Recent theoretical and experimental work has demonstrated how the superconducting critical temperature $\left(T_c\right)$ can be modified by rotating the magnetization of a single homogeneous ferromagnet proximity-coupled to the superconducting layer. This occurs when the superconductor and ferromagnet are separated by a thin heavy normal metal that provides an enhanced interfacial Rashba spin-orbit interaction. In the present work, we consider the reciprocal effect: magnetization reorientation driven by the superconducting phase transition. We solve the tight-binding Bogoliubov–de Gennes equations on a lattice self-consistently and compute the free energy of the system. We find that the relative angle between the spin-orbit field and the magnetization gives rise to a contribution in the free energy even in the normal state, $T>T_c$, due to band-structure effects. For temperatures below $T_c$, superconductivity gives rise to a competing contribution. We demonstrate that by lowering the temperature, in addition to reorientation of the favored magnetization direction from in-plane to out-of-plane, a $\pi /4$ in-plane rotation for thicker ferromagnetic layers is possible. Furthermore, computation of $T_c$ of the structure in the ballistic limit shows a dependence on the in-plane orientation of the magnetization, in contrast to our previous result on the diffusive limit. This finding is relevant with respect to thin-film heterostructures since these are likely to be in the ballistic regime of transport rather than in the diffusive regime. Finally, we discuss the experimental feasibility of observing the magnetic anisotropy induced by the superconducting transition when other magnetic anisotropies, such as the shape anisotropy for a ferromagnetic film, are taken into account. Our work suggests that the superconducting condensation energy in principle can trigger a reorientation of the magnetization of a thin-film ferromagnet upon lowering the temperature below $T_c$, in particular for ferromagnets with weak magnetic anisotropies.

In the all-organic paramagnetic correlated metal ${\beta }^{″}\text{−}{\left(\text{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$ spin fluctuations can be ruled out as the glue to superconductivity below $T_c=5.5$ K. Instead, we demonstrate the coupling of charge degrees of freedom to the superconducting state, which occurs adjacent to a charge-ordered insulating phase. A comparison to isostructural ${\beta }^{″}\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{SF}}_{5}R{\mathrm{SO}}_3$ compounds reveals that superconductivity is suppressed when the system is tuned away from the insulator towards the metal; an enhanced charge imbalance, on the other hand, leads to an increase in $T_c$, suggesting an involvement of charge fluctuations. By locally probing the charge disproportionation through the superconducting transition, we discover slight modifications of the Fano coupling constant and intensity at $T_c$, indicating the effect of the paired electrons on the molecular vibrations.

The lattice dynamics of low-dimensional ${\mathrm{BaVS}}_3$ is reported across the hexagonal-orthorhombic phase transition occurring at $T_S=250\mathrm{K}$ using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density functional theory calculations. We observed the occurrence of strongly temperature-dependent diffuse scattering upon approaching the transition, centered at the $\mathrm{\Gamma }$ points associated with weak Bragg reflections with odd $L$. Inelastic scattering experiments related this observation to the condensation of a low-lying overdamped optical phonon at the Brillouin zone center, evidenced by significant variations of quasielastic scattering. These results unravel the dynamical origin of the zigzag distortion of the V chains at $T_S$, which is probably a prerequisite of the Peierls anomaly driving the metal-insulator transition at $T_{\mathrm{MI}}=70\mathrm{K}$.

We present thermodynamic and neutron-scattering measurements on the quantum spin ice candidate ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. The parametrization of the anisotropic exchange Hamiltonian is refined based on high-energy-resolution inelastic neutron-scattering data together with thermodynamic data using linear spin-wave theory and numerical linked-cluster expansion. Magnetic phase diagrams are calculated using classical Monte Carlo simulations with fields along [100], [110], and [111] crystallographic directions which agree qualitatively with the experiment. Large hysteresis and irreversibility for [111] is reproduced and the microscopic mechanism is revealed by mean-field calculations to be the existence of metastable states and domain inversion. Our results shed light on the explanations of the recently observed dynamical kagome ice in ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ in [111] fields.

