Charge conserving spin singlet and spin triplet superconductors in one dimension are described by the $U\left(1\right)$ symmetric Thirring Hamiltonian. We solve the model with open boundary conditions on a finite line segment by means of the Bethe ansatz. We show that the ground state displays a fourfold degeneracy when the bulk is in the spin triplet superconducting phase. This degeneracy corresponds to the existence of zero energy boundary bound states localized at the edges which may be interpreted, in the light of the previous semiclassical analysis due to Kesselman and Berg, as resulting from the existence of fractional spin $\pm 1/4$ localized at the two edges of the system.

We study interaction-induced Mott insulators, and their topological properties in a one-dimensional non-Hermitian strongly correlated spinful fermionic superlattice system with either nonreciprocal hopping or complex-valued interaction. For the nonreciprocal hopping case, the low-energy neutral excitation spectrum is sensitive to boundary conditions, which is a manifestation of the non-Hermitian skin effect. However, unlike the single-particle case, particle density of strongly correlated system does not suffer from the non-Hermitian skin effect due to the Pauli exclusion principle and repulsive interactions. Moreover, the anomalous boundary effect occurs due to the interplay of nonreciprocal hopping, superlattice potential, and strong correlations, where some in-gap modes, for both the neutral and charge excitation spectra, show no edge excitations defined via only the right eigenvectors. We show that these edge excitations of the in-gap states can be correctly characterized by only biorthogonal eigenvectors. Furthermore, the topological Mott phase, with gapless particle excitations around boundaries, exists even for the purely imaginary-valued interaction, where the continuous quantum Zeno effect leads to the effective on-site repulsion between two-component fermions.

We report the Raman spectroscopy of ${}^{12}\mathrm{C}$/${}^{13}\mathrm{C}$ graphene isotope superlattices (SLs) synthesized by chemical vapor deposition. At large periods the Raman spectrum corresponds to the sum of the bulk ${}^{12}\mathrm{C}$ and ${}^{13}\mathrm{C}$ contributions. However, at small periods we observe the formation of mixed ${}^{12}\mathrm{C}$/${}^{13}\mathrm{C}$ modes for Raman processes that involve two phonons, which results in the tripling of the $2D$ and $2D^{\prime }$ Raman peaks. This tripling can be well understood in the framework of real-space Raman spectroscopy, where the two emitted phonons stem from different regions of the SL. The intensity of the mixed peak increases as the SL half-period approaches the mean free path of the photoexcited electron-hole pairs. By varying the SL period between 6 and 225 nm we have a direct measure of the photoexcited electron mean free path, which is found to be 18 nm for suspended graphene and 7 nm for graphene on $\mathrm{Si}{\mathrm{O}}_2$ substrates.

The Eilenberger equation is a standard tool in the description of superconductors with an arbitrary degree of disorder. It can be generalized to systems with linear-in-momentum spin-orbit coupling (SOC), by exploiting the analogy of SOC with a non-Abelian background field. Such a field mixes singlet and triplet components and yields the rich physics of magnetoelectric phenomena. In this work we show that the application of this equation extends further, beyond superconductivity. In the normal state, the linearized Eilenberger equation describes the coupled spin-charge dynamics. Moreover, its resolvent corresponds to the so-called Cooperons, and can be used to calculate the weak-localization corrections. Specifically, we show how to solve this equation for any source term and provide a closed-form solution for the case of Rashba SOC. We use this solution to address several problems of interest for spintronics and superconductivity. First, we study spin injection from ferromagnetic electrodes in the normal state, and describe the spatial evolution of spin density in the sample, and the complete crossover from the diffusive to the ballistic limit. Second, we address the so-called superconducting Edelstein effect, and generalize the previously known results to arbitrary disorder. Third, we study weak-localization correction beyond the diffusive limit, which can be a valuable tool in experimental characterization of materials with very strong SOC. We also address the so-called pure gauge case where the persistent spin helices form. Our work establishes the linearized Eilenberger equation as a powerful and a very versatile method for the study of materials with spin-orbit coupling, which often provides a simpler and more intuitive picture compared to alternative methods.

