Recent photoemission experiments [1] reveal that the excitations along the nodal region in the strange metal phase of the cuprates, rather than corresponding to poles in the single-particle Green function, exhibit power-law scaling as a function of frequency and temperature. Because such power-law scaling is indicative of a scale-invariant sector, as a first step, we perturbatively evaluate the electron self-energy due to interactions with scale-invariant unparticles. We focus on a $G_{0}W$-type diagram with an interaction $W$ mediated by a bosonic scalar unparticle. We find that in the high-temperature limit, the imaginary part of the self-energy $\mathrm{Im}\mathrm{\Sigma }$ is linear in temperature. In the low-temperature limit, $\mathrm{Im}\mathrm{\Sigma }$ exhibits the same power law in both temperature and frequency, with an exponent that depends on the scaling dimension of an unparticle operator. Such behavior is qualitatively consistent with the experimental observations. We then expand the unparticle propagator into coherent and incoherent contributions and study how the incoherent part violates the density of states (DOS) and density-density correlation function sum rules (f-sum rule). Such violations, in principle, can be observed experimentally. Our work indicates that the physical mechanism for the origin of the power-law scaling is the incoherent background, which is generated from the Mott-scale physics.

Many phases of matter, including superconductors, fractional quantum Hall fluids, and spin liquids, are described by gauge theories with constrained Hilbert spaces. However, thermalization and the applicability of quantum statistical mechanics has primarily been studied in unconstrained Hilbert spaces. In this paper, we investigate whether constrained Hilbert spaces permit local thermalization. Specifically, we explore whether the eigenstate thermalization hypothesis (ETH) holds in a pinned Fibonacci anyon chain, which serves as a representative case study. We first establish that the constrained Hilbert space admits a notion of locality by showing that the influence of a measurement decays exponentially in space. This suggests that the constraints are no impediment to thermalization. We then provide numerical evidence that ETH holds for the diagonal and off-diagonal matrix elements of various local observables in a generic disorder-free nonintegrable model. We also find that certain nonlocal observables obey ETH.

We show that under the effect of an external magnetic field, a photogalvanic effect and the generation of a second harmonic wave can be induced in inversion symmetric and time-reversal invariant Dirac semimetals and it is linear with the magnetic field. The mechanisms responsible for these nonlinear optical responses are the magnetochiral effect and the chiral magnetic effect. What makes possible that these two effects give rise to the discussed nonlinear optical effects is the presence of band bending effects in the dispersion relation in real Dirac semimetals. Some observable consequences of this phenomenon are the appearance of a dc current on the surface of the system when it is irradiated with linearly polarized light or a rotation of the polarization plane of the reflected second harmonic wave.

The preadsorption of Ag on Si(111) drastically changes the growth of Ge. In a temperature range from $400{}^{\circ }\mathrm{C}$ to $650{}^{\circ }\mathrm{C}$, Ag adsorption on Si leads to the formation of a $\sqrt{3}\times \sqrt{3}$-R ${30}^{\circ }$ reconstruction that exhibits a maze-like morphology on the mesoscopic scale, as observed by low-energy electron diffraction (LEED) and low-energy electron microscopy. This maze morphology can be attributed to a surface roughening on an atomic scale, induced by the re-arrangement of top layer atoms during the $7\times 7$ to $\sqrt{3}\times \sqrt{3}$-R ${30}^{\circ }$ transition. The subsequent deposition of Ge results in the formation of a wetting layer, the evolution of which has been found to be governed by the Ag/Si(111)-$\sqrt{3}\times \sqrt{3}$-R ${30}^{\circ }$ template's maze structure, as the latter offers a high density of heterogeneous nucleation sites. Upon further Ge growth, three-dimensional islands with diameters in the micrometer range are formed, which exhibit a large and flat (111) top facet. X-ray photoemission electron microscopy reveals that during Ge growth, Ag is segregating to the surface very efficiently. Grazing-incidence x-ray diffraction and transmission electron microscopy have been used to study the composition, strain state and defect structure of the Ge islands in dependence of the growth temperature. The strain induced by lattice mismatch is found to be largely relaxed (80–90% relaxation) in the investigated growth temperature range from 400 to $600{}^{\circ }\mathrm{C}$, which is confirmed by high-resolution LEED measurements. As a main relaxation mechanism, the formation of interfacial misfit dislocations has been identified. Interdiffusion of Si into the Ge islands becomes more and more pronounced for increasing growth temperature, whereas the formation of twinned Ge regions can drastically be suppressed at higher temperature.