We consider a simple Ising magnetic model in two dimensions with Einstein site phonons and study it using Monte Carlo simulations that take into account both degrees of freedom simultaneously. In nonfrustrated systems, like the square lattice with ferromagnetic and antiferromagnetic interactions, we find that the coupling of the magnetic to the elastic degrees of freedom gradually lowers the magnetic ordering transition until it is completely suppressed at a critical value of the coupling constant. Above this the system suffers a simultaneous magnetic and structural transition into a dimerized state with lower crystalline symmetry and ferromagnetic clusters antiferromagnetically aligned. In the case of the Kagomé lattice with antiferromagnetic interactions, which is frustrated, we show that a similar ordered state takes place when the coupling constant is above a critical value.

The dynamics of magnetization and energy densities are studied in the two-leg spin-$1/2$ ladder. Using an efficient pure-state approach based on the concept of typicality, we calculate spatiotemporal correlation functions for large systems with up to 40 lattice sites. In addition, two subsequent Fourier transforms from real to momentum space as well as from the time to frequency domain yield the respective dynamical structure factors. Summarizing our main results, we unveil the existence of genuine diffusion for both spin and energy. In particular, this finding is based on four distinct signatures which can all be equally well detected: (i) Gaussian density profiles, (ii) time-independent diffusion coefficients, (iii) exponentially decaying density modes, and (iv) Lorentzian line shapes of the dynamical structure factor. The combination of (i)–(iv) provides a comprehensive picture of high-temperature dynamics in this archetypal nonintegrable quantum model.

The temperature-dependence of the in-plane optical properties of $({\mathrm{CaFe}}_{1-x}{\mathrm{Pt}}_x{\mathrm{As})}_{10}{\mathrm{Pt}}_3{\mathrm{As}}_8$ have been investigated for the undoped ($x=0$) parent compound, and the optimally doped ($x=0.1$) superconducting material ($T_c\simeq 12$ K) over a wide frequency range. The optical conductivity has been described using two free-carrier (Drude) components, in combination with oscillators to describe interband transitions. At room temperature, the parent compound may be described by a strong, broad Drude term, as well as a narrow, weaker Drude component. Below the structural and magnetic transitions at $\simeq 96$ and 83 K, respectively, strength is transferred from the free-carrier components into a bound excitation at $\simeq 1000{\mathrm{cm}}^{-1}$, and the material exhibits semiconducting-like behavior. In the optimally doped sample, at room temperature the optical properties are again described by narrow and broad Drude responses comparable to the parent compound; however, below $T^{*}\simeq 100$ K, strength from the narrow Drude is transferred into a newly emergent low-energy peak at $\simeq 120{\mathrm{cm}}^{-1}$, which arises from a localization process, resulting in semiconducting-like behavior. Interestingly, below $T_c$, this peak also contributes to the superfluid weight, indicating that some localized electrons condense into Cooper pairs; this observation may provide insight into the pairing mechanism in iron-based superconductors.

Transition metal oxides have been one of the central objects in the studies of electron correlation effects because of their rich variety of physical properties mainly depending on the transition metal element. On the other hand, exploiting the anion degrees of freedom is less popular but can be another promising way to control properties of strongly correlated materials. In particular, oxyhydrides offer a unique playground of strongly correlated low-dimensional electronic structure, where the $s$ orbitals of hydrogen breaks a chemical bond between the cation $t_{2g}$ orbitals. In this study, we evaluate the effective interaction, i.e., the screened Coulomb interaction parameters in low-energy effective models, for vanadium oxyhydrides ${\mathrm{Sr}}_{n+1}{\mathrm{V}}_n{\mathrm{O}}_{2n+1}{\mathrm{H}}_n$ ($n=1,\infty$) using the constrained-random-phase approximation (cRPA). We find that the effective interaction in the $t_{2g}$ model, where only the $t_{2g}$ orbitals are explicitly considered, is strongly screened by the $e_g$ bands compared with that for oxides, because the $e_g$ bands are very entangled with the $t_{2g}$ bands in the oxyhydrides. On the other hand, the effective interaction is rather strong in the $d$ model, where all the vanadium $d$ orbitals are explicitly considered, owing to a large energy separation between the $\mathrm{V}\text{−}d$ bands and the anion bands ($\mathrm{O}\text{−}p$ and $\mathrm{H}\text{−}s$), because the $\mathrm{O}\text{−}p$ states are stabilized by the existence of the hydrogen atoms. These findings suggest that nontrivial and unique correlation effects can take place in vanadium oxyhydrides.

