The gapped local minimum in the magnon dispersion, located at a finite wave number and frequency, has been observed in the amorphous ferromagnet ${\mathrm{Co}}_4\mathrm{P}$. The feature is called a “rotonlike” excitation and has eluded explanation for decades. We overcome the limitations of previous theories by combining the reverse Monte Carlo method, to determine the atomic structure, with large-scale atomistic spin simulations. This method enables us to include atomic order and spin correlations on an equal footing. We find the rotonlike feature is actually gapless, in contrast to the gapped structure found in previous studies. The gapless feature is attributed to amorphous umklapp scattering caused by residual structural order.

We implement a recursive Green's function method to extract the Fock space (FS) propagator and associated self-energy across the many-body localization (MBL) transition, for one-dimensional interacting fermions in a random on-site potential. We show that the typical value of the imaginary part of the local FS self-energy, ${\mathrm{\Delta }}_t$, related to the decay rate of an initially localized state, acts as a probabilistic order parameter for the thermal to MBL phase transition and can be used to characterize critical properties of the transition as well as the multifractal nature of MBL states as a function of disorder strength $W$. In particular, we show that a fractal dimension $D_s$ extracted from ${\mathrm{\Delta }}_t$ jumps discontinuously across the transition, from $D_s<1$ in the MBL phase to $D_s=1$ in the thermal phase. Moreover, ${\mathrm{\Delta }}_t$ follows an asymmetrical finite-size scaling form across the thermal-MBL transition, where a nonergodic volume in the thermal phase diverges with a Kosterlitz-Thouless–like essential singularity at the critical point $W_c$ and controls the continuous vanishing of ${\mathrm{\Delta }}_t$ as $W_c$ is approached. In contrast, a correlation length ($\xi$) extracted from ${\mathrm{\Delta }}_t$ exhibits a power-law divergence on approaching $W_c$ from the MBL phase.

We introduce the projected Green's function technique to study quasiperiodic systems such as the Aubry-André-Harper (AAH) model and beyond. In particular, we use projected Green's functions to construct a rational approximate sequence of transfer matrix equations consistent with quasiperiodic topology, where convergence of these sequences corresponds to the existence of extended eigenfunctions. We motivate this framework by applying it to a few well-studied cases such as the almost-Mathieu operator (AAH model), as well as more generic no-dual models that challenge standard routines. The technique is flexible and can be used to extract both analytic and numerical results, e.g., we analytically extract a modified phase diagram for Liouville irrationals. As a numerical tool, it does not require the fixing of boundary conditions and circumvents a primary failing of numerical techniques in quasiperiodic systems—extrapolation from finite size. Instead, it uses finite-size scaling to define convergence bounds on the full irrational limit.

A skyrmion in a frustrated magnetic system has the helicity degree of freedom. A skyrmion string is formed in a frustrated layered system, which is well described by the $XY$ model owing to the exchange coupling between adjacent layers. We consider a system where the interlayer exchange couplings are alternating, where the dimerized $XY$ model is materialized, whose linear limit is the Su-Schrieffer-Heeger model. We argue that it is a nonlinear topological system. We study the quench dynamics of the helicity wave under the initial condition that the helicity of the skyrmion in the bottommost layer is rotated. It yields a good signal to detect whether the system is topological or trivial. Our results show that the helicity dynamics of the skyrmion string have a rich physics in the modulated exchange-coupled system.

Crystal structure and thermodynamic properties of ${\mathrm{Na}}_2\mathrm{SrCo}{\left({\mathrm{PO}}_4\right)}_2$, the chemical sibling of the triangular quantum spin-liquid candidate ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$, are reported. From single crystal x-ray diffraction and high-resolution synchrotron x-ray powder diffraction, the compound was found to crystallize in the monoclinic space group $P{2}_1/a$ at room temperature, in contrast to the trigonal ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$. Above 650 K, the symmetry of ${\mathrm{Na}}_2\mathrm{SrCo}{\left({\mathrm{PO}}_4\right)}_2$ changes to $C2/m$, while around 1025 K a further transformation toward trigonal symmetry is observed. The monoclinic symmetry leads to a small deformation of the ${\mathrm{CoO}}_6$ octahedra beyond the trigonal distortion ubiquitous in this structure type and results in the stronger $g$-tensor anisotropy ($g_{\text{z}}/g_{\text{xy}}=1.6$) as well as the increased XXZ anisotropy ($J_{\text{z}}/J_{\text{xy}}=2.1$) compared to the Ba compound ($g_{\text{z}}/g_{\text{xy}}=1.1, J_{\text{z}}/J_{\text{xy}}=1.5$), while the average coupling strength, $J_{\text{av}}/k_{\text{B}}=(2J_{\text{xy}}+J_{\text{z}})/3k_{\text{B}}\simeq 1.3\text{K}$, remains unchanged. The Néel temperature increases from 140 mK (Ba) to 600 mK (Sr), and an uncompensated in-plane moment of $0.066\left(4\right){\mu }_{\text{B}}/\text{f.u.}$ appears. We show that the ordering temperature of a triangular antiferromagnet is capably controlled by its structural distortions.

