In the vicinity of the type-I/type-II crossover in conventional superconductors, vortices exhibit a nonmonotonic interaction, which leads to exotic vortex matter states. We perform molecular dynamics simulations on a model superconductor in the intertype regime. In a field cooled approach, we examine the transition of a homogeneous vortex lattice (VL) into a structure consisting of VL domains and Meissner-state domains. The results show extraordinary stability of the transition against vortex pinning and a strong dependence on the external magnetic field.

Point defects that exist widely in solid materials can strongly suppress thermal conductivity κ, a key property that determines the performance of electronic devices, photovoltaics, thermoelectric materials, etc. The κ of materials with point defects is essentially determined by the competition between the intrinsic and extrinsic phonon scattering. In this work, we have employed an ab initio Green's function method combined with the Boltzmann transport equation to investigate the influence of typical point defects on the κ and phonon-scattering landscape in cubic BP and BAs, which have recently attracted intense interest due to their high κ. We show the magnitude and temperature dependence of κ can be strongly suppressed by point defects in both materials due to the weak intrinsic phonon-phonon scattering, even for extremely low defect fractions at low temperatures, e.g., 0.001% P vacancy reduces the κ of BP by 23% at 100 K. The competition between phonon-defect and phonon-phonon scattering depends on the type and fraction of point defects, as well as temperature. For both materials, vacancies result in stronger phonon scattering than substitutions. For the ${\mathrm{Si}}_{\mathrm{B}}$ substitution in BP, the mass difference ($V^M$) results in several times smaller phonon-scattering rates than force-constant perturbation ($V^K$) for most phonons, while the $V^M$ produces comparable or even stronger phonon scattering than $V^K$ within 3.5–9.4 THz for ${\mathrm{Si}}_{\mathrm{As}}$ in BAs mainly due to the large As-to-Si mass ratio. Also, the frequency dependence of phonon-defect scattering rates by vacancies and substitutions follows $\sim {\omega }^4$ in the low-frequency range for both materials, a typical behavior of Rayleigh scattering. Furthermore, the scattering strength of different phonon branches by point defects varies and depends on the defect type. These results provide deep insights into phonon scattering in materials with point defects and will be helpful for manipulating the thermal properties of materials by defect engineering for relevant applications.

We developed a path-integral quantum Monte Carlo-based methodology for calculation of polaron mobility in systems with electron-phonon interaction. Within the method, the current-current correlation function in both the imaginary and real time is calculated in a numerically exact way. The choice of basis for representation of the path integral enabled us to reduce the sign problem and perform real-time calculations for longer times. The DC polaron mobility was extracted by performing analytic continuation that makes use of both the real and imaginary-time data. The method was applied to the Holstein polaron model in one dimension. We obtained reliable results for the temperature dependence of the Holstein polaron mobility for interactions ranging from weak to strong and temperatures that are not too low.

We consider a model for the motion of an impurity interacting with two parallel, one-dimensional baths, described as two Tomonaga-Luttinger liquid systems. The impurity is able to move along the baths, and to jump from one to the other. We provide a perturbative expression for the evolution of the system when the impurity is injected in one of the baths, with a given wave packet. We obtain an approximation formally of infinite order in the impurity-bath coupling, which allows us to reproduce the orthogonality catastrophe. We monitor and discuss the dynamics of the impurity and its effect on the baths, in particular for a Gaussian wave packet. Besides the motion of the impurity, we also analyze the dynamics of the bath density and momentum density (i.e., the particle current), and show that it fits an intuitive semiclassical interpretation. We also quantify the correlation that is established between the baths by calculating the interbath, equal-time spatial correlation functions of both bath density and momentum, finding a complex pattern. We show that this pattern contains information on both the impurity motion and on the baths, and that these can be unveiled by taking appropriate “slices” of the time evolution.

