We study the effects of bond and site disorder in the classical $J_1\text{−}{J}_2$ Heisenberg model on a square lattice in the order-by-disorder frustrated regime $2J_2>\left|J_1\right|$. Combining symmetry arguments, numerical energy minimization, and large-scale Monte Carlo simulations, we establish that the finite-temperature Ising-type transition of the clean system is destroyed in the presence of any finite concentration of impurities. We explain this finding via a random-field mechanism which generically emerges in systems where disorder locally breaks the same real-space symmetry spontaneously globally broken by the associated order parameter. We also determine that the phase replacing the clean one is a paramagnet polarized in the nematic glass order with nontrivial magnetic response. This is because disorder also induces noncollinear spin-vortex-crystal order and produces a conjugated transverse dipolar random field. As a result of these many competing effects, the associated magnetic susceptibilities are nonmonotonic functions of the temperature. As a further application of our methods, we show the generation of random axes in other frustrated magnets with broken SU(2) symmetry. We also discuss the generality of our findings and their relevance to experiments.

In this paper we present the effects of the composition on structural, vibrational, magnetic, and electronic properties of double perovskite 3d based manganese oxides ${\mathrm{Gd}}_2{\mathrm{Ni}}_x{\mathrm{Co}}_{1-x}\mathrm{Mn}{\mathrm{O}}_6$ (GNCMO) powder (for $0<x<1$) prepared by high temperature solid state chemistry. These compounds have a great research interest due to their specific physical properties. We have shown that the synthesized compounds exhibit ferromagnetic interaction between the spins of (${\mathrm{Mn}}^{4+}, {\mathrm{Co}}^{2+}$), and (${\mathrm{Mn}}^{4+}, {\mathrm{Ni}}^{2+}$) cations via oxygen ions and antiferromagnetic interactions between (${\mathrm{Gd}}^{3+}, {\mathrm{Co}}^{2+}$), (${\mathrm{Gd}}^{3+}, {\mathrm{Ni}}^{2+}$) and (${\mathrm{Gd}}^{3+}, {\mathrm{Gd}}^{3+}$) and present a paramagnetic to ferromagnetic phase transition. The paramagnetic Curie-Weiss temperature of GNCMO ($T_{\mathrm{c}}$) increases from 121to 128K as the nickel content increases from 0.2 to 0.8. Further, temperature dependent inverse magnetic susceptibility $\chi {\left(T\right)}^{–1}$ shows deviation from Curie Weiss behavior around $T_G$ which depicts that Griffiths phase already observed in several perovskite compounds. The value of the GP percentage and the exponent of ($\lambda <1$) are found to increase as the nickel content increases. The hysteresis curve at $T=20\mathrm{K}$ for $x=0.2$ displays a metamagnetic transition explained by the reorientation of the Gd spin moments towards the Mn, Co, and Ni spin moments. The imaginary part of susceptibility ${\chi }^{\prime \prime }$ shows a small shift of the peak $\sim 1\mathrm{K}$ indicating the presence of spin or cluster glass behavior.

Transient (in time) uniformly precessing magnetization structures (TUPS) on finite-length ferromagnetic wires are discussed. These rotating around the wire magnetization solutions of the Landau-Lifshits-Slonczewski equation are generalizations of traveling uniformly precessing domain walls of infinite range described by Goussev et al. [Phys. Rev. Lett. 104, 147202 (2010)] and comprise a single parameter family of magnetization profiles controlled by the precession frequency. The latter slowly varies in time resulting in passage of the magnetization structure through different evolution stages. The modulational stability of the TUPS is also discussed. An autoresonance approach using chirped frequency transverse spin polarized current is applied to initiate TUPS with a desired precession frequency. The process of complete inversion of the magnetization of the wire involving passage from spatially uniform to nonuniform and then back to uniform precessing state with opposite magnetization is analyzed.

