Using single-crystal neutron diffraction we show that the magnetic structure ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ at fields above 8.6 T along the $c$ axis and low temperature changes from a commensurate collinear antiferromagnetic structure with spins along $c$ and ordering vector $Q_{\mathrm{C}}=\left(001.5\right)$ to a conical spiral with propagation vector $Q_{\mathrm{IC}}=(001.5\pm \delta ), \delta \sim 0.18$, having a significant spin component in the $(a,b)$ plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along $c$ and show the phase transition between the low field and conical spiral phases is of first order by observing a discontinuous jump of the ordering vector. $Q_{\mathrm{IC}}$ is found to drift both as a function of magnetic field and temperature. Preliminary inelastic neutron-scattering data reveal that the spin-wave gap in zero field has minima exactly at $Q_{\mathrm{IC}}$ and a gap of about 1.1 meV consisting with a crossover around 8.6 T. Further, a simple magnetic Hamiltonian accounting in broad terms for these is presented. Our findings confirm the exclusion of the inverse Dzyaloshinskii-Moriya interaction as a cause for the giant magnetoelectric due to symmetry arguments. In its place we advocate for the symmetric exchange striction as the origin of this effect.

The temperature dependent measurements of the thermodynamic critical field and the specific heat for the pressure stabilized Ga-II phase of elemental gallium are presented. The discussion of these and other Ga phases data in the context of elemental and binary phonon-mediated superconductors allowed to establish simple scaling relations between BCS quantities such as $B_{\mathrm{c}}\left(0\right)/T_{\mathrm{c}}\sqrt{{\gamma }_{\mathrm{e}}}$ and the specific heat jump at $T_{\mathrm{c}}$ versus the coupling strength $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{c}}$ [$\mathrm{\Delta }$ and $B_{\mathrm{c}}\left(0\right)$ are the zero-temperature values of the superconducting energy gap and the thermodynamic critical field, respectively, $T_{\mathrm{c}}$ is the transition temperature, and ${\gamma }_{\mathrm{e}}$ is the electronic specific heat]. The scaling relations can be analytically expressed by taking into account strong-coupling corrections to BCS theory. Such correlations can naturally explain the linear relation between $B_{\mathrm{c}}\left(0\right)$ and $T_{\mathrm{c}}$, which holds for conventional superconducting materials.

The first-order charge density wave (CDW) phase transition of ${\mathrm{Er}}_2{\mathrm{Ir}}_3{\mathrm{Si}}_5$ is characterized by a crystal structure analysis, and electrical resistivity, magnetic susceptibility and specific heat measurements. The incommensurate CDW is accompanied by a strong lattice distortion, from which it is shown that the CDW resides on zigzag chains of iridium atoms. The CDW transition affects the magnitude of the local magnetic moments on ${\mathrm{Er}}^{3+}$, implying strong coupling between CDW and magnetism. This could account for the observation that magnetic order is suppressed down to at least 0.1 K in the high-quality single crystal presently studied. Any disorder in the crystallinity, as in ceramic material, broadens and suppresses the CDW transition, while magnetic order appears at 2.1 K.

We perform a direct comparison between Kramers' method in many dimensions, i.e., Langer's theory, adapted to magnetic spin systems, and a path sampling method in the form of forward flux sampling, as a means to compute the collapse rates of metastable magnetic skyrmions. We show that a good agreement is obtained between the two methods. We report variations of the attempt frequency associated with skyrmion collapse by three to four orders of magnitude when varying the applied magnetic field by $5\%$ of the exchange strength, which confirms the existence of a strong entropic contribution to the lifetime of skyrmions. This demonstrates that in complex systems, the knowledge of the rate prefactor, in addition to the internal energy barrier, is essential in order to properly estimate a lifetime.

