Mechanical resonators with high quality factors are widely used in precision experiments, ranging from gravitational wave detection and force sensing to quantum optomechanics. Beams and membranes are well known to exhibit flexural modes with enhanced quality factors when subjected to tensile stress. The mechanism for this enhancement has been a subject of debate, but is typically attributed to elastic energy being “diluted” by a lossless potential. Here we clarify the origin of the lossless potential to be the combination of tension and geometric nonlinearity of strain. We present a general theory of dissipation dilution that is applicable to arbitrary resonator geometries and discuss why this effect is particularly strong for flexural modes of nanomechanical structures with high aspect ratios. Applying the theory to a nonuniform doubly clamped beam, we show analytically how dissipation dilution can be enhanced by modifying the beam shape to implement “soft clamping,” thin clamping, and geometric strain engineering, and derive the ultimate limit for dissipation dilution.

We show how the spin-transfer torque generated by an ac voltage may be used to excite a paramagnetic resonance of an atomic spin deposited on a metallic surface. This mechanism is independent of the environment of the atom and may explain the ubiquity of the paramagnetic resonance reported by Baumann et al. [S. Baumann, W. Paul, T. Choi, C. P. Lutz, A. Ardavan, and A. J. Heinrich, Science 350, 417 (2015)]. The current and spin dynamics are modeled by a time-dependent Redfield master equation generalized to account for the periodic driven voltage. Our approach shows that the resonance effect is a consequence of the nonlinearity of the coupling between the magnetic moment and the spin-polarized current which generates a large second-harmonic amplitude that can be measured in the current signal.

We report on the crystal and magnetic structures and magnetic and transport properties of ${\mathrm{SrMnSb}}_2$ single crystals grown by the self-flux method. Magnetic susceptibility measurements reveal an antiferromagnetic (AFM) transition at $T_{\mathrm{N}}=295\left(3\right)$ K. Above $T_{\mathrm{N}}$, the susceptibility slightly increases and forms a broad peak at $T\sim 420$ K, which is a typical feature of two-dimensional magnetic systems. Neutron diffraction measurements on single crystals confirm the previously reported C-type AFM structure below $T_{\mathrm{N}}$. Both de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects are observed in ${\mathrm{SrMnSb}}_2$ single crystals. Analysis of the oscillatory component by a Fourier transform shows that the prominent frequencies obtained by the two different techniques are practically the same within error regardless of sample size or saturated magnetic moment. Transmission electron microscopy (TEM) reveals the existence of stacking faults in the crystals, which result from a horizontal shift of Sb atomic layers suggesting possible ordering of Sb vacancies in the crystals. Increase of temperature in susceptibility measurements leads to the formation of a strong peak at $T\sim 570$ K that upon cooling under magnetic field the susceptibility shows a ferromagnetic transition at $T_{\mathrm{C}}\sim 580$ K. Neutron powder diffraction on crushed single crystals does not support a ferromagnetic phase above $T_{\mathrm{N}}$. Furthermore, x-ray magnetic circular dichroism (XMCD) measurements of a single crystal at the $L_{2,3}$ edge of Mn shows a signal due to induced canting of AFM moments by the applied magnetic field. All evidence strongly suggests that a chemical transformation at the surface of single crystals occurs above 500 K concurrently producing a minute amount of ferromagnetic impurity phase.

The Fredkin model describes a spin-half chain segment subject to three-body, correlated-exchange interactions and twisted boundary conditions. The model is frustration free, and its ground-state wave function is known exactly. Its low-energy physics is that of a strong $xy$ ferromagnet with gapless excitations and an unusually large dynamical exponent. We study a generalized spin chain model that includes the Fredkin model as a special tuning point and otherwise interpolates between the conventional ferromagnetic and antiferromagnetic quantum Heisenberg models. We solve for the low-lying states, using exact diagonalization and density-matrix renormalization group calculations, in order to track the properties of the system as it is tuned away from the Fredkin point; we also present exact analytical results that hold right at the Fredkin point. We identify a zero-temperature phase diagram with multiple transitions and unexpected ordered phases. The Fredkin ground state turns out to be particularly brittle, unstable to even infinitesimal antiferromagnetic frustration. We remark on the existence of an “anti-Fredkin” point at which all the contributing spin configurations have a spin structure exactly opposite to those in the Fredkin ground state.

