$\mathrm{C}{\mathrm{a}}_3\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ is an experimentally confirmed hybrid improper ferroelectric material, in which the electric polarization is induced by a combination of the coherent $\mathrm{Ti}{\mathrm{O}}_6$ octahedral rotation and tilting. In this work, we investigate the tuning of ferroelectricity of $\mathrm{C}{\mathrm{a}}_3\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ using isovalent substitutions on Ca sites. Due to the size mismatch, larger/smaller alkaline earths prefer $A^{\prime }/A$ sites, respectively, allowing the possibility for site-selective substitutions. Without extra carriers, such site-selected isovalent substitutions can significantly tune the $\mathrm{Ti}{\mathrm{O}}_6$ octahedral rotation and tilting, and thus change the structure and polarization. Using the first-principles calculations, our study reveals that three substituted cases (Sr, Mg, and Sr+Mg) show divergent physical behaviors. In particular, ${\left(\mathrm{CaTi}{\mathrm{O}}_3\right)}_2\mathrm{SrO}$ becomes nonpolar, which can reasonably explain the suppression of polarization upon Sr substitution observed in experiment. In contrast, the polarization in ${\left(\mathrm{MgTi}{\mathrm{O}}_3\right)}_2\mathrm{CaO}$ is almost doubled upon substitutions, while the estimated coercivity for ferroelectric switching does not change. The ${\left(\mathrm{MgTi}{\mathrm{O}}_3\right)}_2\mathrm{SrO}$ remains polar but its structural space group changes, with moderate increased polarization and possible different ferroelectric switching paths. Our study reveals the subtle ferroelectricity in the $A_3\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ family and suggests one more practical route to tune hybrid improper ferroelectricity, in addition to the strain effect.

We developed the model of the internal phonon bottleneck to describe the energy exchange between the acoustically soft ultrathin metal film and acoustically rigid substrate. Discriminating phonons in the film into two groups, escaping and nonescaping, we show that electrons and nonescaping phonons may form a unified subsystem, which is cooled down only due to interactions with escaping phonons, either due to direct phonon conversion or indirect sequential interaction with an electronic system. Using an amplitude-modulated absorption of the sub-THz radiation technique, we studied electron-phonon relaxation in ultrathin disordered films of tungsten silicide. We found an experimental proof of the internal phonon bottleneck. The experiment and simulation based on the proposed model agree well, resulting in ${\tau }_{\mathrm{e}-\mathrm{ph}}\sim 140–190$ ps at $T_{\mathrm{C}}=3.4\mathrm{K}$, supporting the results of earlier measurements by independent techniques.

The single crystal growth and the physical properties of the intermetallic compounds $R{\mathrm{Ni}}_4\mathrm{Cd}$ ($R=\text{Y}$ and Yb) which crystallize in the face-centered cubic (fcc) ${\mathrm{MgCu}}_4\mathrm{Sn}$-type structure (space group $F\overline{4}3m$) are discussed. Thermodynamic and transport properties of ${\mathrm{YbNi}}_4\mathrm{Cd}$ are studied by measuring the magnetization, electrical resistivity, and specific heat. The magnetic susceptibility measurement shows that the $4f$ electrons of ${\mathrm{Yb}}^{3+}$ ions are well localized. The electrical resistivity and specific heat exhibits an antiferromagnetic ordering below $T_N=0.97$ K. Applying the field along the [111] direction results in the suppression of $T_N$ below 0.4 K at the critical field $H_c\sim 4.5$ kOe. No non-Fermi liquid behavior has been observed in the vicinity of $H_c$. Above $H_c$, the magnetoresistivity shows an unconventional temperature dependence $\rho \left(T\right)={\rho }_0+A{T}^n$ with $n>2$, suggesting that an additional scattering mechanism in the resistivity needs to be considered. Based on the analysis of experimental results, we conclude that the ${\mathrm{Yb}}^{3+}$ moments and conduction electrons are weakly coupled. Despite the antiferromagnetic ordering below $T_N$, ${\mathrm{YbNi}}_4\mathrm{Cd}$ exhibits a large frustration parameter $|{\theta }_p/T_N|\sim 16$, where the magnetic ${\mathrm{Yb}}^{3+}$ ions occupy the tetrahedra on the fcc lattice.

