In this paper we study topological phases in a two-dimensional photonic crystal with broken time ($\mathcal{T}$) and parity ($\mathcal{P}$) symmetries by performing calculations of band structures, Berry curvatures, Chern numbers, edge states, and also numerical simulations of light propagation in the edge modes. Specifically, we consider a hexagonal lattice consisting of triangular gyromagnetic rods. Here the gyromagnetic material breaks $\mathcal{T}$ symmetry while the triangular rods break $\mathcal{P}$ symmetry. Interestingly, we find that the crystal could host quantum anomalous Hall (QAH) phases with different gap Chern numbers ($C_g$) including $|C_g|>1$ as well as quantum valley Hall (QVH) phases with contrasting valley Chern numbers ($C_v$), depending on the orientation of the triangular rods. Furthermore, phase transitions among these topological phases, such as from QAH to QVH and vice versa, can be engineered by a simple rotation of the rods. Our band theoretical analyses reveal that the Dirac nodes at the $K$ and $K^{\prime }$ valleys in the momentum space are produced and protected by the mirror symmetry ($m_y$) instead of the $\mathcal{P}$ symmetry, and they become gapped when either $\mathcal{T}$ or $m_y$ symmetry is broken, resulting in a QAH or QVH phase, respectively. Moreover, a high Chern number ($C_g=-2$) QAH phase is generated by gapping triply degenerate nodal points rather than pairs of Dirac points by breaking $\mathcal{T}$ symmetry. Our proposed photonic crystal thus provides a platform for investigating intriguing topological phenomena which may be challenging to realize in electronic systems, and also has promising potentials for device applications in photonics such as reflection-free one-way waveguides and topological photonic circuits.

We study the transport properties of a superconductor–quantum spin Hall insulator–superconductor hybrid system in the presence of microwave radiation. Instead of adiabatic analysis or use of the resistively shunted junction model, we start from the microscopic Hamiltonian and calculate the d.c. current directly with the help of the nonequilibrium Green's function method. The numerical results show that (i) the I-V curves of background current due to multiple Andreev reflections exhibit a different structure from those in the conventional junctions, and (ii) all Shapiro steps are visible and appear one by one at high frequencies, while at low frequencies, the steps evolve exactly as the Bessel functions and the odd steps are completely suppressed, implying a fractional Josephson effect.

We explore a thermal mechanism of changing the magnetic anisotropy by using femtosecond laser pulses in a low-symmetry dielectric ferrimagnetic garnet ${\left(\mathrm{YBiPrLu}\right)}_3{\left(\mathrm{FeGa}\right)}_5{\mathrm{O}}_{12}$ film grown on the (210)-type ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ substrate as a model media. Employing spectral magneto-optical pump-probe technique and phenomenological analysis, we demonstrate that the magnetization precession in this film is a result of laser-induced changes of the growth-induced magnetic anisotropy along with the ultrafast inverse Faraday effect. The change of magnetic anisotropy relies on the lattice heating induced by laser pulses of any polarization on a picosecond time scale. We show that the orientation of the external magnetic field with respect to the magnetization easy plane affects the precession noticeably. Importantly, the relative contributions from the ultrafast inverse Faraday effect and the change of different growth-induced anisotropy parameters can be controlled varying the applied magnetic field strength and direction. As a result, the amplitude and the initial phase of the excited magnetization precession can be gradually tuned.