Experimentally, we establish the quantitative phase diagram of 1/4-filled ${\beta }^{″}$-(BEDT-${\mathrm{TTF})}_2{\mathrm{SF}}_5$R${\mathrm{SO}}_3$ with a tendency toward charge order. Comprehensive optical, transport, and susceptibility measurements reveal the insulating nature and magnetic properties of the charge-ordered ground state. Going from $R=\mathrm{CHF}$ via ${\mathrm{CH}}_2{\mathrm{CF}}_2$ and ${\mathrm{CHFCF}}_2$ toward ${\mathrm{CH}}_2$, a finite charge disproportionation appears and grows up to $2\delta =0.5e$, as evidenced by charge-sensitive molecular vibrations. This is accompanied by an increase of electronic correlation strength, i.e., the intersite Coulomb repulsion $V$ becomes more pronounced in relation to the bandwidth $W$. The broadband electronic excitations and their anisotropy unveil a distinct charge pattern for $R={\mathrm{CH}}_2$ (checkerboard type) as compared to the other compounds (stripelike). Our results validate theoretical predictions for 1/4-filled systems, substantiating the importance of charge fluctuations for unconventional superconductivity at the verge between metal and insulator.

We develop an approach to design, engineer, and measure band structures in a synthetic crystal composed of electric circuit elements. Starting from a nodal analysis of a circuit lattice in terms of currents and voltages, our Laplacian formalism for synthetic matter allows us to investigate arbitrary tight-binding models in terms of wave-number-resolved Laplacian eigenmodes, yielding an admittance band structure of the circuit. For illustration, we model and measure a honeycomb circuit featuring a Dirac cone admittance bulk dispersion as well as flat band admittance edge modes at its bearded and zigzag terminations. We further employ our circuit band analysis to measure a topological phase transition in the topolectrical Su-Schrieffer-Heeger circuit.

Many proposals to generate a time-reversal invariant topological superconducting phase are based on imposing a $\pi$-phase difference between the superconducting leads proximitizing a nanostructure. We show that this phase can be induced on a thin film of a topological insulator such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in proximity to a single $s$-wave superconductor. In our analysis we take into account the parity degree of freedom of the electronic states which is not included in effective Dirac-like surface theories. We find that the topological phase can be reached when the induced interparity pairing dominates over the intraparity one. Application of an electric field perpendicular to the film extends the range of parameters where the topological phase occurs.

We investigated the magnetotransport properties of Fe-doped topological insulator $\mathrm{S}{\mathrm{b}}_{1.96}\mathrm{F}{\mathrm{e}}_{0.04}\mathrm{T}{\mathrm{e}}_3$ single crystals. With doping, the band structure changes significantly and multiple Fermi pockets become evident in the Shubnikov–de Haas oscillations, in contrast to the single frequency detected for pure $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$. Using complementary density functional theory calculations, we identify an additional bulk hole pocket introduced at the Γ point which originates from the chemical distortion associated with the Fe dopant. Experimentally, both doped and undoped samples are hole-carrier dominated; however, Fe doping also reduces the carrier density and mobility. The angle dependent quantum oscillations of $\mathrm{S}{\mathrm{b}}_{1.96}\mathrm{F}{\mathrm{e}}_{0.04}\mathrm{T}{\mathrm{e}}_3$ were analyzed to characterize the complex Fermi surface and isolate the dimensionality of each SdH feature. Among those components, we found two oscillations frequencies, which related to two Fermi pockets are highly angle dependent. Moreover, the fermiology changes via Fe doping and may also provide a different Berry phase, as demonstrated by the Landau fan diagram, thus indicating a rich complexity in the underlying electronic structure.

Computing momentum-dependent susceptibilities in the dynamical mean-field theory (DMFT) requires solving the Bethe-Salpeter equation, which demands large computational cost. Exploiting the strong-coupling feature of local fluctuations, we derive a simplified formula that can be solved at a considerably lower cost. The validity and the physical meaning of the formula are confirmed by deriving the effective intersite interactions in the strong-coupling limit, such as the kinetic exchange and RKKY interactions. Furthermore, numerical calculations for single-orbital and multiorbital models demonstrate surprisingly wider applicability including a weak-coupling region. Based on this formula, we propose three levels of practical approximations that can be chosen depending on complexity of problems. Simpler evaluations of spin and orbital susceptibilities in multiorbital systems thus become possible within DMFT.