Topological magnetic states are promising for ultradense memory and logic devices. Recent progress in two-dimensional magnets encourages the idea to realize topological states, such as skyrmions and merons, in freestanding monolayers. However, monolayers such as ${\mathrm{CrI}}_3$ lack Dzyaloshinskii-Moriya interactions (DMIs) and thus do not naturally exhibit skyrmions/merons but rather a ferromagnetic state. Here we propose the fabrication of $\mathrm{Cr}{(\mathrm{I},X)}_3$ Janus monolayers, in which the Cr atoms are covalently bonded to the underlying I ions and top-layer Br or Cl atoms. By performing first-principles calculations and Monte Carlo simulations, we identify strong enough DMIs, which leads to not only helical cycloid phases, but also to topologically nontrivial states, such as the intrinsic domain wall skyrmions in $\mathrm{Cr}{(\mathrm{I},\mathrm{Br})}_3$ and the magnetic-field-induced bimerons in $\mathrm{Cr}{(\mathrm{I},\mathrm{Cl})}_3$. Microscopic origins of such spin textures are revealed as well.

Externally driven glassy systems may undergo nonequilibrium phase transitions (NEPTs). In particular, ac-driven systems display special features, such as those observed in the vortex matter of ${\mathrm{NbSe}}_2$, where oscillatory drives reorganize the system into partially ordered vortex lattices. We provide experimental evidence for this dynamic reorganization and we show an unambiguous signature of its connection with an NEPT driven by ac forces. We perform a scaling analysis and we estimate critical exponents for this transition. Our results share similarities with some glass-to-viscous-liquid NEPTs and invite a search for similar physics in other elastic disordered media.

We observe the onset of nonergodicity from ballistic propagation of a two-body bound state in an interacting discrete-time quantum walk (DTQW) due to time-dependent disorder in the interaction. The effect of the disorder on the two-body state can be interpreted as Anderson-type localization in a projected DTQW, but without saturation of entanglement entropy. We characterize the two-body localization in terms of the rate of growth of entanglement entropy and compute the localization length. We find indications for two distinct growth laws for the entanglement: A logarithmic law in the delocalized phase and a double-logarithmic law in the localized phase. We discuss similarities with many-body localization.

The stability of two-dimensional chiral skyrmions in a tilted magnetic field is studied. It is shown that by changing the direction and magnitude of the field, one can continuously transform a chiral skyrmion into a skyrmion with opposite polarity and vorticity. This turned inside out skyrmion can be considered as an antiparticle for an ordinary axisymmetric skyrmion. For any tilt angle of the magnetic field, there is a range of its absolute values where two types of skyrmions may coexist. In a tilted field the potentials for interskyrmion interactions are characterized by the presence of local minima suggesting an attractive interaction between the particles. The potentials of interparticle interactions also have so-called fusion channels allowing either the annihilation of two particles or the emergence of a another particle. The presented results are general for a wide class of magnetic crystals with both easy-plane and easy-axis anisotropies.

We investigate the effect of Ni doping on the Fe site in single crystals of the magnetic superconductor $\mathrm{Rb}\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{As}}_4$ for doping concentrations of up to 4%. A clear suppression in the superconducting transition temperature is observed in specific-heat, resistivity, and magnetization measurements. Upon Ni doping, the resistivity curves shift up in a parallel fashion, indicating a strong increase of the residual resistivity due to scattering by charged dopant atoms while the shape of the curve and thus the electronic structure appear largely unchanged. The observed step $\mathrm{\Delta }C/T_c$ at the superconducting transition decreases strongly for increasing Ni doping in agreement with expectations based on a model of multiband superconductivity and strong interband pairing. The upper critical field slopes are reduced upon Ni doping for in-plane as well as out-of-plane fields, leading to a small reduction in the superconducting anisotropy. The specific-heat measurements of the magnetic transition reveal the same Berezinskii-Kosterlitz-Thouless behavior close to the transition temperature $T_m$ for all doping levels. The transition temperature is essentially unchanged upon doping. The in-plane to out-of-plane anisotropy of Eu magnetism observed at small magnetic fields is unaltered as compared to the undoped compound. All of these observations indicate a decoupling of the Eu magnetism from superconductivity and essentially no influence of Ni doping on the Eu magnetism in this compound.

A universal feature of topological insulators is that they cannot be adiabatically connected to an atomic limit, where individual lattice sites are completely decoupled. This property is intimately related to a topological obstruction to constructing a localized Wannier function from Bloch states of an insulator. Here, we generalize this characterization of topological phases toward periodically driven systems. We show that nontrivial connectivity of hybrid Wannier centers in momentum space and time can characterize various types of topology in periodically driven systems, which include Floquet topological insulators, anomalous Floquet topological insulators with micromotion-induced boundary states, and gapless Floquet states realized with topological Floquet operators. In particular, nontrivial time dependence of hybrid Wannier centers indicates impossibility of continuous deformation of a driven system into an undriven insulator, and a topological Floquet operator implies an obstruction to constructing a generalized Wannier function which is localized in real and frequency spaces. Our results pave a way to a unified understanding of topological states in periodically driven systems as a topological obstruction in Floquet states.

