We report a ${}^{35}\mathrm{Cl}$ nuclear magnetic resonance (NMR) study of the diluted Kitaev material $\alpha \text{−}{\mathrm{Ru}}_{1-x}{\mathrm{Ir}}_x{\mathrm{Cl}}_3$ ($x=0.1$ and 0.2) where nonmagnetic ${\mathrm{Ir}}^{3+}$ dopants substitute ${\mathrm{Ru}}^{3+}$ ions. Upon dilution, the ${}^{35}\mathrm{Cl}$ spectra exhibit unusual large magnetic inhomogeneity, which sets in at temperatures below the Kitaev exchange energy scale. At the same time, the ${}^{35}\mathrm{Cl}$ spin-lattice relaxation rate $T_1^{-1}$ as a function of dilution and magnetic field unravels a critical doping of $x_c\approx 0.22$, towards which both the field-induced spin gap and the zero-field magnetic ordering are simultaneously suppressed, while novel gapless low-energy spin excitations dominate the relaxation process. These NMR findings point to the stabilization of a random singlet phase in $\alpha \text{−}{\mathrm{Ru}}_{1-x}{\mathrm{Ir}}_x{\mathrm{Cl}}_3$, arising from the interplay of dilution and exchange frustration in the quantum limit.

In contrast to all other members of the $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ family with nonzero $4f$ electrons ($R$ = Nd to Lu), ${\mathrm{PrMn}}_2{\mathrm{O}}_5$ does not show any spin driven ferroelectricity in the magnetically ordered phase. By means of high-field electric polarization measurements up to 45 T, we have found that this exceptional candidate undergoes a spin driven multiferroic phase under magnetic field. X-ray magnetic circular dichroism studies up to 30 T at the Pr $L_2$ edge show that this ferroelectricity originates from and directly couples to the ferromagnetic component of the ${\mathrm{Pr}}^{3+}$ spins. Experimental observations along with our generalized gradient-approximation $+U$ calculations reveal that this exotic ferroelectric-ferromagnetic combination stabilizes through the exchange-striction mechanism solely driven by a $3d-4f$-type coupling, as opposed to the other $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ members with $3d-3d$ driven ferroelectric-antiferromagnetic-type conventional type-II multiferroicity.

UCoGa and URhGa, two isostructural compounds, show opposite signs of the initial response of Curie temperature to applied hydrostatic pressure. To determine the physical origin of this difference the magnetization data measured with respect to temperature, magnetic field, and hydrostatic pressure were analyzed in the framework of the Takahashi spin-fluctuation theory. The parameters $T_0$ and $T_{\mathrm{A}}$ characterizing the distribution widths of the spin-fluctuation spectrum in the energy and wave-vector space, respectively, and $T_{\mathrm{C}}/T_0$, the degree of the $5f$-electron localization, have been determined. Examination of available experimental data for the other UTX ($T=\mathrm{a}$ transition metal, $X=\mathrm{Al}$, Ga) ferromagnets having the ZrNiAl-type structure revealed some correlations between the degree of the $5f$-electron localization represented by the spin-fluctuation parameters and the response of Curie temperature on the applied pressure. The extent to which this correlation can be used to describe the localization and magnetic behavior of other uranium ferromagnetic compounds is discussed.

The nature of symmetry-protected topological phases of Heisenberg spin chains in totally symmetric representations of rank $N$ of the $\mathrm{SU}(N$) group is investigated through a Majorana fermion study starting from an integrable point. The latter approach generalizes the one pioneered by Tsvelik [Phys. Rev. B 42, 10499 (1990)] to describe the low-energy properties of the Haldane phase of the spin-1 Heisenberg chain from three massive Majorana fermions. We find, for all $N$'s, the emergence of a nondegenerate gapped phase with edge states whose topological protection depends on the parity of $N$. Whereas for $N$ odd, there is no such protection, the phase with even $N$ is shown to be topologically protected. We find that the phase belongs to the same topological class as the phase with edge states living in self-conjugate fully antisymmetric representation of the $\mathrm{SU}\left(N\right)$ group.

We observe significantly enhanced spin Seebeck effect (SSE) and spin Hall magnetoresistance (SMR) values by inserting 0.3–0.6-nm-thick layers of magnetic materials with different composition (Cr, ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$, ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}$, ${\mathrm{Fe}}_{50}{\mathrm{Co}}_{50}$, and Fe) at the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$/Pt interface. To study the actual magnetic state of these insertion layers, we, then, investigated the magnetization of these layers via generalized magneto-optical ellipsometry. Magnetic insertion layers in this thickness range did not exhibit a measurable magnetization, even though our method should have detected even a small fraction of the bulk magnetization value for the utilized materials easily. Therefore, the observed SSE and SMR enhancement generated by the insertion of thin magnetic material layers did not result from a net interface magnetization but might, instead, be related to the paramagnetic state of the inserted layers.