We study ab initio a pyrochlore compound, ${\mathrm{CsW}}_2{\mathrm{O}}_6$, which exhibits a yet unexplained metal-insulator transition. We find that (1) the reported low-$T$ structure is likely inaccurate and the correct structure has a twice larger cell; (2) the insulating phase is not of a Mott or dimer-singlet nature, but a rare example of a three-dimensional Peierls transition, with a simultaneous condensation of three density waves; (3) the spin-orbit interaction plays a crucial role, forming well-nested bands. The high-$T$ (HT) phase, if stabilized, could harbor a unique $e_g+i{e}_g$ superconducting state that breaks time reversal symmetry, but is not chiral. This state was predicted in 1999, but not observed. We speculate about possible ways to stabilize the HT phase while keeping the conditions for superconductivity.

We investigate the interplay of many-body and band-structure effects of interacting Weyl semimetals (WSMs). Attractive and repulsive Hubbard interactions are studied within a model for a time-reversal-breaking WSM with tetragonal symmetry, where we can approach the limit of weakly coupled planes and coupled chains by varying the hopping amplitudes. Using a slab geometry, we employ the variational cluster approach to describe the evolution of WSM Fermi arc surface states as a function of interaction strength. We find spin and charge density wave instabilities which can gap out Weyl nodes. We identify scenarios where the bulk Weyl nodes are gapped while the Fermi arcs still persist, hence realizing a quantum anomalous Hall state.

We present an extension of the functional renormalization group to Floquet space, which enables us to treat the long time behavior of interacting time periodically driven quantum dots. It is one of its strength that the method is neither bound to small driving amplitudes nor to small driving frequencies, i.e., very general time periodic signals can be considered. It is applied to the interacting resonant level model, a prototype model of a spinless, fermionic quantum dot. The renormalization in several setups with different combinations of time periodic parameters is studied, where the numerical results are complemented by analytic expressions for the renormalization in the limit of small driving amplitude. We show how the driving frequency acts as an infrared cutoff of the underlying renormalization group flow which manifests in novel power laws. We utilize the tunability of the effective reservoir distribution function in a periodically driven onsite energy setup to show how its shape is directly reflected in the renormalization group flow. This allows us to flexibly tune the power-law renormalization generically encountered in quantum dot structures. Finally, an in-phase quantum pump as well as a single parameter pump are investigated in the whole regime of driving frequency, demonstrating that the new power law in the driving frequency is reflected in the mean current of the latter.

Employing a low-temperature growth mode, we fabricated ultrathin $\mathrm{S}{\mathrm{i}}_{1-x}\mathrm{G}{\mathrm{e}}_x$/Si multiple quantum well structures with a well thickness of less than 1.5 nm and a Ge concentration above 60% directly on a Si substrate. We identified an unusual temperature-dependent blueshift of the photoluminescence (PL) and exceptionally low thermal quenching. We find that this behavior is related to the relative intensities of the no-phonon (NP) peak and a phonon-assisted replica that are the main contributors to the total PL signal. To investigate these aspects in more detail, we developed a strategy to calculate the PL spectrum employing a self-consistent multivalley effective mass model, in combination with second-order perturbation theory. Through our investigation, we find that while the phonon-assisted feature decreases with temperature, the NP feature shows a strong increase in the recombination rate. Besides leading to the observed robustness against thermal quenching, this causes the observed blueshift of the total PL signal.

