consequence of the uniaxial-stretching-strengthened interlayer $s$-$s$ interactions. The present finding provides a new perspective in exploring the topological behavior in materials with similar compositions and space group.

Motivated by recent experimental progress towards the measurement and manipulation of Majorana fermions with superconducting circuits, we propose a device interfacing Majorana fermions with circuit quantum electrodynamics. The proposed circuit acts as a charge parity detector changing the resonance frequency of a superconducting $\lambda /4$ resonator conditioned on the parity of charges on nearby gates. Operating at both charge and flux sweet spots, this device is highly insensitive to environmental noise. It enables high-fidelity single-shot quantum nondemolition readout of the state of a pair of Majorana fermions encoding a topologically protected qubit. Additionally, the interaction permits the realization of an arbitrary phase gate on the topological qubit, closing the loop for computational completeness. Away from the charge sweet spot, this device can be used as a highly sensitive charge detector with a sensitivity better than ${10}^{-4}\text{e}/\sqrt{\text{Hz}}$ and bandwidth larger than 1 MHz.