We report diffusion Monte Carlo (DMC) and path integral Monte Carlo (PIMC) calculations of the properties of a one-dimensional (1D) Bose quantum fluid. The equation of state, the superfluid fraction ${\rho }_S/{\rho }_0$, the one-body density matrix $n\left(x\right)$, the pair distribution function $g\left(x\right)$, and the static structure factor $S\left(q\right)$ are evaluated. The aim is to test Luttinger liquid (LL) predictions for 1D fluids over a wide range of fluid density and LL parameter $K$. The 1D Bose fluid examined is a single chain of ${}^4\mathrm{He}$ atoms confined to a line in the center of a narrow nanopore. The atoms cannot exchange positions in the nanopore, the criterion for 1D. The fluid density is varied from the spinodal density where the 1D liquid is unstable to droplet formation to the density of bulk liquid ${}^4\mathrm{He}$. In this range, $K$ varies from $K>2$ at low density, where a robust superfluid is predicted, to $K<0.5$, where fragile 1D superflow and solidlike peaks in $S\left(q\right)$ are predicted. For uniform pore walls, the ${\rho }_S/{\rho }_0$ scales as predicted by LL theory. The $n\left(x\right)$ and $g\left(x\right)$ show long range oscillations and decay with $x$ as predicted by LL theory. The amplitude of the oscillations is large at high density (small $K)$ and small at low density (large $K)$. The $K$ values obtained from different properties agree well verifying the internal structure of LL theory. In the presence of disorder, the ${\rho }_S/{\rho }_0$ does not scale as predicted by LL theory. A single $v_J$ parameter in the LL theory that recovers LL scaling was not found. The one body density matrix (OBDM) in disorder is well predicted by LL theory. The “dynamical” superfluid fraction, ${\rho }_S^D/{\rho }_0$, is determined. The physics of the deviation from LL theory in disorder and the “dynamical” ${\rho }_S^D/{\rho }_0$ are discussed.

We report on the structural and superconducting electronic properties of the heavy-electron superconductor ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$. By using a variable-temperature scanning tunneling microscopy (VT-STM) the coexistence of $(\sqrt{2}\times \sqrt{2})\mathrm{R}{45}^{\circ }$ and $(2\times 1)$ surface reconstructions is observed. Similar to earlier observations on the “122” family of Fe-based superconductors, we find that their respective surface fraction strongly depends on the temperature during cleavage, the measurement temperature, and the sample's history. Cleaving at low temperature predominantly results in the $(\sqrt{2}\times \sqrt{2})\mathrm{R}{45}^{\circ }$-reconstructed surface. A detailed analysis of the $(\sqrt{2}\times \sqrt{2})\mathrm{R}{45}^{\circ }$-reconstructed domains identifies $(2\times 1)$-ordered dimers, tertramers, and higher order even multimers as domain walls. Higher cleaving temperatures and the warming of low-temperature–cleaved samples increases the relative weight of the $(2\times 1)$ surface reconstruction. By slowly increasing the sample temperature $T_{\mathrm{s}}$ inside the VT-STM we find that the $(\sqrt{2}\times \sqrt{2})\mathrm{R}{45}^{\circ }$ surface reconstructions transforms into the $(2\times 1)$ structure at $T_{\mathrm{s}}=123$ K. We identify the polar nature of the ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$(001) surface as the most probable driving mechanism of the two reconstructions, as both lead to a charge density $\rho =0.5e^{-}$, thereby avoiding divergent electrostatic potentials and the resulting “polar catastrophe.” Low-temperature scanning tunneling spectroscopy (STS) performed with normal metal and superconducting probe tips shows a superconducting gap which is best fit with an isotropic $s$ wave. We could not detect any correlation between the local surface reconstruction, suggesting that the superconductivity is predominantly governed by ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$ bulk properties. Correspondingly, temperature- and field-dependent data reveal that both the critical temperature and critical magnetic field are in good agreement with bulk values obtained earlier from transport measurements. In the superconducting state the formation of an Abrikosov lattice is observed without any zero bias anomaly at the vortex core.

We study the density of states (DOS) and the transition temperature $T_c$ in a dirty superconducting film with rare classical magnetic impurities of an arbitrary strength described by the Poissonian statistics. We take into account that the potential disorder is a source of mesoscopic fluctuations of the local DOS, and, consequently, of the effective strength of magnetic impurities. We find that these mesoscopic fluctuations result in a nonzero DOS for all energies in the region of the phase diagram where without this effect the DOS is zero within the standard mean-field theory. This mechanism can be more efficient in filling the mean-field superconducting gap than rare fluctuations of the potential disorder (instantons). Depending on the magnetic impurity strength, the suppression of $T_c$ by spin-flip scattering can be faster or slower than in the standard mean-field theory.