Superconductivity in ${\mathrm{PrIr}}_2{\mathrm{Zn}}_{20}$ appears at $T_{\mathrm{c}}=0.05$ K in the presence of an antiferroquadrupolar order below $T_{\mathrm{Q}}=0.11$ K. We have studied pressure dependences of $T_{\mathrm{c}}$, $T_{\mathrm{Q}}$, and non-Fermi-liquid behaviors in the resistivity $\rho \left(T\right)$ by using two pressure-transmitting media: argon, maintaining highly hydrostatic pressure, and glycerol, which solidifies above 5 GPa, producing nonhydrostatic pressure. Upon applying $P$ with argon up to 10.6 GPa, $T_{\mathrm{c}}$ hardly changes, while $T_{\mathrm{Q}}$ monotonically increases from 0.11 to 0.23 K. With glycerol, however, $T_{\mathrm{Q}}$ and $T_{\mathrm{c}}$ simultaneously fall below 0.04 K at 6.3 GPa. The contrasting results indicate that on-site quadrupolar fluctuations induce superconductivity in this compound.

We study unconventional superconductivity in a two-dimensional locally noncentrosymmetric triangular lattice. The model is relevant to bilayer transition metal dichalcogenides with $2{\mathrm{H}}_b$ stacking structure, for example. The superconducting instability is analyzed by solving the linearized Eliashberg equation within the random phase approximation. We show that ferromagnetic fluctuations are dominant owing to the existence of disconnected Fermi pockets near van Hove singularity, and hence odd-parity spin-triplet superconductivity is favored. In the absence of the spin-orbit coupling, we find that odd-parity $f$-wave superconducting state is stabilized in a wide range of carrier density and interlayer coupling. Furthermore, we investigate impacts of the layer-dependent staggered Rashba and Zeeman spin-orbit coupling on the superconductivity. Multiple odd-parity superconducting phase diagrams are obtained as a function of the spin-orbit coupling and Coulomb interaction. Especially, a topological chiral $p$-wave pairing state is stabilized in the presence of a moderate Zeeman spin-orbit coupling. Our results shed light on a possibility of odd-parity superconductivity in various ferromagnetic van der Waals materials.

Reconnections between quantum vortex filaments in the presence of trapped particles are investigated using numerical simulations of the Gross-Pitaevskii equation. Particles are described with classical degrees of freedom and modeled as highly repulsive potentials which deplete the superfluid. First, the case of a vortex dipole with a single particle trapped inside one of the vortices is studied. It is shown that the reconnection takes place at the position of the particle as a consequence of the symmetry breaking induced by it. The separation rate between the reconnecting points is compatible with the known dynamics of quantum vortex reconnections, and it is independent of the particle mass and size. After the reconnection, the particle is pushed away with a constant velocity, and its trajectory is deflected because of the transverse momentum exchange with the vortex filaments. The momentum exchanges between the particle, the vortex, and a density pulse are characterized. Finally, the reconnection of two linked rings, each of them with several initially randomly distributed particles, is studied. It is observed that generically, reconnections take place at the location of trapped particles. It is shown that reconnection dynamics is unaffected for light particles.

Existing theoretical works differ on whether three-dimensional Dirac and Weyl semimetals are stable to a short-range-correlated random potential. Numerical evidence suggests the semimetal to be unstable, while some field-theoretic instanton calculations have found it to be stable. The differences go beyond method: the continuum field-theoretic works use a single, perfectly linear Weyl cone, while numerical works use tight-binding lattice models which inherently have band curvature and multiple Weyl cones. In this work, we bridge this gap by performing exact numerics on the same model used in analytic treatments, and we find that all phenomena associated with rare regions near the Weyl node energy found in lattice models persist in the continuum theory: The density of states is nonzero and exhibits an avoided transition. In addition to characterizing this transition, we find rare states and show that they have the expected behavior. The simulations utilize sparse matrix techniques with formally dense matrices; doing so allows us to reach Hilbert space sizes upwards of ${10}^7$ states, substantially larger than anything achieved before.

We study the behavior of an extended fermionic wire coupled to a local stochastic field. Since the quantum jump operator is Hermitian and quadratic in fermionic operators, it renders the model soluble, allowing investigation of the properties of the nonequilibrium steady state and the role of dissipation-induced fluctuations. We derive a closed set of equations of motion solely for the two-point correlator; on the other hand, we find, surprisingly, that the many-body state exhibits non-Gaussian correlations. Density-density correlation function demonstrates a crossover from a regime of weak dissipation characterized by moderate heating and stimulated fluctuations to a quantum Zeno regime ruled by strong dissipation, which tames quantum fluctuations. Instances of soluble dissipative impurities represent an experimentally viable platform to understand the interplay between dissipation and Hamiltonian dynamics in many-body quantum systems.