We develop a theoretical method to calculate x-ray magnetic circularly polarized emission (XMCPE) spectra for itinerant ferromagnets, in particular, $K\alpha$ emission for transition metals. The effects of electron excitations in the $d$ bands are treated by means of the Keldysh Green's function. In our theoretical framework, the ferromagnetic ground state and the excitations are described by a mean-field approximation and a single-bubble diagram, respectively. We apply our method to $K\alpha$ XMCPE spectra of metallic iron, and reproduce the observed tail structures on the low-energy side of each peak. Contributions from the excitations in the $d$ bands depend strongly on the exchange interaction as well as the direct interaction between the $2p$ and $3d$ electrons. The dependence of the spectral intensity on emission angles with respect to the magnetization direction is cosinusoidal, indicating that $K\alpha$ XMCPE can be used to investigate the magnetization directions in bulk ferromagnets.

The layered $\mathrm{Mn}{\mathrm{Bi}}_{2n}{\mathrm{Te}}_{3n+1}$ family represents the first intrinsic antiferromagnetic (AFM) topological insulator (protected by a combination symmetry $S$) ever discovered, providing an ideal platform to explore novel areas of physics such as the quantum anomalous Hall effect at elevated temperature and axion electrodynamics. Some of the recent angle-resolved photoemission spectroscopy (ARPES) experiments on this family have revealed that all terminations exhibit (nearly) gapless topological surface states (TSSs) in the AFM state. The gapless behavior is inconsistent with the theoretical expectation, as the surfaces being studied are $S$-breaking and shall therefore open a gap. Here we explain this curious paradox using a surface-bulk band hybridization picture. Combining circular dichroism ARPES and first-principles calculations on $\mathrm{Mn}{\mathrm{Bi}}_6{\mathrm{Te}}_{10}$, we prove that gaplike features are induced through hybridization between TSSs and certain bulk bands with Rashba character. The observed (nearly) gapless features are consistently reproduced by tight-binding simulations where TSSs are coupled to a pair of Rashba-split bands (RSBs). The Dirac-cone-like spectral features actually originate from the RSBs. Our findings highlight the role of band hybridization, superior to magnetism in this case, in shaping the general surface band structure in this family of magnetic topological materials.

Crystals form regular and robust structures that under extreme conditions can melt and recrystallize into different arrangements in a process that is called crystal metamorphism. While crystals exist due to the breaking of a continuous translation symmetry in space, it was recently proposed that discrete crystalline order can also emerge in time and give rise to a novel phase of matter named the discrete time crystal (DTC). In this paper, we join these two ideas and propose a model for quantum metamorphism between two DTCs with different periodicities, $2T$- and $4T$-DTCs. In our model the conditions for metamorphism come from the modulation of perturbative terms in the $4T$-DTC Hamiltonian that gradually melt its structure and transform it into a $2T$-DTC. This process is studied in detail from the viewpoint of many-body physics of periodically driven systems. We also propose a protocol to experimentally observe quantum metamorphism using current quantum technology.