The nature of the superfluid-insulator transition in one dimension has been much debated recently. In particular, to describe the strong disorder regime characterized by weak link proliferation, a scratched XY model has been proposed [New J. Phys. 18, 045018 (2016)], where the transport is dominated by a single anomalously weak link and is governed by Kane-Fisher weak link physics. In this article, we consider the simplest problem to which the scratched XY model relates: a single weak link in an otherwise clean system, with an intensity $J_W$ which decreases algebraically with the size of the system $J_W\sim L^{-\alpha }$. Using a renormalization group approach and a vortex energy argument, we describe the Kane-Fisher physics in this model and show that it leads to a transition from a transparent regime for $K>K_c$ to a perfect cut for $K<K_c$, with an adjustable $K_c=1/(1-\alpha )$ depending on $\alpha$. We check our theoretical predictions with Monte Carlo numerical simulations complemented by finite-size scaling. Our results clarify two important assumptions at the basis of the scratched XY scenario, the behaviors of the crossover length scale from weak link physics to transparency and of the superfluid stiffness.

We study the collisionless dynamics of two classes of nonintegrable pairing models. One is a Bardeen-Cooper-Schrieffer model with separable energy-dependent interactions, and the other is a two-dimensional topological superconductor with spin-orbit coupling and a band-splitting external field. The long-time quantum quench dynamics at integrable points of these models are well understood. Namely the squared magnitude of the time-dependent order parameter $\mathrm{\Delta }\left(t\right)$ can vanish (Phase I), reach a nonzero constant (Phase II), or periodically oscillate as an elliptic function (Phase III). We demonstrate that nonintegrable models, too, exhibit some or all of these nonequilibrium phases. Remarkably, elliptic periodic oscillations persist, even though both their amplitude and functional form change drastically with integrability breaking. Striking new phenomena accompany loss of integrability. First, an extremely long timescale emerges in the relaxation to Phase III, such that short-time numerical simulations risk erroneously classifying the asymptotic state. This timescale diverges near integrable points. Second, an entirely new Phase IV of quasiperiodic oscillations of $\left|\mathrm{\Delta }\right|$ emerges in the quantum quench phase diagrams of nonintegrable pairing models. As integrability techniques do not apply for the models we study, we develop the concept of asymptotic self-consistency and a linear stability analysis of the asymptotic phases. With the help of these new tools, we determine the phase boundaries, characterize the asymptotic state, and clarify the physical meaning of the quantum quench phase diagrams of BCS superconductors. We also propose an explanation of these diagrams in terms of bifurcation theory.

The high-pressure structural properties of bismuth oxide ${\mathrm{Bi}}_2{\mathrm{SiO}}_5$ have been investigated up to 29 GPa using in situ powder synchrotron x-ray diffraction (XRD) and up to 50 GPa with density functional theory (DFT) calculations. We found a compressibility anomaly at about 20 GPa that is observed both in our XRD and DFT results. The rotation of the corner-connected ${\mathrm{SiO}}_4$ tetrahedra produces a straightening of the silicate chains, yielding a lower compressibility above 20 GPa for the structure with straight chains. We analyzed the stereochemical activity of the ${\mathrm{Bi}}^{3+}$ lone electron pair, which is found to decrease with increasing pressure, but it can still be identified in the calculated electron density difference maps at 50 GPa.