We present diffuse x-ray scattering data on the strong negative thermal expansion (NTE) material ${\mathrm{ScF}}_3$ and find that two-dimensional nanoscale correlations exist at momentum-space regions associated with possibly rigid rotations of the perovskite octahedra. We address the extent to which rigid octahedral motion describes the dynamical fluctuations behind NTE by generalizing a simple model supporting a single floppy mode that is often used to heuristically describe instances of NTE. We find this model has tendencies toward dynamic inhomogeneities and its application to recent and existing experimental data suggest an intricate link between the nanometer correlation length scale, the energy scale for octahedral tilt fluctuations, and the coefficient of thermal expansion in ${\mathrm{ScF}}_3$. We then investigate the breakdown of the rigid limit and propose a resolution to an outstanding debate concerning the role of molecular rigidity in strong NTE materials.

We report a Raman scattering study of six rare-earth orthoferrites $R{\mathrm{FeO}}_3$, with $R$ = La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of ${\mathrm{SmFeO}}_3$ and first-principles calculations enable the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. The assignment of the spectra and their comparison throughout the whole series allow correlating the phonon modes with the orthorhombic structural distortions of $R{\mathrm{FeO}}_3$ perovskites. In particular, the positions of two specific $A{}_g$ modes scale linearly with the two ${\mathrm{FeO}}_6$ octahedra tilt angles, allowing the distortion to be tracked throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective $A{}_g$ modes. This behavior, as well as the general relations between the tilt angles, the frequencies of the associated modes, and the ionic radii are rationalized in a simple Landau model. The reported Raman spectra and associated phonon-mode assignment provide reference data for structural investigations of the whole series of orthoferrites.

We report a neutron diffraction study of the multiferroic mechanism in ${\left({\mathrm{ND}}_4\right)}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at $T_N=7.3$ K, followed by a second transition to an IC cycloidal spin state at $T_{FE}=6.8$ K, the latter of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling, as evidenced by the observations of both odd and even higher-order harmonics associated with the cycloid wave vector, and a weak commensurate phase that coexists with the IC phase. The second-order harmonic appears at $T_{FE}$, thereby providing unambiguous evidence that the onset of the electric polarization is accompanied by a lattice modulation due to spin-lattice interaction. The neutron results, in conjunction with the negative thermal expansion and large magnetostriction observed in Ref. [19], indicate that spin-lattice coupling plays a critical role in the ferroelectric mechanism of ${\left({\mathrm{ND}}_4\right)}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$.

$\mathrm{GeC}{\mathrm{u}}_2{\mathrm{O}}_4$ spinel exhibits a tetragonal structure due to the strong Jahn-Teller distortion associated with $\mathrm{C}{\mathrm{u}}^{2+}$ ions. We show that its magnetic structure can be described as slabs composed of a pair of layers with orthogonally oriented spin-$\frac{1}{2}$ Cu chains in the basal $ab$ plane. The spins between the two layers within a slab are collinearly aligned while the spin directions of neighboring slabs are perpendicular to each other. Interestingly, we find that spins along each chain form an unusual up-up-down-down (UUDD) pattern, suggesting a non-negligible nearest-neighbor biquadratic exchange interaction in the effective classical spin Hamiltonian. We hypothesize that spin-orbit coupling and orbital mixing of $\mathrm{C}{\mathrm{u}}^{2+}$ ions in this system are non-negligible, which calls for future calculations using perturbation theory with extended Hilbert (spin and orbital) space and calculations based on density functional theory including spin-orbit coupling and looking at the global stability of the UUDD state.