We theoretically study the indirect interaction between two ferromagnetic contacts located on the surface of a $d$-wave superconductor. When the magnets are connected to a $\left\{010\right\}$ edge of the superconductor we find an oscillating RKKY interaction that varies in sign as the distance between the magnetic contacts is varied. However, when coupling the magnets to an $\left\{110\right\}$ edge of the superconductor, we find that the presence of midgap states qualitatively changes the results. The ground state of the system is then found to always favor alignment of the magnets as this configuration most strongly suppresses the midgap states, leading to a larger condensation energy, which dominates over the intrinsic RKKY interaction.

We describe the local structural properties of the iron oxychalcogenides, ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Fe}}_2\mathrm{O}{M}_2(M=\mathrm{S},\mathrm{Se})$, by using pair distribution function analysis applied to total scattering data. Our results from neutron powder diffraction show that $M$ = S and Se possess similar nuclear structures at low and room temperatures. The local crystal structures were studied by investigating deviations in atomic positions and the extent of the formation of orthorhombicity. Analysis of the total scattering data suggests that buckling of the ${\mathrm{Fe}}_2\mathrm{O}$ plane occurs below 100 K. The buckling may occur concomitantly with a change in octahedral height. Furthermore, within a typical range of 1–2 nm, we observed a short-range orthorhombiclike structure suggestive of nematic fluctuations in both of these materials.

Using classical Monte Carlo simulation we investigate the evolution of the specific heat in chiral magnets with impurities. Our calculations are made in the framework of a spin lattice model and could be applicable to an analysis of the specific heat in transition-metal monosilicides ${\mathrm{Mn}}_{1-x}$(Fe,${\mathrm{Co})}_x\mathrm{Si}$ with increasing doping. We propose two tentative models for impurity positions in the lattice. In the first one, impurities substitute the regular transition-metal ions in a parent compound. The second one treats impurities as frozen spins placed into interstitial positions of the regular lattice and coupled with their nearest neighbors by random exchange interactions. In both models an increase of doping leads to a quick degradation of the magnetic phase transition, though an evolution of the hump anomaly is not similar. We provide a comparison of the behavior of the specific heat and Bragg scattering amplitude for these two models.

We performed soft x-ray spectroscopic studies of the ferrimagnet ${\mathrm{TbFe}}_5{\mathrm{Al}}_7$ with strong easy-plane anisotropy in pulsed magnetic fields up to 29 T along with bulk magnetization and magnetostriction measurements. We observed pronounced amplitude changes of x-ray magnetic circular dichroism and x-ray absorption spectra at the field-induced magnetic transition. This microscopically evidences the simultaneous rotation of the Tb $4f$ and Fe $3d$ magnetic moments from a collinear ferrimagnetic order along the [100] axis to a state with the moments close to [010], the other easy-axis direction of the tetragonal lattice in magnetic fields applied along the [100] axis. We determined the magnetic-anisotropy constant of ${\mathrm{TbFe}}_5{\mathrm{Al}}_7$ by simulating the high-field macro- and microscopic magnetization process using a two-sublattice model.

Fractons are topological quasiparticles with limited mobility. While there exist a variety of models hosting these excitations, typical fracton systems require rather complicated many-particle interactions. Here, we discuss fracton behavior in the more common physical setting of classical kagome spin models with frustrated two-body interactions only. We investigate systems with different types of elementary spin degrees of freedom (three-state Potts, XY, and Heisenberg spins) which all exhibit characteristic subsystem symmetries and fractonlike excitations. The mobility constraints of isolated fractons and bound fracton pairs in the three-state Potts model are, however, strikingly different compared to the known type-I or type-II fracton models. One may still explain these properties in terms of type-I fracton behavior and construct an effective low-energy tensor gauge theory when considering the system as a two-dimensional cut of a three-dimensional cubic lattice model. Our extensive classical Monte Carlo simulations further indicate a crossover into a low-temperature glassy phase where the system gets trapped in metastable fracton states. Moving on to XY spins, we find that in addition to fractons the system hosts fractional vortex excitations. As a result of the restricted mobility of both types of defects, our classical Monte Carlo simulations do not indicate a Kosterlitz-Thouless transition but again show a crossover into a glassy low-temperature regime. Finally, the energy barriers associated with fractons vanish in the case of Heisenberg spins, such that defect states may continuously decay into a ground state. These decays, however, exhibit a power-law relaxation behavior which leads to slow equilibration dynamics at low temperatures.