Few-level quantum systems driven by $n_{\mathrm{f}}$ incommensurate fundamental frequencies exhibit temporal analogs of noninteracting phenomena in $n_{\mathrm{f}}$ spatial dimensions, a consequence of the generalization of Floquet theory in frequency space. We organize the fundamental solutions of the frequency lattice model for $n_{\mathrm{f}}=2$ into a quasienergy band structure and show that every band is classified by an integer Chern number. In the trivial class, all bands have zero Chern number and the quasiperiodic dynamics is qualitatively similar to Floquet dynamics. The topological class with nonzero Chern bands has dramatic dynamical signatures, including the pumping of energy from one drive to the other, chaotic sensitivity to initial conditions, and aperiodic time dynamics of expectation values. The topological class is however unstable to generic perturbations due to exact level crossings in the quasienergy spectrum. Nevertheless, using the case study of a spin in a quasiperiodically varying magnetic field, we show that topological class can be realized at low frequencies as a prethermal phase, and at finite frequencies using counterdiabatic tools.

The asymmetric responses of the system between the external force of the right and left directions are called “nonreciprocal.” There are many examples of nonreciprocal responses, such as the rectification by the $p\text{−}n$ junction. However, the quantum-mechanical wave does not distinguish between the right and the left directions as long as the time-reversal symmetry is intact, and it is a highly nontrivial issue how the nonreciprocal nature originates in quantum systems. Here we demonstrate by the quantum ratchet model, i.e., a quantum particle in an asymmetric periodic potential, that the dissipation characterized by a dimensionless coupling constant $\alpha$ plays an essential role for nonlinear nonreciprocal response. The temperature ($T$) dependence of the second-order nonlinear mobility ${\mu }_2$ is found to be ${\mu }_2\sim T^{(6/\alpha )-4}$ for $\alpha <1$, and ${\mu }_2\sim T^{2(\alpha -1)}$ for $\alpha >1$, respectively, where ${\alpha }_c=1$ is the critical point of the localization-delocalization transition, i.e., Schmid transition. On the other hand, ${\mu }_2$ shows the behavior ${\mu }_2\sim T^{-11/4}$ in the high-temperature limit. Therefore, ${\mu }_2$ shows the nonmonotonous temperature dependence corresponding to the classical-quantum crossover. The generic scaling form of the velocity $v$ as a function of the external field $F$ and temperature $T$ is also discussed. These findings are relevant to the heavy atoms in metals, resistive superconductors with vortices and Josephson junction system and will pave a way to control the nonreciprocal responses.

The standard equilibrium Green–Kubo and nonequilibrium molecular dynamics (MD) methods for computing thermal transport coefficients in solids typically require relatively long simulation times and large system sizes. To this end, we revisit here the homogeneous nonequilibrium MD method by Evans [Phys. Lett. A 91, 457 (1982)] and generalize it to many-body potentials that are required for more realistic materials modeling. We also propose a method for obtaining spectral conductivity and phonon mean-free path from the simulation data. This spectral decomposition method does not require lattice dynamics calculations and can find important applications in spatially complex structures. We benchmark the method by calculating thermal conductivities of three-dimensional silicon, two-dimensional graphene, and a quasi-one-dimensional carbon nanotube and show that the method is about one to two orders of magnitude more efficient than the Green–Kubo method. We apply the spectral decomposition method to examine the long-standing dispute over thermal conductivity convergence vs divergence in carbon nanotubes.

We show that continuous and spin-lattice models of chiral ferro- and antiferromagnets provide the existence of an infinite number of stable soliton solutions of any integer topological charge. A detailed description of the morphology of new skyrmions and the corresponding energy dependencies are provided. The considered model is general, and is expected to predict a plethora of particlelike states which may occur in various chiral magnets including ultrathin films, e.g., PdFe/Ir(111), rhombohedral ${\mathrm{GaV}}_4{\mathrm{S}}_8$ semiconductor, B20-type alloys as ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Ge}$, ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$, ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$, ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, and acentric tetragonal Heusler compounds.