Single crystalline intermetallics ${\mathrm{CeInCu}}_2$ and ${\mathrm{NdInCu}}_2$, where the rare earth ions occupy geometrically frustrated fcc lattice of Heusler-type structure, have been studied comparatively to shed light on the ground-state magnetism derived from Kondo physics and/or spin frustration competing with the Ruderman-Kittel-Kasuya-Yosida interaction. ${\mathrm{CeInCu}}_2$ is distinct from ${\mathrm{NdInCu}}_2$ due to the significant Kondo effect that is absent in the latter. They order antiferromagnetically at $T_N\approx 1.4$ and 2.0 K with large paramagnetic Curie-Weiss temperature of $-35.4$ K and $-41.1$ K, respectively. The electronic specific-heat coefficient $\gamma$ = $C/T$ ($T\to 0$) = 870 mJ ${\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ in ${\mathrm{CeInCu}}_2$ is significantly enhanced, much larger than 180 mJ ${\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ of the latter that is also enhanced albeit its non-Kondo nature. For both compounds, the Kadowaki-Woods ratio deviates greatly from the standard value expected from a Kramers doublet ground state, which, however, can be restored if one considers a smaller but still sizable $\gamma$ estimated from the paramagnetic state. Likewise, the Kondo temperature of ${\mathrm{CeInCu}}_2$ is revised to be $T_K\approx$ 13 K that is double the literature values, manifesting a major effect of spin frustration to low-temperature thermodynamics beyond the Kondo physics. Similarities between the two compounds is also noticed in their temperature-field phase diagrams in spite of their thermodynamic distinctions in the thermal expansion and the Grüneisen ratio, which are much more sensitive to Kondo hybridization than to geometrical frustration.

Layered materials are the most important class of solid lubricants. Friction on their surfaces has complex origins. Most experimental methods so far only give total friction force and cannot separate contributions from different origins. Here, we report a method to separate anisotropic and isotropic friction forces on atomically flat surfaces such as ${\mathrm{MoS}}_2$, graphite, h-BN, and mica by combining a two-dimensional friction force microscope technology and a two-dimensional friction model. We found that the friction force of most atomically flat surfaces is anisotropic, the total force on the tip misaligns with the scan direction, and the friction anisotropy vanishes under low sliding velocity. Our two-dimensional friction model explains experimental observations. It reveals the existence of elemental hopping combinations and the isotropic component in total friction. The misalignment angle can be used to calculate the ratio of anisotropic and isotropic friction components and the ratio of resistance forces from different lattice directions. The separation of anisotropic and isotropic friction forces will offer an avenue for studying the properties of individual friction components, which can boost the study of friction mechanisms in the future and benefit the application of solid lubricants.

The search for novel topological states of matter remains to be a research focus in the past several decades. While a topology theory based on Bloch bands is thoroughly investigated in systems with finite-range hopping, mostly in the context of condensed matter physics, here we study a generalized one-dimensional Su-Schrieffer-Heeger model with semi-infinite long-range hopping, and demonstrate another type of topological phase referred to as a Hermitian non-Bloch topological phase. This phase presents a pair of symmetry-protected edge modes, and can be characterized by a topological invariant defined upon real-space wave functions. Interestingly, we also find a large number of localized bulk modes near the band edges, residing at specific positions determined by the ratio between hopping range and system size. The proposed phenomena of a Hermitian non-Bloch topological phase can be realized in metamaterials such as topolectrical circuits and mechanical oscillator lattices.

Using a density functional theory (DFT) plus dynamical mean-field theory methodology, we compute the many-body electronic structure and optical conductivity of ${\mathrm{NdNiO}}_2$ under the influence of large scattering rates on the $\mathrm{Nd}(5d$) bands and including dynamical interactions on the $\mathrm{Nd}(5d$) orbitals with shifts of the Nd-Ni $d$-level energy difference. We find a robust conducting pathway in the out-of-plane direction arising from strong hybridization between the Ni-$d_{z^2}$ and $\mathrm{Nd}(5d$) orbitals. This pathway can be “short-circuited” if this hybridization is suppressed through large electronic scattering rates but is not reduced to zero even by very large beyond-DFT shifts of the Nd-Ni $d$-level energy splitting. The computed in-plane conductivity for ${\mathrm{NdNiO}}_2$ predicts the material to be a “good metal” in contrast to experiments indicating the material is a “bad metal” or “weak insulator”. Our results motivate future experiments measuring the $c$-axis resistivity as a proxy for the spectator bands and suggests the essential difference between the infinite-layer nickelates and the cuprates is dimensionality of their electronic structures.