In this paper we present a systematic study of the de Haas–van Alphen (dHvA) oscillations, magnetotransport, and superconductivity in the Kramers-Weyl semimetal candidate ${\mathrm{NbGe}}_2$. We show that ${\mathrm{NbGe}}_2$ is a type-I chiral superconductor with a $T_c$ of 2.06 K, a full gap, a type-II to type-I crossover around 1.5 K, and an enhanced critical field from filamentary superconductivity suggested by resistivity measurements. The study of the dHvA oscillations reveals three distinct frequency branches $\beta$ and the spin-split $\alpha$/${\alpha }^{\prime }$, with cyclotron effective masses of 1.21(5)$m_e$, 0.21(1)$m_e$, and 0.24(1)$m_e$, respectively. The magnetoresistance of ${\mathrm{NbGe}}_2$ exceeds $1000\%$ at 2.4 K and 9 T and exhibits quasilinear behavior at low temperatures. The comparison between experimental results and first-principles calculations shows excellent agreement between the two and suggests that $\alpha$ and ${\alpha }^{\prime }$ correspond to the hole Fermi pockets centered around the Weyl $M$ point, while $\beta$ is associated with the hole pocket near the trivial $H$ point in the Brillouin zone. Furthermore, we show that a Van Hove singularity arising from the contributions of the $M$ and $H$ points at the Fermi level, together with the calculated electron-phonon coupling, is strong enough to account for the observed superconductivity. Last, our calculation of the conductivity tensor within the relaxation time approximation suggests that the observed linear magnetoresistance most likely arises from the presence of open Fermi surfaces.

Transition-metal dichalcogenide (TMD) bilayers are a new class of tunable moiré systems attracting interest as quantum simulators of strongly interacting electrons in two dimensions. In particular, recent theory predicts that the correlated insulator observed in ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ at half filling is a charge-transfer insulator similar to cuprates and, upon further hole doping, exhibits a transfer of charge from anionlike to cationlike orbitals at different locations in the moiré unit cell. In this work, we demonstrate that in this doped charge-transfer insulator, tightly bound charge-$2e$ excitations can form to lower the total electrostatic repulsion. This composite excitation, which we dub a trimer, consists of a pair of holes bound to a charge-transfer exciton. When the bandwidth of doped holes is small, trimers crystallize into insulating pair density waves at a sequence of commensurate doping levels. When the bandwidth becomes comparable to the pair binding energy, itinerant holes and charge-$2e$ trimers interact resonantly, leading to unconventional superconductivity similar to superfluidity in an ultracold Fermi gas near Feshbach resonance. Our theory is broadly applicable to strongly interacting charge-transfer insulators, such as ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ or TMD homobilayers under an applied electric field.

The spin and orbital freedoms of electrons traveling on spin-resolved quantum Hall edge states (quantum Hall ferromagnets) are maximally entangled. The unitary operations on these two freedoms are hence equivalent, which means that one can manipulate the spins with nonmagnetic methods through the orbitals. Taking the quantization axis of the spins along the magnetization axis, the zenith angle is determined by the partition rate of spin-separated edges, while the azimuth angle is defined as the phase difference between the edges. Utilizing these properties, we have realized an electrically controlled unitary operation on the electron spins on quantum Hall ferromagnets. The zenith angle of the spin was controlled through the radius of gyration at a corner by applying voltage to a thin gate placed at one edge. The subsequent rotation in the azimuth angle was controlled via the distance between the edge channels also by a gate voltage. The combination of the two operations constitutes a systematic electric operation on spins in quantum Hall edge channels.

The osmium-hydrogen system was studied at pressures up to 186 GPa by in situ synchrotron x-ray diffraction in a diamond anvil cell. Hydrogen solubility in the hexagonal close-packed (hcp) osmium metal at room temperature was found to be negligibly small in the studied pressure range. After laser heating of the osmium sample at the maximal ${\mathrm{H}}_2$ pressure, it transformed to hydride with a face-centered cubic (fcc) metal lattice. The equation of state $V\left(P\right)$ for this hydride was then measured at room temperature and decreasing pressure. The hydrogen-induced volume expansion of the Os lattice proved to be weakly dependent on the pressure, and its estimated value of $1.35{\AA }^3/\mathrm{Os}$ atom at 100 GPa suggested the formation of monohydride OsH with hydrogen atoms occupying all octahedral interstices in its fcc metal lattice. The OsH sample began to gradually lose hydrogen at pressures below about 55 GPa and completely decomposed to hcp-Os and molecular ${\mathrm{H}}_2$ at about 25 GPa.