Some intricate behaviors have been reported at magnetic compensation temperature in ${\mathrm{Co}}_2{\mathrm{TiO}}_4$ (CTO), but their origins are still unclear. In this paper, we investigate systematically the anomalous properties in CTO crystal by using direct and alternating current magnetic susceptibilities, specific heat, thermal expansion, magnetostriction, and neutron diffraction measurements. CTO exhibits a ferrimagnetic ordering transition at $T_C\approx 48\mathrm{K}$ and a magnetic compensation behavior at $T_{\mathrm{comp}}\approx 31\mathrm{K}$. Under zero field, there is no obvious anomaly in CTO around ferrimagnetic compensation temperature $T_{\mathrm{comp}}$, but a minimum value of M(T), a drop of ${\chi }^{\prime }\left(T\right)$, a step of $\mathrm{\Delta }L/L_{80\mathrm{K}}$ and a sharp specific heat peak all occur at $T_{\mathrm{comp}}$ under the same critical magnetic field of 20 kOe. We propose a phenomenological model to explain these anomalous phenomena, in which the 180 ° reversal of the ferrimagnetic component at $T_{\mathrm{comp}}$ under $H\ge 20\mathrm{kOe}$, as well as the deflection of the oblique spin moments at B sites under magnetic field, play a key role in how to understand them.

Superconducting quantum interference devices (SQUIDs) that incorporate two superconductor/insulator/superconductor (SIS) Josephson junctions in a closed loop form the core of some of the most sensitive detectors of magnetic and electric fields currently available. SQUIDs in these applications are typically operated with a finite voltage which generates microwave radiation through the ac Josephson effect. This radiation may impact the system being measured. We describe here a SQUID in which the Josephson junctions are formed from strips of normal metal (N) in good electrical contact with the superconductor (S). Such SNS SQUIDs can be operated under a finite voltage bias with performance comparable or potentially better than conventional SIS SQUIDs. However, they also permit a mode of operation that is based on the unusual interplay of quasiparticle currents and supercurrents in the normal metal of the Josephson junction. The method allows measurements of the flux dependence of the critical current of the SNS SQUID without applying a finite voltage bias across the SNS junction, enabling sensitive flux detection without generating microwave radiation.

The electronic properties of junctions defined electrostatically on twisted bilayer graphene can be addressed theoretically using lattice models. Recent works have introduced minimal local orbital models to describe twisted bilayer graphene at the magic angle (MATBLG) with different degrees of approximation and accounting for fragile topology. Based on these models and using Green's function techniques we analyze in this work the spectral and transport properties for MATBLG defined along different types of edges. To this end we first study the effect of symmetry breaking perturbations on the bulk bands, identifying their topological character. In a second step we obtain results for the local spectral densities at open boundaries and for two terminal transport on a three region junction where one could control independently the central and lateral regions doping level. Due to the large moiré length we observe that the hybridization of chiral edge states on these junctions gives rise to a signature in the longitudinal transport and to asymmetries in the local densities of states. We further show that these properties are extremely sensitive to the orientation of the junctions along the moiré lattice.

The interlayer van der Waals interaction in twisted bilayer graphene (tBLG) induces both in-plane and out-of-plane atomic displacements showing complex patterns that depend on the twist angle. In particular, for small twist angles, within each graphene layer, the relaxations give rise to a vortexlike displacement pattern which is known to affect the dispersion of the flat bands. Here, we focus on yet another structural property, the chirality of the twisted bilayer. We perform first-principles calculations based on density functional theory to investigate the properties induced by twist chirality in both real and momentum space. In real space, we study the interplay between twist chirality and atomic relaxation patterns. In momentum space, we investigate the spin textures around the $K$ points of the Brillouin zone, showing that alternating vortexlike textures are correlated with the chirality of tBLG. Interestingly, the helicity of each vortex is inverted by changing the chirality while the different twist angles also modify the spin textures. We discuss the origin of the spin textures by calculating the layer weights and using plot regression models.