Understanding instabilities is of vast importance in material science. One such example is the Peierls instability of one-dimensional metals with a single band. The question is why H and Li chains behave so differently in spite of their similar electronic structure. We demonstrate that this is due to the suppression of the susceptibility for Li because of the interatomic, interorbital s-p mixing of the band crossing the Fermi energy. The influence of this on the structure-factor-like term in response functions is critical and depends on the wave function's details also in higher dimensions.

${\mathrm{La}}_{0.92}{\mathrm{Sr}}_{0.08}{\mathrm{MnO}}_3$ films with the thickness ($t$) ranging from 8 to 157 nm, grown on (001)-oriented ${\mathrm{SrTiO}}_3$ substrates by rf magnetron sputtering, undergo a secondary metal-insulator transition (SMIT) below a critical temperature when $t\ge 63\mathrm{nm}$. The transition temperature gradually increases from 25 K at 63 nm to 108 K at 157 nm with increasing film thickness. Raman spectra demonstrate that the occurrence of the SMIT is attributed to the reduction of the Jahn-Teller distortion. Further investigations indicate that the SMIT can take place in the ferromagnetic $\left(63\mathrm{nm}\le t<98\mathrm{nm}\right)$ and $A$-type antiferromagnetic ($t\ge 98\mathrm{nm}$) spin structures, which is ascribed to the generation of two-dimensional conduction bands in the ${\mathrm{MnO}}_2$ planes through the double-exchange interaction. This work suggests that the SMIT provides a pathway to study multiple metal-insulator transitions and demonstrates the potential applications in spintronic devices.

Based on a relationship with continuous-time random walks discovered by Iglói, Turban, and Rieger [Phys. Rev. E 59, 1465 (1999)], we derive exact lower and upper bounds on the lowest energy gap of open transverse-field Ising chains, which are explicit in the parameters and are generally valid for arbitrary sets of possibly random couplings and fields. In the homogeneous chain and in the random chain with uncorrelated parameters, both the lower and upper bounds are found to show the same finite-size scaling in the ferromagnetic phase and at the critical point, demonstrating the ability of these bounds to infer the correct finite-size scaling of the critical gap. Applying the bounds to random transverse-field Ising chains with coupling-field correlations, a model which is relevant for adiabatic quantum computing, the finite-size scaling of the gap is shown to be related to that of sums of independent random variables. We determine the critical dynamical exponent of the model and reveal the existence of logarithmic corrections at special points.

The ever expected canted antiferromagnetic (CAFM) $Pb\prime n\prime m:{\mathrm{\Gamma }}_4({\mathit{G}}_{\mathit{x}},A_y,F_Z;F_Z^R)$ to $Pbn\prime m\prime :{\mathrm{\Gamma }}_2(F_x,C_y,{\mathit{G}}_{\mathit{Z}};F_x^R, {\mathit{C}}_{\mathit{y}}^{\mathit{R}})$ spin reorientation phase transition (SRPT) has only recently been confirmed through high-resolution time-of-flight neutron scattering studies by Sau et al. [Phys. Rev. B 103, 144418 (2021)]. Despite several studies on $\mathrm{SmCr}{\mathrm{O}}_3$, the nature of its SRPT still remains debatable. In the present study, we revisit the issue through dc $M\left(T\right)$ and ac-susceptibility, ${\chi }_{\mathrm{ac}}\left(T\right)$, measurements. Repeated cycle field-cooled-cooling and field-cooled-warming dc $M\left(T\right)$ measurements clearly expose a temperature point differentiating the regimes of continuous and discontinuous parts of the SRPT. The discontinuous part has a tiny but clear hysteresis in $M\left(T\right)$, confirming the first-order nature of the SRPT with supercooling $\left(T^{*}\right)$ and superheating $\left(T^{**}\right)$ temperatures to be $\sim 33$ and $\sim 36\mathrm{K}$, respectively. The hysteresis in the $M\left(T\right)$ is strongly supported by the occurrence of hysteresis in the nondispersing peaks in ${\chi }_{\mathrm{ac}}\left(T\right)$, measured using a 3 Oe ac signal amplitude during cooling and heating under zero dc-bias. Below SRPT, the complete reversibility of $M\left(T\right)$ and ${\chi }_{\mathrm{ac}}\left(T\right)$ confirms the second-order nature of the ${\mathrm{Sm}}^{3+}$ ordering at $T_{\mathrm{N}2}$, which arises due to independent ${\mathrm{Sm}}^{3+}\text{−}{\mathrm{Sm}}^{3+}$ interaction. Similarly, the absence of hysteresis in $M\left(T\right)$ as well as in ${\chi }_{\mathrm{ac}}\left(T\right)$, across the paramagnetic to CAFM ${\mathrm{\Gamma }}_4$ phase transition, proves the second-order nature of this phase transition.