We analyze the weak-coupling instabilities that may arise when multiple high-order Van Hove points are present inside the Brillouin zone. The model we consider is inspired by twisted bilayer graphene, although the analysis should be more generally applicable. We employ a parquet renormalization group analysis to identify the leading weak-coupling instabilities, supplemented with a Ginzburg-Landau treatment to resolve any degeneracies. Hence we identify the leading instabilities that can occur from weak repulsion with the power-law divergent density of states. Five correlated phases are uncovered along distinct stable fixed trajectories, including $s$-wave ferromagnetism, $p$-wave chiral/helical superconductivity, $d$-wave chiral superconductivity, $f$-wave valley-polarized order, and $p$-wave polar valley-polarized order. The phase diagram is stable against band deformations which preserve the high-order Van Hove singularity.

We report the observation of magnon spin currents generated by the spin Seebeck effect (SSE) in a bulk single crystal of the easy-plane antiferromagnet NiO. A magnetic field induces a nondegeneracy and thereby an imbalance in the population of magnon modes with opposite spins. A temperature gradient then gives rise to a nonzero magnon spin current. This SSE is measured in both a local and a nonlocal geometry at 5 K in bulk NiO. The magnetic field dependence of the obtained signal is modeled by magnetic field splitting of the low-energy magnon modes, affecting the spin Seebeck coefficient. The relevant magnon modes at this temperature are linked to cubic anisotropy and magnetic dipole-dipole interactions. The nonlocal signal deviates from the expected quadratic Joule heating by saturating at a current from around $75\mu \mathrm{A}$ in the injector. The magnon chemical potential does not decay exponentially with distance and inhomogeneities may be the result of local magnon accumulations.

Berry phase is a powerful concept that unravels the nontrivial role of topology in phenomena observed in chiral magnetic materials and structures. A celebrated example is the anomalous Hall effect (AHE) driven by the nonvanishing Berry phase in momentum space. As the AHE is dependent on details of the band structure near the Fermi edge, the Berry phase and AHE can be altered in thin films whose chemical potential is tunable by dimensionality and disorder. Here, we demonstrate how Berry phase in ultrathin ${\mathrm{SrRuO}}_3$ films provides a comprehensive explanation for the nontrivial Hall response which is conventionally attributed to the topological Hall effect (THE). Particularly, the Berry phase contribution to this effect can be altered, enhanced, and even change signs in response to the number of layers, temperature, and importantly, disorder. By comparing the effects of disorder theoretically on a skyrmion model and a spin-orbit coupled model, we show that disorder suppresses the THE while it can enhance the AHE. The experiments on more disordered samples confirm this interpretation, and proposed multichannel analysis judiciously explains the observed THE-like feature.

The Fermi-Hubbard model is of fundamental importance in condensed-matter physics, yet is extremely challenging to solve numerically. Finding the ground state of the Hubbard model using variational methods has been predicted to be one of the first applications of near-term quantum computers. Here we carry out a detailed analysis and optimization of the complexity of variational quantum algorithms for finding the ground state of the Hubbard model, including costs associated with mapping to a real-world hardware platform. The depth complexities we find are substantially lower than previous work. We performed extensive numerical experiments for systems with up to 12 sites. The results suggest that the variational ansätze we used—an efficient variant of the Hamiltonian variational ansatz and a generalization thereof—will be able to find the ground state of the Hubbard model with high fidelity in relatively low quantum circuit depths. Our experiments include the effect of realistic measurements and depolarizing noise. If our numerical results on small lattice sizes are representative of the somewhat larger lattices accessible to near-term quantum hardware, they suggest that optimizing over quantum circuits with a gate depth less than a thousand could be sufficient to solve instances of the Hubbard model beyond the capacity of classical exact diagonalization.