We have studied the pressure effect on the rattling of tetrahedrite ${\mathrm{Cu}}_{10}{\mathrm{Zn}}_2{\mathrm{Sb}}_4{\mathrm{S}}_{13}$ (CZSS) and type-I clathrate ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Sn}}_{30}$ (BGS) by specific heat and x-ray diffraction measurements. By applying pressure $P$, the rattling energy for CZSS initially decreases and steeply increases for $P>1\mathrm{GPa}$. By contrast, the energy for BGS increases monotonically with $P$ up to 6.5 GPa. An analysis of the pressure-dependent specific heat and x-ray diffraction indicates that the out-of-plane rattling of the Cu atoms in the ${\mathrm{S}}_3$ triangle of CZSS originates from the chemical pressure, unlike the rattling of the Ba ions among off-center sites in an oversized cage of BGS. The rattling in CZSS ceases upon further increasing $P$ above 2 GPa, suggesting that Cu atoms escape away from the ${\mathrm{S}}_3$ triangle plane.

${\mathrm{PbZrO}}_3$ is regarded as the first antiferroelectric and currently is under intense reexamination. We propose a multiscale approach that combines classical and first-principles density functional theory based simulations to explore polar phases in this material. Application of this approach to ${\mathrm{PbZrO}}_3$ predicts three polar phases, none of which is the common $R3c$. The lowest-energy polar phase $Cc$ is metastable at 0 K but can be stabilized by application of electric field. Its structural and polarization response to the electric field is in good agreement with experimental data from the literature. The other two phases are $Ima2$ and $I4cm$ of orthorhombic and tetragonal symmetry, respectively, and predicted to stabilize at finite temperatures and under application of larger electric fields. These two phases exhibit very large strain which is of technological significance.

During the last years, topologically protected collective modes of magnetization have called much attention. Among these, skyrmions and merons have been objects of intense study. In particular, topological skyrmions are structures with an integer skyrmion number $Q$ while merons have a half-integer skyrmion charge $q$. In this paper, we consider a $Q=1$ skyrmion, composed of a meron and an antimeron (bimeron), displacing in a ferromagnetic racetrack, disputing a long-distance competition with its more famous counterpart, the typical $Q=1$ cylindrically symmetrical skyrmion. Both types of topological structures induce a Magnus force, and then they are subject to the skyrmion Hall effect. The influence of the Dzyaloshinskii-Moriya interaction DMI present in certain materials and able to induce DMI skyrmions is also analyzed. Our main aim is to compare the motions (induced by a spin-polarized current) of these objects along with their own specific racetracks. We also investigate some favorable factors which are able to give breath to the competitors, impelling them to remain in the race for longer distances before their annihilation at the racetrack lateral border. An interesting result is that the DMI skyrmion loses this hypothetical race due to its larger rigidity.

We report magnetic, transport, neutron diffraction, and muon spin rotation data showing that ${\mathrm{Pb}}_2{\mathrm{CoOsO}}_6$, a metallic double-perovskite with a centrosymmetric space group at room temperature exhibits a continuous second-order phase transition at 45 K to a magnetically ordered state with a noncentrosymmetric space group. The absence of inversion symmetry is very uncommon in metals, particularly metallic oxides. In contrast to the recently reported ferroelectriclike structural transition in ${\mathrm{LiOsO}}_3$, the phase transition in ${\mathrm{Pb}}_2{\mathrm{CoOsO}}_6$ is driven by a long-range collinear antiferromagnetic order, with propagation vector $\mathbf{k}=(\frac{1}{2},0,\frac{1}{2})$, which relieves the frustration associated with the symmetry of the magnetic exchanges. This magnetically driven loss of inversion symmetry represents another frontier in the search for novel metallic behavior.

In this paper, we report the theoretical possibility of generating magnon-polaron excitations through a space-varying magnetic field. The spatial dependence of the magnetic field in the Zeeman interaction gives rise to a magnon-phonon coupling when a magnetic field gradient is applied, and such a coupling depends directly on the strength of the gradient. It is also predicted that the direction of the magnetic field gradient allows control over which phonon polarization couples to the magnons in the material. Here, we develop the calculations of the magnon-phonon coupling for an arbitrary (anti)ferromagnet, which are later used to numerically study its consequences. These results are compared to the ones obtained with the phenomenological magnetoelastic coupling in yttrium iron garnet (YIG) where we show that the magnon-polaron band gap seen in YIG can be also obtained with a magnetic field gradient of $\sim 0.1\mathrm{T}/\mathrm{m}$ which can be achieved with the current experimental techniques. Our results propose a different way of controlling the magnetoelastic coupling in an arbitrary material and open a route to exploit the magnon-phonon interaction in magnonic and spintronic devices.