We study superconductivity in the Hubbard model on various quasi-one-dimensional lattices with coexisting wide and narrow bands originating from multiple sites within a unit cell, where each site corresponds to a single orbital. The systems studied are the two-leg and three-leg ladders, the diamond chain, and the crisscross ladder. These one-dimensional lattices are weakly coupled to form two-dimensional (quasi-one-dimensional) ones, and the fluctuation exchange approximation is adopted to study spin-fluctuation-mediated superconductivity. When one of the bands is perfectly flat and the Fermi level intersecting the wide band is placed in the vicinity of, but not within, the flat band, superconductivity arising from the interband scattering processes is found to be strongly enhanced owing to the combination of the light electron mass of the wide band and the strong pairing interaction due to the large density of states of the flat band. Even when the narrow band has finite bandwidth, the pairing mechanism still works since the edge of the narrow band, due to its large density of states, plays the role of the flat band. The results indicate the wide applicability of the high-$T_c$ pairing mechanism due to coexisting wide and “incipient” narrow bands in quasi-one-dimensional systems.

By combining polarized pump-probe spectroscopic and Laue x-ray diffraction measurements, we have observed nonequivalent transient optical responses with the probe beam polarized along the $x$ and $y$ axes in overdoped $\mathrm{NaF}{\mathrm{e}}_{1\text{–}x}\mathrm{C}{\mathrm{o}}_x\mathrm{As}$ superconductors. Such transient anisotropic behavior has been uncovered in the tetragonal phase with the doping level and temperature range far from the borders of static nematic phases. The measured transient anisotropy can be well explained as a result of nematic fluctuation driven by an orbital order with energy splitting of the $d_{xz}$- and $d_{yz}$-dominant bands. In addition, the doping level dependence and the pressure effect of the crossover temperature show significant differences between the transient nematic fluctuation and static nematic phase, implying spin and orbital orders may play different roles in static and transient nematic behaviors.

In this paper we investigate a hybrid quantum system comprising a mechanical oscillator coupled via magnetic induced electromotive force to an $LC$ resonator. We derive the Lagrangian and Hamiltonian for this system and find that the interaction can be described by a charge-momentum coupling with a strength that has a strong geometry dependence. We focus our study on a mechanical resonator with a thin-film magnetic coating which interacts with a nanofabricated planar coil. We determine that the coupling rate between these two systems can enter the strong and ultrastrong coupling regimes with experimentally feasible parameters. This magnetomechanical configuration allows for a range of applications including electromechanical state transfer and weak-force sensing.

The hyperfine structure of the ground state of erbium-doped yttrium orthosilicate is analyzed with the use of electron paramagnetic resonance experiments in a tunable microwave resonator. This work was prompted by the disagreement between a recent measurement made at zero magnetic field and a previously published spin Hamiltonian. The ability to vary magnetic field strength, resonator frequency, and the orientation of our sample enabled us to monitor how the frequencies of hyperfine transitions change as a function of a vector magnetic field. We arrived at a different set of spin Hamiltonian parameters, which are also broadly consistent with the existing data. We discuss the reliability of our spin Hamiltonian parameters to make predictions outside the magnetic field and frequency regimes of our data. We also discuss why it proved to be difficult to determine spin Hamiltonian parameters for this material and present data collection strategies that improve the model reliability.

We present an implementation of time-dependent density functional perturbation theory for spin fluctuations, based on plane waves and pseudopotentials. We compute the dynamic spin susceptibility self-consistently by solving the time-dependent Sternheimer equation, within the adiabatic local density approximation to the exchange and correlation kernel. We demonstrate our implementation by calculating the spin susceptibility of representative elemental transition metals, namely bcc Fe, fcc Ni, and bcc Cr. The calculated magnon dispersion relations of Fe and Ni are in agreement with previous work. The calculated spin susceptibility of Cr exhibits a soft-paramagnon instability, indicating the tendency of the Cr spins to condense in an incommensurate spin density wave phase, in agreement with experiment.