Searching for thermoelectrics with low lattice thermal conductivity ${\kappa }_l$ is crucial to address needs for waste heat recovery, but few light element dominated materials provide a low ${\kappa }_l$. Herein, we propose an effective strategy, i.e., a split-anion approach, to reduce the ${\kappa }_l$ of the antifluorite. After introducing the split-anion approach, the stable quasiantifluorite ${\mathrm{Li}}_4\mathrm{OSe}$ is obtained, and ${\kappa }_l$ is reduced by $\sim 90\%$ compared to the antifluorite ${\mathrm{Li}}_2\mathrm{Se}$. The ${\kappa }_l$ value of ${\mathrm{Li}}_4\mathrm{OSe}$ reaches as low as 0.49–1.75 W ${\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, the range of thermoelectric applications, despite numerous light Li atoms and a large mass mismatch among the constituent atoms. This significant reduction of the thermal conductivity mainly results from the soft phonon branches and large optical bandwidth induced by lattice symmetry breaking after the split-anion approach. These findings offer ample scope for tuning the phonon band structure and thermal transport by reducing lattice symmetry like in the split-anion approach, leading to ultralow ${\kappa }_l$ of materials.

We calculate the polarization-controlled Rayleigh scattering response of twisted bilayer graphene (tBLG) based on the continuum electronic band model developed by Bistritzer and MacDonald while considering its refinements which address the effects of structural corrugation, doping-dependent Hartree interactions and particle-hole asymmetry. The dominant wave vectors for the Rayleigh scattering process emanate from various regions of the moiré Brillouin zone (MBZ) in contrast to single-layer graphene (SLG) and AB-stacked bilayer graphene (AB-BLG), where the dominant contributions always stem from the vicinity of the $\mathbf{K}$ point for optical laser energies and below. Compared to SLG, the integrated Rayleigh intensity is strongly enhanced for small twist angles (e.g., at a twist angle $\theta =1.2^{\circ }$, the integrated Rayleigh intensity at laser energy $E_l=2$ eV enhances by a factor of $\sim 100$ for the case of parallel polarization). While for the case of cross-polarization, it exhibits a markedly complex behavior suggestive of strong interference effects mediated by the optical matrix elements. We find that at small twist angles, e.g., $\theta =1.{05}^{\circ }$, the corrugation effects strongly enhances the ratio $R_A=\frac{\text{integrated}\text{Rayleigh}\text{intensity}\text{for}\text{parallel}\text{polarization}}{\text{integrated}\text{Rayleigh}\text{intensity}\text{for}\text{cross-polarization}}$ by $\sim 1300$ times viz-à-viz SLG or AB-BLG. Measured as a function of the incoming laser energy $E_l, R_A$ exhibits a characteristic evolution as the twist angle reduces, thus providing a unique fingerprint of the prevailing twist angle of the tBLG sample under study, which would be interesting to verify experimentally.

X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) measurements at the oxygen (O) $K$ edge were performed to investigate the magnetic polarization of ligand O atoms in the weak ferromagnetic (WFM) phase of the Ir perovskite compound ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. With the onset of the WFM phase below $T_{\mathrm{N}}\simeq 240$ K, XMCD signals corresponding to XAS peaks respectively identified as originating from the magnetic moments of apical and planar oxygen (${\mathrm{O}}_{\mathrm{A}}$ and ${\mathrm{O}}_{\mathrm{P}}$) in the ${\mathrm{IrO}}_6$ octahedra were observed. The observation of magnetic moments at ${\mathrm{O}}_{\mathrm{A}}$ sites is consistent (except for the relative orientation) with that suggested by prior muon spin rotation ($\mu \mathrm{SR}$) experiment in the noncollinear antiferromagnetic (NC-AFM) phase below $T_{\mathrm{M}}\approx 100$ K. Assuming that the ${\mathrm{O}}_{\mathrm{A}}$ magnetic moment observed by $\mu \mathrm{SR}$ is also responsible for the corresponding XMCD signal, the magnetic moment of ${\mathrm{O}}_{\mathrm{P}}$ is estimated to be consistent with the previous $\mu \mathrm{SR}$ result. Since the ${\mathrm{O}}_{\mathrm{A}}$ XMCD signal is mainly contributed by Ir $5dzx$ and $yz$ orbitals which also hybridize with ${\mathrm{O}}_{\mathrm{P}}$, it is inferred that the relatively large ${\mathrm{O}}_{\mathrm{P}}$ magnetic moment is induced by Ir $5dxy$ orbitals. Moreover, the inversion of ${\mathrm{O}}_{\mathrm{A}}$ moments relative to Ir moments between the two magnetic phases revealed by XMCD suggests the presence of competing magnetic interactions for ${\mathrm{O}}_{\mathrm{A}}$, with which the ordering of ${\mathrm{O}}_{\mathrm{A}}$ moments in the NC-AFM phase may be suppressed to $T_{\mathrm{M}}$.