Magnetic impurities on the surface of Rashba spin-orbit-coupled, but otherwise conventional, superconductors provide a promising way to engineer topological superconductors with Majorana bound states as boundary modes. In this work we show that the spin-polarization in the interior of both one-dimensional impurity chains and two-dimensional islands in these systems can be used to determine the superconducting topological phase, as it changes sign exactly at the topological phase transition. Thus, spin polarization offers an alternative method to detect the topological phase in magnetic impurity chains and islands deposited on conventional superconductors, beyond the zero-energy Majorana bound states.

We present scanning tunneling microscope (STM) measurements of the local electronic structure of superconducting granular aluminium films. The STM spectra show a homogeneously increased superconducting gap compared to that of aluminum, both near and above the Mott resistivity ${\rho }_{\mathrm{M}}\approx 400\mu \mathrm{\Omega }\mathrm{cm}$. Above ${\rho }_{\mathrm{M}}$ we find Coulomb charging effects, a first indication of electrical decoupling, and in-gap states on individual grains, which could contribute to flux noise and dielectric loss in quantum devices. We also observe multiple low-energy states outside the gap, which indicate bosonic excitations of an energy below twice the superconducting gap.

We present a theoretical model of an on-chip three-level maser in a superconducting circuit based on a single artificial atom and pumped by a temperature gradient between thermal baths coupled to different interlevel transitions. We show that maser powers of the order of a few femtowatts, well exceeding the resolution of the sensitive bolometry, can be achieved with typical circuit parameters. We also demonstrate that population inversion in the artificial atom can be detected without measuring coherent radiation output of the maser. For that purpose, the system should operate as a three-terminal heat transport device. The hallmark of population inversion is the influx of heat power into the weakly coupled output terminal even though its temperature exceeds the temperatures of the two other terminals. The proposed method of on-chip conversion of heat into microwave radiation and control of energy-level populations by heating provide additional useful tools for circuit quantum electrodynamics experiments.

The manipulation of two-dimensional materials via their dielectric environment offers novel opportunities to control electronic as well as optical properties and allows one to imprint nanostructures in a noninvasive way. Here we asses the potential of monolayer semiconducting transition-metal dichalcogenides (TMDCs) for Coulomb engineering in a material realistic and quantitative manner. We compare the response of different TMDC materials to modifications of their dielectric surrounding, analyze effects of dynamic substrate screening, i.e., frequency dependencies in the dielectric functions, and discuss inherent length scales of Coulomb-engineered heterojunctions. We find symmetric and rigid-shift-like quasiparticle band-gap modulations for both instantaneous and dynamic substrate screening. From this, we derive short-ranged self-energies for an effective multiscale modeling of Coulomb-engineered heterojunctions composed of a homogeneous monolayer placed on a spatially structured substrate. For these heterojunctions, we show that band-gap modulations on the length scale of a few lattice constants are possible, rendering external limitations of the substrate structuring more important than internal effects. We find that all semiconducting TMDCs are similarly well suited for these external and noninvasive modifications.

We study electrical and thermoelectrical properties for a double quantum dot system. We consider the cases of both single-level and multilevel quantum dots whatever the way they are coupled, either in a series or in a parallel arrangement. The calculations are performed by using the nonequilibrium Green function theory. In the case of a single-level double quantum dot, the problem is exactly solvable whereas for a multilevel double quantum dot, an analytical solution is obtained in the limit of energy-independent hopping integrals. We present a detailed discussion about the dependences of electrical conductance, zero-frequency charge susceptibility, and Seebeck coefficient on the gate voltages applied to the dots, allowing us to derive the charge stability diagram. The findings are in agreement with the experimental observations notably with the occurrence of successive sign changes of the Seebeck coefficient when varying the gate voltages. We interpret the results in terms of the bonding and antibonding states produced by the level anticrossing effect which occurs in the presence of a finite interdot coupling. We show that at equilibrium the boundary lines between the domains with different dot occupancies in the charge stability diagram take place when the bonding and antibonding state levels are aligned with the chemical potentials in the leads. Finally the total dot occupancy is found to be considerably reduced in the case in parallel compared with the case in series, whenever the level energies in each dot are equal. We interpret this dip as a direct manifestation of the interference effects occurring in the presence of the two electronic transmission paths provided by each dot.