The coupling between spins and charge disproportionation states has been investigated in the $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ oxide with neutron powder diffraction. This $A$-site layer ordered triple perovskite $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ undergoes charge disproportionation on cooling and shows two different charge ordering patterns. At 230 K, $\mathrm{F}{\mathrm{e}}^{3.67+}$ disproportionates into a 2:1 ratio of $\mathrm{F}{\mathrm{e}}^{3+}:\mathrm{F}{\mathrm{e}}^{5+}$, which order in a layered manner along the $\langle 010\rangle$ direction of the pseudocubic unit cell. At lower temperatures $(T<170\mathrm{K})$, the charge ordering pattern changes to a layered arrangement along the $\langle 111\rangle$ direction. Neutron powder diffraction data show that in the intermediate temperature range ($170\mathrm{K}<T<230\mathrm{K}$) the spins order into a cycloidal structure on the $ac$ plane for the $\mathrm{F}{\mathrm{e}}^{3+}$ cations while the $\mathrm{F}{\mathrm{e}}^{5+}$ cations remain paramagnetic. For the lowest temperature range ($2\mathrm{K}<T<190\mathrm{K}$), the spin structure follows the charge ordering and evolves to a $\langle 111\rangle$ layered magnetic structure.

The temperature dependence of the London penetration depth $\mathrm{\Delta }\lambda \left(T\right)$ in the superconducting doped topological crystalline insulator ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ was measured down to 450 mK for two different doping levels, $x\approx 0.45$ (optimally doped) and $x\approx 0.10$ (underdoped), bookending the range of cubic phase in the compound. The results indicate no deviation from fully gapped BCS-like behavior, eliminating several candidate unconventional gap structures. Critical field values below 1 K and other superconducting parameters are also presented. The introduction of disorder by repeated particle irradiation with 5 MeV protons does not enhance $T_c$, indicating that ferroelectric interactions do not compete with superconductivity.

We consider a Weyl semimetal with finite doping and tilt within a continuum model Hamiltonian with both broken time-reversal and inversion symmetry. We calculate the absorptive part of the anomalous ac Hall conductivity as a function of photon energy $\mathrm{\Omega }$ for both type-I and type-II Weyl semimetals. For a given Weyl node, changing the sign of its chirality or of its tilt changes the sign of its contribution to the absorptive Hall conductivity with no change in magnitude. For a noncentrosymmetric system we find that there are ranges of photon energies for which only the positive or only the negative-chirality node contributes to the imaginary (absorptive) part of the Hall conductivity. There are also other photon energies where both chiralities contribute, and there can be other ranges of $\mathrm{\Omega }$ where there is no absorption associated with the ac Hall conductivity in type-I semimetals and regions where it is instead constant for type-II semimetals. We comment on implications for the absorption of circularly polarized light.

The extremely high-power densities and short durations of single pulses of x-ray free electron lasers (XFELs) have opened new opportunities in atomic physics, where complex excitation-relaxation chains allow for high ionization states in atomic and molecular systems, and in dense plasma physics, where XFEL heating of solid-density targets can create unique dense states of matter having temperatures on the order of the Fermi energy. We focus here on the latter phenomena, with special emphasis on the problem of optimum target design to achieve high x-ray heating into the warm dense matter (WDM) state. We report fully three-dimensional simulations of the incident x-ray pulse and the resulting multielectron relaxation cascade to model the spatial energy density deposition in multicomponent targets, with particular focus on the effects of nonlocal heat transport due to the motion of high energy photoelectrons and Auger electrons. We find that nanoscale high-$Z$/low-$Z$ multicomponent targets can give much improved energy density deposition in lower-$Z$ materials, with enhancements reaching a factor of 100. This has three important benefits. First, it greatly enlarges the thermodynamic parameter space in XFEL x-ray heating studies of lower-$Z$ materials. Second, it allows the use of higher probe photon energies, enabling higher-information content x-ray diffraction (XRD) measurements such as in two-color XFEL operations. Third, while this is merely one step toward optimization of x-ray heating target design, the demonstration of the importance of nonlocal heat transport establishes important common ground between XFEL-based x-ray heating studies and more traditional laser plasma methods.