Using the combined theoretical approaches, the structural, electronic, and magnetic properties of the two-dimensional (2D) $1T$ phase of monolayer ${\mathrm{RhO}}_2$ were studied. Density-functional theory plus $U$ calculation indicate that $1T\text{−}{\mathrm{RhO}}_2$ favors a ferromagnetic metallic phase with an out-of-plane magnetization, and it can be achieved by exfoliation from the layered bulk oxides containing monolayer $1T\text{−}{\mathrm{RhO}}_2$. Monte Carlo simulation shows that the ferromagnetic phase is stable below the Curie temperature of 73.9 K based on the Heisenberg Hamiltonian model. Magnetic anisotropy energy and its cause were further investigated to understand the microscopic origin of the 2D magnetization. Our results indicate that spin-orbit coupling interaction between Rh $4d$ stabilizes perpendicular magnetic anisotropy, resulting in the out-of-plane magnetization, over in-plane magnetic anisotropy. In addition, mechanical tensile strain can strengthen perpendicular magnetic anisotropy by increasing positive contribution of the interaction to the magnetic anisotropy energy.

We examine the impact of random potential due to remote impurites (RIs) and its in situ controlled screening on fractional quantum Hall effects (FQHEs) around Landau-level filling factor $\nu =1/2$. The experiment is made possible by using a dual-gate GaAs quantum well (QW) that allows for the independent control of the density $n_e$ of the two-dimensional electron system in the QW and that ($n_{\mathrm{SL}}$) of excess electrons in the modulation-doping superlattice. As the screening is reduced by decreasing $n_{\mathrm{SL}}$ at a fixed $n_e$, we observe a decrease in the apparent energy gap of the FQHEs deduced from thermal activation, which signifies a corresponding increase in the disorder broadening $\mathrm{\Gamma }$ of composite fermions (CFs). Interestingly, the increase in $\mathrm{\Gamma }$ is accompanied by a noticeable increase in the longitudinal resistivity at $\nu =1/2$ (${\rho }_{1/2}$), with a much stronger correlation with $\mathrm{\Gamma }$ than electron mobility $\mu$ has. The in situ control of RI screening enables us to disentangle the contributions of RIs and background impurities (BIs) to ${\rho }_{1/2}$ with the latter in good agreement with the CF theory. We construct a scaling plot that helps in estimating the BI contribution to ${\rho }_{1/2}$ for a given set of $n_e$ and $\mu$.

We derive the fundamental limit to near-field radiative thermal rectification mediated by an intrinsic semiconductor film within the framework of fluctuational electrodynamics. By leveraging the electromagnetic local density of states, we identify ${\varepsilon }_{\mathrm{H}}^{″}/{\varepsilon }_{\mathrm{L}}^{″}$ as an upper bound on the rectification magnitude, where ${\varepsilon }_{\mathrm{H}}^{″}$ and ${\varepsilon }_{\mathrm{L}}^{″}$ are the imaginary parts of the film permittivity at high and low temperatures, respectively. This bound is tight and can be approached regardless of whether the film is suspended or supported. For intrinsic silicon the limit can in principle exceed ${10}^9$. Our work highlights the possibility of controlling heat flow as effectively as electric current, and offers guidelines to potentially achieve this goal.

The photonic spin Hall effect is a manifestation of the spin-orbit interaction of light and can be measured by a transverse shift $\delta$ of photons with opposite spins. The precise measurement of transverse shifts can enable many spin-related applications, such as precise metrology and optical sensing. However, this transverse shift is generally small (i.e., $\delta /\lambda <{10}^{-1}$, where $\lambda$ is the wavelength), which impedes its precise measurement. To date, proposals to generate a giant spin Hall effect (namely, with $\delta /\lambda >{10}^2$) have severe limitations, particularly its occurrence only over a narrow angular cone (with a width of $\mathrm{\Delta }\theta <1^{\circ }$). Here we propose a universal scheme to realize the wide-angle giant photonic spin Hall effect with $\mathrm{\Delta }\theta >{70}^{\circ }$ by exploiting the interface between free space and uniaxial epsilon-near-zero media. The underlying mechanism is ascribed to the almost-perfect polarization splitting between $s$ and $p$ polarized waves at the designed interface. Remarkably, this almost-perfect polarization splitting does not resort to the interference effect and is insensitive to the incident angle, which then gives rise to the wide-angle giant photonic spin Hall effect.