Dynamical quantum phase transitions (DQPTs), which refer to the criticality in time of a quantum many-body system, have attracted much theoretical and experimental research interest recently. Despite that DQPTs are defined and signaled by the nonanalyticities in the Loschmidt rate, its interrelation with various correlation measures such as the equilibrium order parameters of the system remains unclear. In this work, by considering the quench dynamics in an interacting topological model, we find that the equilibrium order parameters of the model in general exhibit signatures around the DQPT, in the short time regime. The first extrema of the equilibrium order parameters are connected to the first Loschmidt rate peak. By studying the unequal-time two-point correlation, we also find that the correlation between the nearest neighbors decays while that with neighbors further away builds up as time grows in the noninteracting case, and upon the addition of repulsive intracell interactions. On the other hand, the intercell interaction tends to suppress the two-site correlations. These findings could provide us insights into the characteristic of the system around DQPTs, and pave the way to a better understanding of the dynamics in nonequilibrium quantum many-body systems.

We study dynamical properties of the finite-size fully connected Ising model with a transverse field at zero temperature. In a quench dynamics, we study the time period and the first critical time, which play important roles in the dynamical phase transitions, based on a dynamical order parameter and the Loschmidt rate, respectively. When all the spins are initially polarized in the direction of their mutual interaction, we show that both the time period and critical time diverge logarithmically with the system size at the dynamical critical point. When all the spins are initially in the direction of transverse field, both the time period and critical time exhibit logarithmic or power-law divergences depending on the final field strength. In the case of convergence, we provide estimates for the finite-size scaling and converged value. We also investigate the equilibrium phase transition, presenting approximate ground and first excited states away from the criticality, and compare their energy gap and bipartite and multipartite entanglements with the exact eigenstates.

The noncentrosymmetric nature of single-layer (SL) transition-metal dichalcogenides (TMD) manifests itself in the finite piezoelectricity and valley-Zeeman coupling. We microscopically model nonlinear exciton transport in nanobubbles of SL TMDs. Thanks to the giant piezoelectric effect, we obtain an enormous internal electric field, $E_{\mathrm{piezo}}\sim {10}^7\mathrm{V}/\mathrm{m}$, resulting in a built-in dipole moment of excitons. We demonstrate that the piezo-induced dipole-dipole interaction provides a novel channel for the nonlinear exciton transport distinct from the conventional isotropic funneling of excitons and leads to the formation of a hexagon-shaped exciton droplet on the top of circularly symmetric nanobubbles. Moreover, we found that the hexagonal distribution of exciton density is preserved even for strongly elliptic nanobubbles. The effect is tunable via the bubble-size dependence of the piezoelectric field $E_{\mathrm{piezo}}\sim h_{\max }^2/R^3$ with $h_{\max }$ and $R$ being the bubble height and radius, respectively.

Noncollinear chiral spin textures in ferromagnetic multilayers are at the forefront of recent research in nanomagnetism with the promise of fast and energy-efficient devices. The recently demonstrated possibilities to stabilize such chiral structures in synthetic antiferromagnets (SAFs) has raised interest as they are immune to dipolar field, hence favoring the stabilization of ultrasmall textures. They also improve mobility and avoid the transverse deflections of moving skyrmions limiting the efficiency in some foreseen applications. However, such systems with zero net magnetization are difficult to characterize by most of the standard techniques. Here, we report that the relevant parameters of a magnetic SAF texture, those being its period, its type (Néel or Bloch), and its chirality (clockwise or counterclockwise), can be directly determined using the circular dichroism in x-ray resonant scattering at half-integer multilayer Bragg peaks in reciprocal space. The analysis of the dependence in temperature down to 40 K allows us moreover to address the question of the temperature stability of a spin spiral in a SAF sample and of the temperature scaling of the symmetric and antisymmetric exchange interactions.