Antiferromagnetic materials, whose time-reversal symmetry is broken, can be classified into the $Z_2$ topology if they respect some specific symmetry. Since the theoretical proposal, however, no materials have been found to host such $Z_2$ antiferromagnetic topological ($Z_2-\mathrm{AFT}$) phase to date. Here we demonstrate that the topological Kondo insulator ${\mathrm{SmB}}_6$ can be a $Z_2-\mathrm{AFT}$ system when pressurized to undergo an antiferromagnetic phase transition. In addition to proposing the possible candidate for a $Z_2-\mathrm{AFT}$ material, in this work we also illustrate the anomalous topological surface states of the $Z_2-\mathrm{AFT}$ phase which have not been discussed before. Originating from the interplay between the topological properties and the antiferromagnetic surface magnetization, the topological surface states of the $Z_2-\mathrm{AFT}$ phase behave differently as compared with those of a topological insulator. Besides, the $Z_2-\mathrm{AFT}$ insulators are also found promising in the generation of tunable spin currents, which is an important application in spintronics.

The solid solution $\mathrm{Eu}{\left({\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\right)}_4$ was grown in single crystal form to reveal a rich variety of crystallographic, magnetic, and electronic properties that differ from the isostructural end compounds ${\mathrm{EuGa}}_4$ and ${\mathrm{EuAl}}_4$, despite the similar covalent radii and electronic configurations of Ga and Al. Here we report the onset of magnetic spin reorientation and metamagnetic transitions for $x=0–1$ evidenced by magnetization and temperature-dependent specific heat measurements. $T_{\mathrm{N}}$ changes nonmonotonously with $x$, and it reaches a maximum around 20 K for $x=0.50$, where the $a$ lattice parameter also shows an extreme (minimum) value. Anomalies in the temperature-dependent resistivity consistent with charge density wave behavior exist only for $x=0.50$ and 1. Density functional theory calculations show increased polarization between the Ga-Al covalent bonds in the $x=0.50$ structure compared to the end compounds, such that crystallographic order and chemical pressure are proposed as the causes of the charge density wave behavior.

The energy dependence of the relaxation rate of hot electrons due to interaction with the Fermi sea is studied. We consider 2D and 3D systems, quasi-1D quantum wires with multiple transverse bands, as well as single-channel 1D wires. Our analysis includes both spinful and spin-polarized setups, with short-range and Coulomb interactions. We show that, quite generally, the relaxation rate is a nonmonotonic function of the electron energy and decays as a power law at high energies. In other words, ultrahot electrons regain their coherence with increasing energy. Such a behavior was observed in a recent experiment on multiband quantum wires, J. Reiner et al., Phys. Rev. X 7, 021016 (2017).

We analyze the effect of environmental dephasing on the electrical current in an ac-driven triple quantum dot system in a symmetric $\mathrm{\Lambda }$ configuration. The current is explored by solving the time evolution equation of the density matrix as a function of the frequency and amplitude of the driving field. Two characteristic spectra are observed depending on the field amplitude. At the resonance condition, when the frequency matches the interdot energy difference, one spectrum shows a distinctive Fano-type peak, while the other, occurring at larger values of the field amplitude, exhibits a strong current suppression due to dynamic localization. In the former case we observe that the current maximum is reduced due to dephasing, while in the latter it is shown that dephasing partially alleviates the localization. In both cases, away from resonance, we observe current oscillations which are dephasing-enhanced for a wide range of frequencies. These effects are also discussed using Floquet theory, and analytical expressions for the electrical current are obtained within the rotating wave approximation.

We study theoretically the fluctuations of the nuclear spins in quantum dots employing the central spin model which accounts for the hyperfine interaction of the nuclei with the electron spin. These fluctuations are calculated both with an analytical approach using homogeneous hyperfine couplings (box model) and with a numerical simulation using a distribution of hyperfine coupling constants. The approaches are in good agreement. The box model serves as a benchmark with low computational cost that explains the basic features of the nuclear spin noise well. We also demonstrate that the nuclear spin noise spectra comprise a two-peak structure centered at the nuclear Zeeman frequency in high magnetic fields with the shape of the spectrum controlled by the distribution of the hyperfine constants. This allows for direct access to this distribution function through nuclear spin noise spectroscopy.

We study the effect of system-bath coupling strength on quantum thermal transport through the interface of two weakly coupled anharmonic molecular chains by using a quantum self-consistent phonon approach. The approach inherently assumes that the two segments (anharmonic molecular chains) are approximately in local thermal equilibrium with respect to the baths that they are connected to and transforms the strongly anharmonic system into an effective harmonic one with a temperature-dependent transmission. Despite the approximations, the approach is ideal for our setup, wherein the weak interfacial coupling guarantees an approximate local thermal equilibrium of each segment and short chain length (less than the phonon mean-free path) ensues from the effective harmonic approximation. Remarkably, the heat current shows a resonant to bi-resonant transition due to the variations in the interfacial coupling and temperature, which is attributed to the delocalization of phonon modes. Delocalization occurs only in the strong system-bath coupling regime and we utilize it to model a thermal rectifier whose ratio can be nonmonotonically tuned not only with the intrinsic system parameters but also with the external temperature.