Quantum spin systems with strong geometric restrictions give rise to rich quantum phases such as valence bond solids and spin liquid states. However, the geometric restrictions often hamper the application of sophisticated numerical approaches. Based on the stochastic series expansion method, we develop an efficient and exact quantum Monte Carlo “sweeping cluster” algorithm which automatically satisfies the geometrical restrictions. Here we use the quantum dimer model as a benchmark to demonstrate the reliability and power of this algorithm. Comparing to existing numerical methods, we can obtain higher accuracy results for a wider parameter region and much more substantial system sizes.

$\mathrm{Ti}{\mathrm{O}}_2$ is an intensively studied photocatalytic material owing to its low cost and high activity. The anatase/rutile (A/R) mixed-phase $\mathrm{Ti}{\mathrm{O}}_2$ is recognized as an effective strategy to achieve high photocatalytic efficiency by the type-II band alignment favorable to spatial charge separation. However, the atomic structure, as well as the exact band alignment of the A/R mixed-phase $\mathrm{Ti}{\mathrm{O}}_2$, is very difficult to identify either in experimental measurements or theoretical simulations. Moreover, the time-dependent photogenerated carrier dynamics, which can determine the photocatalytic efficiency, has not been studied at the atomic scale. In this paper, we use an adaptive genetic algorithm to search the stable interface structures. We find that the band alignment is determined by the interfacial atomic structures. Especially, with oxygen vacancy $\left({\mathrm{O}}_{\mathrm{V}}\right)$ at the interface, band alignment can be reversed as compared to that of the stoichiometric interface. Then, we select one stoichiometric and one defective structure to study the photogenerated carrier dynamics using the time-dependent ab initio nonadiabatic molecule dynamics. We find that in the stoichiometric system, for both the electron and the hole, the charge transfer happens within 400 fs, which is much shorter than the electron-hole recombination timescale at nanosecond-to-microsecond magnitude, which suggests that the charge transfer can occur efficiently at the interface before they recombine. For the defective A/R system with ${\mathrm{O}}_{\mathrm{V}}$, we find that the electron will be trapped by the defect state within 1 ps, while the hole dynamics is not affected. Our study provides atomic insights into the understanding of the band alignment and photogenerated carrier dynamics at the mixed A/R $\mathrm{Ti}{\mathrm{O}}_2$ interface, which provides valuable guidance for functional material design for solar energy conversion.

Systems that break transport reciprocity have recently opened exciting possibilities for wave manipulation. Here we report nonreciprocal acoustic transmission in cascaded resonators that are modulated in space and time. An analytic approach is developed and the design strategy is discussed for realizing a physical system based on this approach. The theory is verified numerically by finite-difference time-domain (FDTD) simulations, with a one-way isolation factor greater than 25 dB out of just two resonators. Our work provides a feasible route to achieve nonreciprocal acoustic transmission in a compact manner.

Ultrafast optical pump-probe studies of uranium dioxide ($\mathrm{U}{\mathrm{O}}_2$) under pressure were performed in order to better understand the material's response to ionizing radiation. Photoexcitation generates oscillations in the time-resolved reflectivity at two distinct GHz-scale frequencies. The higher-frequency mode is attributed to a coherent longitudinal acoustic mode. The lower-frequency mode does not correspond to any known excitation under equilibrium conditions. The frequency and lifetime of the low-frequency mode are studied as a function of pressure. Abrupt changes in the pressure-dependent slopes of these attributes are observed at $\sim 10$ GPa, which correlates with an electronic transition in $\mathrm{U}{\mathrm{O}}_2$. Variation of probe wavelength reveals that the low-$k$ dispersion of the low-frequency mode does not fit into either an optical or acoustic framework. Rather, we propose that this mode is related to the dynamical magnetic structure of $\mathrm{U}{\mathrm{O}}_2$. The implications of these results help account for the anomalously small volume of damage known to be caused by ionizing radiation in $\mathrm{U}{\mathrm{O}}_2$; we propose that the existence of the low-frequency mode enhances the material's transient thermal conductivity, while its long lifetime lengthens the timescale over which energy is dissipated. Both mechanisms enhance damage recovery.