The incorporation of driven auxiliary nodes in Floquet lattices can substantially enhance their topological nature, resulting in unique nontrivial phases. Here, we develop an alternative design approach in Floquet topology by introducing a dual-helix modulation scheme that can simultaneously support a clockwise and a counterclockwise helicity in a singular unit cell. As a result, the band structure of a dual-helix configuration can parametrically deform between a conventional phase that exhibits crossed edge states and an antichiral phase with tilted unidirectional states, showcasing for the first time an antichiral phase in Floquet topological systems. An extended family of tilted and overtilted edge states is identified and topologically characterized, revealing that integer Chern and winding numbers can successfully characterize lattices that lack global topological gaps. This results in a rich dynamic response where light wave-packets can acquire different velocities at adjacent edges, and even halt or reverse their direction into unidirectional bulk channels, preserving their topological protection. To investigate these effects, we study a dual-helix photonic lattice of evanescently coupled waveguides where a nontrivial topology can now emerge on a completely stationary configuration, departing from conventional schemes that require helical variations of the lattice sites.

We study the magnetic interactions in transition-metal multilayers composed of Co layers and Co/Fe bilayers sandwiched between Ir and Rh layers based on density functional theory (DFT). By mapping our total energy DFT calculations of collinear and noncollinear spin states including spin-orbit coupling to an atomistic spin model we extract the intra- and interlayer exchange constants, the intra- and interlayer Dzyaloshinskii-Moriya interaction constants and the magnetocrystalline anisotropy energies of several multilayers. We demonstrate that the exchange frustration, which stabilizes zero-field sub-10 nm magnetic skyrmions in Rh/Co films on the Ir(111) surface [S. Meyer et al., Nat. Commun. 10, 3823 (2019)], can be transferred to multilayers formed by a repetition of Rh/Co/Ir trilayers. Increasing the number of Co layers reduces the exchange frustration whereas with adding Fe, a sufficient exchange frustration and strength of the Dzyaloshinskii-Moriya interaction can be obtained to stabilize topological spin structures such as skyrmions in Rh/Co/Fe/Ir multilayers. We show that the exchange interaction in Rh/Co/Fe/Ir multilayers results from the interplay of strong frustration of intralayer exchange in the Fe layer, weak ferromagnetic intralayer exchange in the Co layer, and ferromagnetic interlayer Fe-Co exchange. We analyze the Dzyaloshinskii-Moriya interaction in terms of inter- and intralayer contributions as well as its sign based on the electronic structure of Co/Fe based multilayers with different stacking sequences.

We present an extension to simple $s\text{−}d$ models, aiming at simulating ultrafast magnetization dynamics and spin transport in metallic heterostructures. In particular, we consider an alternative spin dissipation channel due to a finite exchange splitting of the $s$ band. From this theory, we show three different mechanisms governing the dynamics of spin accumulation. On top of the already widely discussed “$-dM/dt$” electron-magnon mechanism, we study the role of a dynamic change of exchange splitting (of conduction electrons) as well as the rotation of spins reflected at an interface with a ferromagnet. Finally, we use the presented theory to explain the recent observation of subpicosecond reversal of a ferromagnet in rare-earth free spin valves. Our conclusion agrees with the one of J. Igarashi et al. [Nat. Mater. (2023)] favoring magnetization reversal due to the rotation of the spin polarization of a reflected spin current.