Neutron scattering from single crystals has been used to determine the magnetic structure and magnon dynamics of ${\text{FePS}}_3$, an $S=2$ Ising-like quasi-two-dimensional antiferromagnet with a honeycomb lattice. The magnetic structure has been confirmed to have a magnetic propagation vector of ${\mathbf{k}}_M=\left[01\frac{1}{2}\right]$ and the moments are collinear with the normal to the $ab$ planes. The magnon data could be modeled using a Heisenberg Hamiltonian with a single-ion anisotropy. Magnetic interactions up to the third in-plane nearest neighbor needed to be included for a suitable fit. The best fit parameters for the in-plane exchange interactions were $J_1=1.46, J_2=-0.04$, and $J_3=-0.96$ meV. The single-ion anisotropy is large, $\mathrm{\Delta }=2.66$ meV, explaining the Ising-like behavior of the magnetism in the compound. The interlayer exchange is very small, $J^{\prime }=-0.0073$ meV, proving that ${\text{FePS}}_3$ is a very good approximation to a two-dimensional magnet.

We present an angular-dependent magnetotunneling technique, which facilitates unambiguous separation of superconducting (supporting circulating screening currents) and nonsuperconducting (not supporting screening currents) contributions to the pseudogap phenomenon in layered ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ cuprates. Our data indicate persistence of superconducting correlations at temperatures up to $1.5T_c$ in a form of both phase and amplitude fluctuations of the superconducting order parameter. However, despite a profound fluctuations region, only a small fraction of the pseudogap spectrum is caused by superconducting correlations, while the dominating part comes from a competing nonsuperconducting order, which does not support circulating orbital currents.

Inelastic neutron scattering combined with the dynamic pair density function (DPDF) analysis were used to investigate the magnetic and lattice dynamics in the orbitally active ${\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ as it crosses the antiferromagnetic (AFM) to ferromagnetic (FM) phase boundary. Upon doping, the FM state present in ${\mathrm{YTiO}}_3$ is suppressed on approaching a critical concentration of $x_c\sim 0.3$ in which $T_C\simeq 0$, and is replaced by the AFM phase of ${\mathrm{LaTiO}}_3$. Below $x_c$, magnetic scattering from spin waves is dominant at low energies. At $x_c$ with a $T_C\simeq 0$, magnetic scattering is also observed and is most likely due to AFM fluctuations. At the same time, local atomic fluctuations extending to 50 meV are observed above and below the magnetic transitions from $0\le x\le 1$ that show distinct characteristics with $x$. From Y to La, a clear difference is observed in the phonon density of states as a function of doping as well. At $x=0.15$ and 0.3, low-energy modes involving predominantly the rare-earth ion become suppressed with increasing temperature, while in $x=1$, strong suppression of phonon modes across a wide range in energy is observed above $T_N$. It is likely that in the Y heavy samples, phonon modes below 20 meV have a stronger influence on the orbital excitations, while in ${\mathrm{LaTiO}}_3$, a strong phonon dependence is observed upon cooling up to $T_N$.

Rare-earth intermetallics play a critical yet often obscure role in numerous technological applications, including sensors, actuators, permanent magnets, and rechargeable batteries; therefore, understanding their basic science is of utmost importance. Here we report structural behaviors, specific heat, and magnetism of $\mathrm{P}{\mathrm{r}}_{1\text{–}x}\mathrm{E}{\mathrm{r}}_x\mathrm{A}{\mathrm{l}}_2$ studied by means of temperature-dependent x-ray powder diffraction, heat capacity, and magnetization measurements, in addition to first-principles calculations. Although the cubic lattice of $\mathrm{PrA}{\mathrm{l}}_2$ distorts tetragonally at the Curie temperature $T_C$, the distortion is rhombohedral in $\mathrm{ErA}{\mathrm{l}}_2$, creating a potential for instability in the pseudobinary $\mathrm{PrA}{\mathrm{l}}_2\text{−}\mathrm{ErA}{\mathrm{l}}_2$ system. When $0.05\le x\le 0.5$, materials show complex magnetization behaviors, including metamagnetic transitions and Griffith-like phase. Unique among other mixed-lanthanide dialuminides, the substitution of Er for Pr in $\mathrm{P}{\mathrm{r}}_{1\text{–}x}\mathrm{E}{\mathrm{r}}_x\mathrm{A}{\mathrm{l}}_2$ results in unexpected ferrimagnetic behavior, and the ferrimagnetic interactions become strongest around $x=0.25$, where the compound shows unusual metamagnetic like transitions observed only in the odd-numbered quadrants of the full magnetic field cycles. The electronic structure calculations, including exchange interactions and crystal field splitting, magnetic moments, anisotropic $4f$ energy density, and magnetic surface potentials rationalize the interesting physics observed experimentally.