We develop a model for the magnonic contribution to the unidirectional spin Hall magnetoresistance (USMR) of heavy metal/ferromagnetic insulator bilayer films. We show that diffusive transport of Holstein-Primakoff magnons leads to an accumulation of spin near the bilayer interface, giving rise to a magnoresistance which is not invariant under inversion of the current direction. Unlike the electronic contribution described by Zhang and Vignale [Phys. Rev. B 94, 140411 (2016)], which requires an electrically conductive ferromagnet, the magnonic contribution can occur in ferromagnetic insulators such as yttrium iron garnet. We show that the magnonic USMR is, to leading order, cubic in the spin Hall angle of the heavy metal, as opposed to the linear relation found for the electronic contribution. We estimate that the maximal magnonic USMR in Pt|YIG bilayers is on the order of ${10}^{-8}$ but may reach values of up to ${10}^{-5}$ if the magnon gap is suppressed and can thus become comparable to the electronic contribution in, e.g., Pt|Co. We show that the magnonic USMR at a finite magnon gap may be enhanced by an order of magnitude if the magnon diffusion length is decreased to a specific optimal value that depends on various system parameters.

Magnetic skyrmions, topological spin textures observed in chiral magnets, have attracted huge interest due to their applications in the field of spintronics. In this work we study the stability of circular isolated skyrmions in ferromagnetic chiral magnets under the influence of different perturbations and external fields. To this end we develop a general systematic procedure based in a harmonic expansion series of the skyrmion boundary which allows the identification of the breakdown of the skyrmion circular shape on each instability channel independently. We apply our approach to a few representative spin models of actual interest in order to obtain the zero temperature phase diagram, where isolated skyrmions emerge as metastable states. The results presented in this paper are in agreement with properties of isolated skyrmions observed in recent experiments, opening the possibility of extending the analysis to more complex situations.

Evidence of nematic effects in the mixed superconducting phase of slightly underdoped $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ is reported. We have found strong in-plane resistivity anisotropy for crystals in different strain conditions. For these compositions, there is no magnetic long-range order, so the description may be ascribed to the interplay between the superconducting and nematic order parameters. A piezoelectric-based apparatus is used to apply tensile or compressive strain to tune nematic domain orientation in order to examine intrinsic nematicity. Measurements are done under a rotating magnetic field, and the analysis of the angular dependence of physical quantities identifies the cases in which the sample is detwinned. Furthermore, the angular dependence of the data allows us to evaluate the effects of nematicity on the in-plane superconductor stiffness. Our results show that although nematicity contributes in a decisive way to the conduction properties, its contributions to the anisotropy properties of the stiffness of the superconducting order parameter is not as significant in these samples.

We studied the electronic structure of the heavy-fermion compound $\mathrm{Yb}{\left({\mathrm{Ru}}_{1-x}{\mathrm{Rh}}_x\right)}_2{\mathrm{Ge}}_2$ with $x=0$ and nominally $x=0.125$ using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a noninteger valence close to $3+$. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasiquartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru $4d$ character is well reproduced by our calculations. ${\mathrm{YbRu}}_2{\mathrm{Ge}}_2$ undergoes a nonmagnetic phase transition into a ferroquadrupolar ordered state below 10.2 K and then to an antiferromagnetically ordered state below 6.5 K. A small hole Fermi surface shows nesting features in our calculated band structure and its size determined by ARPES is close to the magnetic ordering wave vector found in neutron scattering. The transitions are suppressed when ${\mathrm{YbRu}}_2{\mathrm{Ge}}_2$ is doped with 12.5% Rh. The electron doping leads to a shift of the band structure and successive Lifshitz transitions.

Newly synthesized Janus transition-metal dichalcogenides MXY ($M=\mathrm{Mo}$, W; $X\ne Y=\mathrm{S}$, Se, Te) possess intrinsic Rashba spin splitting and out-of-plane dipole moment due to the breaking of mirror symmetry. Taking WSSe as an example, we present a first-principles investigation of the structural stability and electronic properties of mono-, bi-, and multilayer MXY. Results show that S atoms contribute more than Se atoms in the valence-band maximum at the $\mathrm{\Gamma }$ point, which can be greatly affected by interlayer interactions. The high-symmetry AA′ stacking is still the most stable pattern, but there are various orders of chalcogen atomic layers in each stacking. The most preferred order of two adjacent layers is S-Se-Se-S, followed by Se-S-Se-S. The Se-S-Se-S–ordered WSSe bilayer is found to have significant layer splitting due to the net dipole moment, which has great potential for solar cells. Layer-dependent Rashba splittings exist in asymmetry-ordered WSSe bilayers, that can be tuned by changing the interlayer distance, originating from the regulation of interlayer electrostatic interaction. However, there is not layer splitting in a symmetrically stacked WSSe bilayer and opposite Rashba splitting appears in the two layers at a sufficiently large interlayer distance. The electronic structures and spin splittings can be easily modulated by controlling the chalcogen atomic-layer order, so that we can obtain the desired properties from mono-, bi-, and multilayer MXY.