We present a method to treat the two-dimensional (2D) Hubbard model for parameter regimes which are relevant for the physics of the high-$T_c$ superconducting cuprates. Unlike previous attempts to attack this problem, our approach takes into account all fluctuations in different channels on equal footing and is able to treat reasonable large lattice sizes up to $32\times 32$. This is achieved by the following three-step procedure: (i) We transform the original problem to a representation (dual fermions) in which all purely local correlation effects from the dynamical mean field theory are already considered in the bare propagator and bare interaction of the new problem. (ii) The strong $1/{\left(i\nu \right)}^2$ decay of the bare propagator allows us to integrate out all higher Matsubara frequencies aside from the lowest using low-order diagrams. The new effective action depends only on the two lowest Matsubara frequencies which allows us to (iii) apply the two-particle self-consistent parquet formalism, which takes into account the competition between different low-energy bosonic modes in an unbiased way, on much finer momentum grids than usual. In this way, we were able to map out the phase diagram of the 2D Hubbard model as a function of temperature and doping. Consistently with the experimental evidence for hole-doped cuprates and previous dynamical cluster approximation calculations, we find an antiferromagnetic region at low doping and a superconducting dome at higher doping. Our results also support the role of the van Hove singularity as an important ingredient for the high value of $T_c$ at optimal doping. At small doping, the destruction of antiferromagnetism is accompanied by an increase of charge fluctuations supporting the scenario of a phase-separated state driven by quantum critical fluctuations.

When semiconductors are optically excited above or slightly below the band gap, the linear and nonlinear responses originate predominantly from the interband polarization. Here, we demonstrate that intraband excitations, i.e., rapidly oscillating currents that originate from the electric-field-induced acceleration of electrons and holes, contribute strongly to transient four-wave mixing when it is performed with center frequencies near half the band gap frequency. Within a two-band model we show that the presence of several pathways arising from different combinations of inter- and intraband excitations and their interference give rise to characteristic signatures in time- and spectrally resolved signals. Our approach is based on the semiconductor Bloch equations and includes the dynamics of off-resonant electron-hole excitations on a microscopic level. The predicted significant broadening and structure appearing in the four-wave-mixing spectra are in good qualitative agreement with experimental results.

We consider the relaxation dynamics of two spins coupled to a common bosonic bath. The time evolution is simulated by a generalized master equation derived within a real-time diagrammatic approach. Interference effects due to the coherent coupling to the common bath give rise to characteristic features in the relaxation dynamics after a quench or during a periodic external driving. In particular, we find that the long-time behavior during periodic driving depends sensitively on the initial state as well as on system parameters such as coupling asymmetries. When coupled to more than a single reservoir, the interference effects can lead to a cooling mechanism for one of the bosonic reservoirs.

We investigate a Rabi-Kondo model describing an optically driven two-channel quantum dot device featuring a non-Fermi-liquid Kondo effect. Optically induced Rabi oscillation between the valence and conduction levels of the dot gives rise to a two-stage Kondo effect: Primary screening of the local spin is followed by secondary nonequilibrium screening of the local orbital degree of freedom. Using bosonization arguments and the numerical renormalization group, we compute the dot emission spectrum and residual entropy. Remarkably, both exhibit two-stage Kondo screening with non-Fermi-liquid properties at both stages.

The average spectrum method is a promising approach for the analytic continuation of imaginary time or frequency data to the real axis. It determines the analytic continuation of noisy data from a functional average over all admissible spectral functions, weighted by how well they fit the data. Its main advantage is the apparent lack of adjustable parameters and smoothness constraints, using instead the information on the statistical noise in the data. Its main disadvantage is the enormous computational cost of performing the functional integral. Here we introduce an efficient implementation, based on the singular value decomposition of the integral kernel, eliminating this problem. It allows us to analyze the behavior of the average spectrum method in detail. We find that the discretization of the real-frequency grid, on which the spectral function is represented, biases the results. The distribution of the grid points plays the role of a default model while the number of grid points acts as a regularization parameter. We give a quantitative explanation for this behavior, point out the crucial role of the default model and provide a practical method for choosing it, making the average spectrum method a reliable and efficient technique for analytic continuation.