The natural and true band profiles at heterojunctions formed by hexagonal ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ alloys are investigated by a variety of methods: density-functional theory for atomic geometries, approximate quasiparticle treatments for electronic structures, different band-edge alignment procedures, and construction of various hexagonal unit cells to model alloys and heterojunctions. We demonstrate that the natural band offsets are rather unaffected by the choice to align the vacuum level or the branch point energy, as well as by the use of a hybrid or the Tran-Blaha functional. At interfaces between Ge-rich alloys we observe a type-I heterocharacter with direct band gaps, while Si-rich junctions are type-I but with an indirect band gap. The true band lineups at pseudomorphically grown heterostructures are strongly influenced by the generated biaxial strain of opposite sign in the two adjacent alloys. Our calculations show that the type-I character of the interface is reduced by strain. To prepare alloy heterojunctions suitable for active optoelectronic applications, we discuss how to decrease the compressive biaxial strain at Ge-rich alloys.

Light-hole (lh) excitons (Xlhs) in quantum wells (QWs) are hardly studied compared with heavy-hole (hh) excitons (Xhhs), mainly due to the difficulties of their experimental observation. In this paper, a comprehensive study of both types of excitons in high-quality GaAs/AlGaAs QWs of different widths is performed. We focus on the energy positions of exciton resonances and the exciton-light interaction. The effect of mixing lhs and hhs in GaAs-based structures on these exciton characteristics is investigated. The corrections to the exciton energy due to mixing are only a fraction of millielectronvolts for wide QWs, in which the energy levels of Xhhs and Xlhs are close to each other. It is also experimentally found that the oscillator strength of Xlhs is $\sim 2.5$ times less than that of Xhhs. This value noticeably deviates from the 3:1 oscillator strength ratio known for optical transitions between free electron and hole states. This deviation originates from the different squeezing of the Xlh and Xhh wave functions due to distinct values of the effective masses of the Xlh and Xhh in the heterostructure. Mixing of hh and lh valence bands is not so important.

Low-field microwave absorption techniques are ultrasensitive, nondestructive methods for probing electric and magnetic properties of solids. Nonresonant low-field microwave absorption techniques such as magnetic field modulated microwave spectroscopy (MFMMS) can easily detect electromagnetic phase transitions in minute and inhomogeneous samples. While this technique can easily and almost selectively identify superconducting transitions, magnetic phase transitions produce more varied responses. Here, we present a technique to investigate the electric and magnetic properties of a sample with complex electromagnetic responses. This technique involves taking a series of magnetic hysteresis loops and magnetoresistance measurements. These can be compared to MFMMS data to identify features having electric or magnetic origin. This approach is applied to magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$, which possesses an electric, magnetic, and structural phase transition across its Verwey transition. By measuring high-quality ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin films in MFMMS and complementary techniques, the previously inscrutable MFMMS signal is analyzed. Furthermore, a model of the MFMMS signal can be calculated from the magnetic and electric data, which reproduces most of the features of the experimentally obtained MFMMS signal. This technique broadens the capabilities of MFMMS beyond the detection of superconductors.

We synthesized ${\mathrm{Ti}}_2{\mathrm{O}}_3$ epitaxial films with continuously varying ratios of the c-axis to a-axis lattice constants ($c/a$ ratios) on 4H-SiC (0001) substrates and investigated their structural and electronic properties. ${\mathrm{Ti}}_2{\mathrm{O}}_3$ films with a wide range of $c/a$ ratios were fabricated in a controllable fashion by changing the growth temperature. As the $c/a$ ratio at room temperature increased, the metal-insulator transition (MIT) temperature systematically decreased and eventually the MIT disappeared. Detailed analyses revealed that the MIT occurred at a critical $c/a$ ratio of 2.68. The critical $c/a$ ratio for the occurrence of the MIT was also reproduced by density functional theory calculations. These results provide evidence for the origin of the MIT in ${\mathrm{Ti}}_2{\mathrm{O}}_3$. The MIT is not a Mott transition induced by temperature, but a gradual semimetal-to-semiconductor transition induced by lattice deformation.

By taking inspiration from the backflow transformation for correlated systems, we introduce a tensor network Ansatz which extends the well-established matrix product state representation of a quantum many-body wave function. This structure provides enough resources to ensure that states in dimensions larger than or equal to one obey an area law for entanglement. It can be efficiently manipulated to address the ground-state search problem by means of an optimization scheme which mixes tensor-network and variational Monte Carlo algorithms. We benchmark the Ansatz against spin models both in one and two dimensions, demonstrating high accuracy and precision. We finally employ our approach to study the challenging $S=1/2$ two-dimensional (2D) $J_1\text{−}{J}_2$ model, demonstrating that it is competitive with the state-of-the-art methods in 2D.