We have performed micromagnetic simulations of low-amplitude gyrotropic dynamics of magnetic vortices in the presence of spatially uniform out-of-plane magnetic fields. For disks having small lateral dimensions, we observe a frequency drop-off when approaching the disk's out-of-plane saturation field. This nonlinear frequency response is shown to be associated with a vortex core deformation driven by nonuniform demagnetizing fields that act on the shifted core. The deformation results in an increase in the average out-of-plane magnetization of the displaced vortex state (contrasting the effect of gyrofield-driven deformation at low field), which causes the exchange contribution to the vortex stiffness to switch from positive to negative. This generates an enhanced reduction of the core stiffness at high field, leading to a nonlinear field dependence of the gyrotropic mode frequency.

Superconducting properties change in confined geometries. Here we study the effects of strong confinement in nanosized Pb islands on Si(111) $7\times 7$. Small hexagonal islands with diameters less than 50 nm and a uniform height of seven atomic layers are formed by depositing Pb at low temperature and annealing at 300 K. We measure the tunneling spectra of individual Pb nanoislands using a low-temperature scanning tunneling microscope operated at 0.6 K and follow the narrowing of the superconducting gap as a function of magnetic field. We find the critical magnetic field, at which the superconducting gap vanishes, reaches several Tesla, which represents a greater than 50-fold enhancement compared to the bulk value. By independently measuring the size of the superconducting gap, and the critical magnetic field that quenches superconductivity for a range of nanoislands, we can correlate these two fundamental parameters and estimate the maximal achievable critical field for 7 ML Pb nanoislands to be 7 T.

Quantum Monte Carlo (QMC) methods are one of the most important tools for studying interacting quantum many-body systems. The vast majority of QMC calculations in interacting fermion systems require a constraint to control the sign problem. The constraint involves an input trial wave function which restricts the random walks. We introduce a systematically improvable constraint which relies on the fundamental role of the density or one-body density matrix. An independent-particle calculation is coupled to an auxiliary-field QMC calculation. The independent-particle solution is used as the constraint in QMC, which then produces the input density or density matrix for the next iteration. The constraint is optimized by the self-consistency between the many-body and the independent-particle calculations. The approach is demonstrated in the two-dimensional Hubbard model by accurately determining the ground state when collective modes separated by tiny energy scales are present in the magnetic and charge correlations. Our approach also provides an ab initio way to predict effective interaction parameters for independent-particle calculations.

We report on magnetotransport properties of the Kondo semiconducting compound ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$, focusing on its exotic phase below $T_0=27$ K. An excess thermal conductivity $\kappa$ emerges below $T_0$ and is gradually suppressed by magnetic field, strikingly resembling those observed in the hidden-order phase of ${\mathrm{URu}}_2{\mathrm{Si}}_2$. Our analysis indicates that low-energy magnetic excitation is the most likely origin, as was also proposed for ${\mathrm{URu}}_2{\mathrm{Si}}_2$ recently based on inelastic x-ray scattering measurements of phonon dynamics, despite the largely reduced magnetic moments. Likewise, other transport properties such as resistivity, thermopower, and Nernst effect exhibit distinct features characterizing the very different charge dynamics above and below $T_0$, sharing similarities to ${\mathrm{URu}}_2{\mathrm{Si}}_2$, too. Given the exotic nature of the ordered phase in both compounds, whether a unified interpretation to all these observations exists appears to be extremely interesting.