Femtosecond optical control of magnetic materials shows promise for future ultrafast data storage devices. To date, most studies in this area have relied on quasimonochromatic light in magneto-optical pump-probe experiments, which limited their ability to probe semiconducting and molecule-based materials with structured optical spectra. Here, we demonstrate the possibility of extracting the magneto-optical spectrum of the electrons in the conduction band in the two-dimensional van der Waals ferromagnet ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT), which is made possible due to broadband probing in the visible spectrum. The magneto-optical signal is a sum of contributions from electrons in the conduction and valence bands, which are of opposite sign for CGT. Depending on the probe wavelength used, this difference could lead to an erroneous interpretation that the magnetization direction is reversed after excitation, which has important consequences for understanding spin toggle switching phenomena.

We present the results of first-principles calculations based on density functional theory estimating the magnitude of the current-induced spin polarization (CISP) at the surfaces of the $5d$ transition metals with fcc and bcc crystal structures. We predict that the largest surface CISP occurs for W and Ta, whereas CISP is considerably weaker for Pt and Au surfaces. We then discuss how CISP emerges over a length scale equal to a few atomic layers as opposed to the spin accumulation characteristic of the spin Hall effect, which is related to the materials' spin diffusion length. Finally, using our estimates for the CISP magnitude, we suggest that the spin density appearing near W surfaces in experiments is mostly due to CISP, whereas that at Pt surfaces stems from the Hall effect.

We report pressure-driven evolution of the superconducting transition temperature ($T_{\mathrm{c}}$) and upper critical field $B_{\mathrm{c}2}$ $\left(0\right)$ of strong-coupling superconductor ${\mathrm{Ti}}_4{\mathrm{Ir}}_2\mathrm{O}$ that possesses an unusually large $B_{\mathrm{c}2} \left(0\right)$ at ambient pressure (AP). Our results reveal an extremely low-pressure coefficients of $T_{\mathrm{c}}$, i.e., $d{T}_c/dP\approx -0.047$ K/GPa for $P<15$ GPa and −0.017 K/GPa for $15<P<50$ GPa, presumably associated with an inherent large bulk modulus of $\sim 252$ GPa. Interestingly, we find that its $B_{\mathrm{c}2}$ $\left(0\right)$ undergoes a smooth crossover at 35.6 GPa from well beyond to less than the Pauli paramagnetic limit $B_{\mathrm{p}}^{\mathrm{BCS}}\left(0\right)=1.84T_c$; i.e., the $B_{\mathrm{c}2}\left(0\right)\approx 18.2 \mathrm{T}=1.7B_{\mathrm{p}}^{\mathrm{BCS}} \left(0\right)$ at AP decreases to $\sim 5.8 \mathrm{T}=0.88B_{\mathrm{p}}^{\mathrm{BCS}} \left(0\right)$ at 50 GPa. The density-functional calculations predict the possible occurrence of pressure-induced Fermi surface reconstruction for the energy bands near the $K$ point with strong spin-orbit coupling in 31–41 GPa. The present work sheds more light on this intriguing superconductor capable of resisting large external compression and strong magnetic fields.

The normal-state transport properties of superconducting infinite-layer nickelates are investigated within an interacting three-orbital model. It includes effective Ni-$d_{z^2}$ and Ni-$d_{x^2-y^2}$ bands as well as the self-doping band degree of freedom. The thermopower, Hall coefficient, and optical conductivity are modeled within a quasiparticle approximation to the electronic states. Qualitative agreement in comparison to experimentally available Hall data is achieved, with notably a temperature-dependent sign change of the Hall coefficient for larger hole doping $x$. The Seebeck coefficient changes from negative to positive in a nontrivial way with $x$, but generally shows only a modest temperature dependence. The optical conductivity shows a pronounced Drude response and a prominent peak structure at higher frequencies due to interband transitions. While the quasiparticle picture is surely approximative to low-valence nickelates, it provides enlightening insights into the multiorbital nature of these challenging systems.