We have investigated the effects of low-energy ion beam irradiations using argon clusters on the chemical and electronic properties of ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ (LAO/STO) heterointerfaces by combining x-ray photoelectron spectroscopy (XPS) and electrical transport measurements. Due to its unique features, we demonstrate that a short-time cluster ion irradiation of the LAO surface induces significant modifications in the chemical properties of the buried STO substrate with (1) a lowering of Ti atoms oxidation states (from ${\mathrm{Ti}}^{4+}$ to ${\mathrm{Ti}}^{3+}$ and ${\mathrm{Ti}}^{2+}$) correlated to the formation of oxygen vacancies at the LAO surface and (2) the creation of new surface states for Sr atoms. Contrary to what is generally observed by using higher energy ion beam techniques, this leads to an increase of the electrical conductivity at the LAO/STO interface. Our XPS data clearly reveal the existence of dynamical processes on the titanium and strontium atoms, which compete with the effect of the cluster ion beam irradiation. These relaxation effects are in part attributed to the diffusion of the ion-induced oxygen vacancies in the entire heterostructure since an increase of the interfacial metallicity is also evidenced far from the irradiated area. This paper highlights the possibility of tuning the electrical properties of LAO/STO interfaces by surface engineering, confirming experimentally the intimate connection between LAO chemistry and electronic properties of LAO/STO interfaces.

Proceeding from the model of a two-dimensional elastic continuum, we describe the characteristic features of the thermal expansion of graphene using an approach that goes beyond the quasiharmonic approximation. The negative value of the thermal expansion coefficient of graphene at low temperatures and its sign reversal at $T\approx 1000\mathrm{K}$ are established. It is shown that the bending vibrational mode plays a decisive role in the peculiarities of the thermal contraction/expansion of graphene and that the contribution of this mode to the thermal expansion coefficient does not depend on the sample size, due to the “sound” nature of the bending mode long-wave dispersion. The obtained results allow giving a quantitative description of the known data of numerical simulations on the thermal expansion of graphene in the entire interval of the molecular dynamics simulations.

We report a single crystal neutron and x-ray diffraction study of the hybrid improper multiferroic ${\mathrm{Ca}}_3{\mathrm{Mn}}_{1.9}{\mathrm{Ti}}_{0.1}{\mathrm{O}}_7$ (CMTO), a prototypical system where the electric polarization arises from the condensation of two lattice distortion modes. With increasing temperature ($T$), the out-of-plane, antiphase tilt of ${\mathrm{MnO}}_6$ decreases in amplitude while the in-plane, in-phase rotation remains robust and experiences abrupt changes across the first-order structural transition. Application of hydrostatic pressure ($P$) to CMTO at room temperature shows a similar effect. The consistent behavior under both $T$ and $P$ reveals the softness of antiphase tilt and highlights the role of the partially occupied $d$ orbital of the transition-metal ions in determining the stability of the octahedral distortion. Polarized neutron analysis indicates the symmetry-allowed canted ferromagnetic moment is less than the $0.04{\mu }_B/\mathrm{Mn}$ site, despite a substantial out-of-plane tilt of the ${\mathrm{MnO}}_6$ octahedra.

The electronic band structure of correlated ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ featuring an antiferromagnetic (AFM) as well as a structural transition has been determined theoretically at high temperatures, which has led to the understanding of the remarkable properties of ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, such as the bulk spin-valve effects. However, its band structure and Fermi surface (FS) below the structural transition have not been resolved, even though a FS consisting of electron pockets was found experimentally. Here we report magnetoelectrical transport and thermoelectric measurements with the electric current and temperature gradient directed along the $a$ and $b$ axes, respectively, of an untwined single crystal of ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. The thermopowers obtained along the two crystal axes were found to show opposite signs at low temperatures, demonstrating the presence of both electron and hole pockets on the FS. In addition, how the FS evolves across $T^{*}=30\mathrm{K}$ at which a distinct transition from coherent to incoherent behavior occurs was also inferred: the Hall and Nernst coefficient results suggest a temperature- and momentum-dependent partial gap opening in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ below the structural transition with a possible Lifshitz transition occurring at $T^{*}$. The experimental demonstration of a correlated semimetal ground state in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ calls for further theoretical studies of this remarkable material.