We consider a quantum dot coupled to both superconducting and spin-polarized electrodes, and we study the triad interplay of the Kondo effect, superconductivity, and ferromagnetism, any two of which compete with and suppress each other. We find that the interplay leads to a mixed-valence quantum phase transition, which for other typical systems is merely a crossover rather than a true transition. At the transition, the system changes from a spin-doublet to -singlet state. The singlet phase is adiabatically connected (through crossovers) to the so-called “charge Kondo state” and to the superconducting state. We analyze in detail the physical characteristics of different states, and we propose that the measurement of the cross-current correlation and the charge relaxation resistance can clearly distinguish between them.

We consider a quench in an infinite spin ladder describing a system with two species of bosons in the limit of strong interactions. If the heavy bosonic species has infinite mass, the model becomes a spin chain with quenched binary disorder which shows true Anderson localization (AL) or many-body localization (MBL). For finite hopping amplitude $J^{\prime }$ of the heavy particles, on the other hand, we find an exponential polarization decay with a relaxation rate which depends monotonically on $J^{\prime }$. Furthermore, the entanglement entropy changes from a constant (AL) or logarithmic (MBL) scaling in time $t$ for $J^{\prime }=0$ to a subballistic power law, $S_{\mathrm{ent}}\sim t^{\alpha }$ with $\alpha <1$, for finite $J^{\prime }$. We do not find a distinct regime in time where the dynamics for $J^{\prime }\ne 0$ shows the characteristics of an MBL phase. Instead, we discover a time regime with distinct dephasing and entanglement times, leading to dynamics in this regime which is different both from a localized and from a fully ergodic phase.

In this work we present the eigenspectra of a novel species of Wannier excitons when exposed to crossed electric and magnetic fields. In particular, we compute the eigenenergies of giant-dipole excitons in ${\mathrm{Cu}}_2\mathrm{O}$ in crossed fields. In our theoretical approach, we calculate the excitonic spectra within both an approximate as well as a numerically exact approach for arbitrary field configurations. We verify that stable bound excitonic giant-dipole states are only possible in the strong magnetic field limit, as this is the only regime providing sufficiently deep potential wells for their existence. Comparing both analytic as well as numerical calculations, we obtain excitonic giant-dipole spectra with level spacings in the range of 0.6–100 $\mu \mathrm{eV}$.

In this work, we show that by using the density functional approach, in which the electronic degrees of freedom are separated by the ionic ones, it is possible to individuate and separately study the elastic and electrostatic step interactions, traditionally introduced in the literature as the only two kinds of step interaction expected at $T=0$. We have applied the method to the technologically important GaAs(001) surface and found some unexpected results for the relatively short step distances accessible to the ab initio approaches, contradicting those of the continuum models so far employed for the study of the elastic step interactions: (i) the sign of the step interaction depends on the step termination and is due to the electrostatic interaction; (ii) the elastic interaction does not contribute to the step interaction, contrary to the common belief of a strong elastic repulsive interaction between like-oriented steps. We show that this is due to the electron behavior. When considering only ion displacements and pointlike steps as in the continuum theories, we recover the classical results and repulsive step elastic interactions; (iii) the experimentally observed $Ab$ step termination shows a weakly attractive step interaction whereby attractive step interactions between like-oriented steps on an unstrained surface are believed not to exist. The proposed method of separating elastic and electrostatic interactions for further analysis of their dependence on the configurational degrees of freedom can be extended to other defective situations.