We study aspects of the Berry phase in gapped many-body quantum systems by means of effective field theory. Once the parameters are promoted to space-time-dependent background fields, such adiabatic phases are described by Wess-Zumino-Witten (WZW) and similar terms. In the presence of symmetries, there are also quantized invariants capturing generalized Thouless pumps. Consideration of these terms provides constraints on the phase diagram of many-body systems, implying the existence of gapless points in the phase diagram, which are stable for topological reasons. We describe such diabolical points, realized by free fermions and gauge theories in various dimensions, which act as sources of “higher Berry curvature” and are protected by the quantization of the corresponding WZW terms or Thouless pump terms. These are analogous to Weyl nodes in a semimetal band structure. We argue that in the presence of a boundary, there are boundary diabolical points—parameter values where the boundary gap closes—which occupy arcs ending at the bulk diabolical points. Thus the boundary has an “anomaly in the space of couplings” in the sense of [C. Cordova, D. S. Freed, H. T. Lam, and N. Seiberg, SciPost Phys. 8, 001 (2020) and SciPost Phys. 8, 002 (2020)]. Consideration of the topological effective action for the parameters also provides some new checks on conjectured infrared dualities and deconfined quantum criticality in 2+1d.

We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure, ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ and ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$. Upon cooling below 80 K, ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ simultaneously undergoes a metal-to-insulator transition and develops antiferromagnetic order. ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structures. The Os atoms in ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ are arranged on an approximately face-centered cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ is heavily distorted up to at least 500 K including antiferroelectriclike displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which, in turn, drives the metal-insulator transition. Our results suggest that the phase transition in ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ is spin driven and could be a rare example of a Slater transition.

We study the transport of classical waves through three-dimensional (3D) anisotropic media close to the Anderson localization transition. Time-, frequency-, and position-resolved ultrasonic measurements are performed on anisotropic slab-shaped mesoglass samples to probe the dynamics and the anisotropy of the multiple scattering halo, and hence to investigate the influence of disorder on the nature of wave transport and its anisotropy. These experiments allow us to address conflicting theoretical predictions that have been made about whether or not the transport anisotropy is affected by the interference effects that lead to Anderson localization. We find that the transport anisotropy is significantly reduced as the mobility edge is approached—a behavior similar to the one predicted recently for matter waves in infinite anisotropic 3D media.

The concept of geometrical frustration has led to rich insights into condensed matter physics, especially as a mechanism to produce exotic low-energy states of matter. Here we show that frustration provides a natural vehicle to generate models exhibiting anomalous thermalization of various types within high-energy states. We consider three classes of nonintegrable frustrated spin models: (I) systems with local conserved quantities where the number of symmetry sectors grows exponentially with the system size but more slowly than the Hilbert space dimension, (II) systems with exact eigenstates that are singlet coverings, and (III) flatband systems hosting magnon crystals. We argue that several one- and two-dimensional models from class I exhibit disorder-free localization in high-energy states so that information propagation is dynamically inhibited on length scales greater than a few lattice spacings. We further show that models of classes II and III exhibit quantum many-body scars: eigenstates of nonintegrable Hamiltonians with finite-energy density and anomalously low-entanglement entropy. Our results demonstrate that magnetic frustration supplies a means to systematically construct classes of nonintegrable models exhibiting anomalous thermalization in mid-spectrum states.

It is generally accepted that the effective magnetic field acting on a magnetic moment is given by the gradient of the energy with respect to the magnetization. However, in ab initio spin dynamics within the adiabatic approximation, the effective field is also known to be exactly the negative of the constraining field, which acts as a Lagrange multiplier to stabilize an out-of-equilibrium, noncollinear magnetic configuration. We show that for Hamiltonians without mean-field parameters both of these fields are exactly equivalent, while there can be a finite difference for mean-field Hamiltonians. For density functional theory (DFT) calculations, the constraining field obtained from the auxiliary Kohn-Sham Hamiltonian is not exactly equivalent to the DFT energy gradient. This inequality is highly relevant for both ab initio spin dynamics and the ab initio calculation of exchange constants and effective magnetic Hamiltonians. We argue that the effective magnetic field and exchange constants have the highest accuracy in DFT when calculated from the energy gradient and not from the constraining field.