We investigate the proximity-induced exchange coupling in transition-metal dichalcogenides (TMDCs), originating from spin injector geometries composed of hexagonal boron-nitride ($h\mathrm{BN}$) and ferromagnetic (FM) cobalt (Co) or nickel (Ni), from first principles. We employ a minimal tight-binding Hamiltonian that captures the low energy bands of the TMDCs around $K$ and $K^{\prime }$ valleys, to extract orbital, spin-orbit, and exchange parameters. The TMDC/$h\mathrm{BN}$/FM heterostructure calculations show that due to the $h\mathrm{BN}$ buffer layer, the band structure of the TMDC is preserved, with an additional proximity-induced exchange splitting in the bands. We extract proximity exchange parameters in the 1–10 meV range, depending on the FM. The combination of proximity-induced exchange and intrinsic spin-orbit coupling (SOC) of the TMDCs, leads to a valley polarization, translating into magnetic exchange fields of tens of Tesla. The extracted parameters are useful for subsequent exciton calculations of TMDCs in the presence of a $h\mathrm{BN}$/FM spin injector. Our calculated absorption spectra show large splittings for the exciton peaks; in the case of ${\mathrm{MoS}}_2/h\mathrm{BN}$/Co we find a value of about 8 meV, corresponding to about 50 T external magnetic field in bare TMDCs. The reason lies in the band structure, where a hybridization with Co $d$ orbitals causes a giant valence band exchange splitting of more than 10 meV. Structures with Ni do not show any $d$ level hybridization features, but still sizable proximity exchange and exciton peak splittings of around 2 meV are present in the TMDCs.

The possibility of inducing superconductivity in type-I Weyl semimetal through coupling its surface to a superconductor was investigated. A single crystal of NbP, grown by chemical vapor transport method, was carefully characterized by x-ray diffraction, scanning electron microscopy and energy-dispersive x-ray spectroscopy, and angle-resolved photoelectron spectroscopy techniques and by electron transport measurements. The mobility spectrum of the carriers was determined and it was found that there are four separate sharp peaks visible, which indicates that the carriers participating in the conductance have four different (almost discrete) mobilities. For the studies of interface transmission, the (001) surface of the crystal was covered by several hundred nanometer-thick metallic layers of either Pb, or Nb, or In. DC current-voltage characteristics and AC differential conductance through the interfaces as a function of the DC bias were investigated. Upon cooling of the devices during which the metals become superconducting, all three types of junctions show conductance increase, pointing out the Andreev reflection as a prevalent contribution to the subgap conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is satisfactorily described by modified Blonder-Tinkham-Klapwijk model. Unfortunately, the absolute value of the conductance is much smaller than that for the bulk crystal, indicating that the transmission occurs through only a small part of the contact area. An opposite situation occurs in In-NbP junction, where we observe very high and narrow peak at zero bias. The conductance at the peak reaches the bulk value, indicating that almost the whole contact area is transmitting and, additionally, a superconducting proximity phase is formed in the material. We interpret this as a result of indium diffusion into NbP, where the metal atoms penetrate the surface barrier and form very transparent superconductor-Weyl semimetal contact inside. However, further diffusion occurring already at room temperature leads to degradation of the effect, so it is observed only in the pristine structures. Despite this, our observation directly demonstrates possibility of inducing superconductivity in a type-I Weyl semimetal.

In this work we propose a two-dimensional carbon-nitrogen allotrope composed of pentagons and octagons in a structure of carbon chains bound by nitrogen dimers. Our first-principles calculations show the system behaves as a small-gap semiconductor which is thermodynamically stable at room temperature confirmed by molecular dynamic simulations. By cutting the sheet along different crystallographic directions, a variety of metallic or semiconductor one-dimensional ribbons can be obtained. We explore the topological properties of these nanoribbons finding that for a particular boundary geometry nontrivial topological phases can be obtained, depending on the ribbon termination. We investigate the emergence of symmetry protected interface topological states in heterojunctions and topological-induced edge states in finite ribbons.