We study the effective interactions between Dirac fermions on the surface of a three-dimensional topological insulator due to the proximity coupling to the magnetic fluctuations in a ferromagnetic or antiferromagnetic insulator. Our results show that the magnetic fluctuations can mediate attractive interactions between Dirac fermions of both Amperean and BCS types. In the ferromagnetic case, we find pairing between fermions with parallel momenta, so-called Amperean pairing, whenever the effective Lagrangian for the magnetic fluctuations does not contain a quadratic term. The pairing interaction also increases with increasing Fermi momentum and is in agreement with previous studies in the limit of high chemical potential. If a quadratic term is present, the pairing is instead of BCS type above a certain chemical potential. In the antiferromagnetic case, BCS pairing occurs when the ferromagnetic coupling between magnons on the same sublattice exceeds the antiferromagnetic coupling between magnons on different sublattices. Outside this region in parameter space, we again find that Amperean pairing is realized.

The heat capacity $C\left(T\right)$ of a 2-bromobenzophenone (2-BrBP) single crystal measured at temperatures from 0.4 to 30 K demonstrates anomalies inherent in disordered solids: Instead of the Debye law $C_{\mathrm{D}}\propto T^3, C\left(T\right)$ shows a linear temperature dependence and a boson peak, i.e., peculiarities typical of solids with disorder. Computations for a pair of interacting 2-BrBP molecules revealed a few low-frequency states which are expected to actively couple to long-wave phonons. Raman scattering spectra demonstrate two strong spikes at energies close to the boson peak center at $26{\mathrm{cm}}^{-1}$. We relate the Raman spikes to low-energy intramolecular modes, which are the rotational oscillations of the substituted phenyl ring around the C-C link to the ketone. Thus, we have found that a completely ordered crystal made up of molecules with low-energy intramolecular modes can show low-$T$ properties, which are inherent in irregular solids such as various glasses, etc.

We investigate the behavior of persistent currents for a fixed number of noninteracting fermions in a periodic quantum ladder threaded by Aharonov-Bohm and transverse magnetic fluxes $\mathrm{\Phi }$ and $\chi$. We show that the coupling between ladder legs provides a way to effectively change the ground-state fermion-number parity, by varying $\chi$. Specifically, we demonstrate that varying $\chi$ by $2\pi$ (one flux quantum) leads to an apparent fermion-number parity switch. We find that persistent currents exhibit a robust $4\pi$ periodicity as a function of $\chi$, despite the fact that $\chi \to \chi +2\pi$ leads to modifications of order $1/N$ of the energy spectrum, where $N$ is the number of sites in each ladder leg. We show that these parity-switch and $4\pi$ periodicity effects are robust with respect to temperature and disorder, and outline potential physical realizations using cold atomic gases and photonic lattices, for bosonic analogs of the effects.

The growing interest in using x-ray spectroscopy for refined materials characterization calls for an accurate electronic-structure theory to interpret the x-ray near-edge fine structure. In this work, we propose an efficient and unified framework to describe all the many-electron processes in a Fermi liquid after a sudden perturbation (such as a core hole). This problem has been visited by the Mahan-Noziéres-De Dominicis (MND) theory, but it is intractable to implement various Feynman diagrams within first-principles calculations. Here, we adopt a nondiagrammatic approach and treat all the many-electron processes in the MND theory on an equal footing. Starting from a recently introduced determinant formalism [Phys. Rev. Lett. 118, 096402 (2017)], we exploit the linear dependence of determinants describing different final states involved in the spectral calculations. An elementary graph algorithm, breadth-first search, can be used to quickly identify the important determinants for shaping the spectrum, which avoids the need to evaluate a great number of vanishingly small terms. This search algorithm is performed over the tree-structure of the many-body expansion, which mimics a path-finding process. We demonstrate that the determinantal approach is computationally inexpensive even for obtaining x-ray spectra of extended systems. Using Kohn-Sham orbitals from two self-consistent fields (ground and core-excited state) as input for constructing the determinants, the calculated x-ray spectra for a number of transition metal oxides are in good agreement with experiments. Many-electron aspects beyond the Bethe-Salpeter equation, as captured by this approach, are also discussed, such as shakeup excitations and many-body wave function overlap considered in Anderson's orthogonality catastrophe.

The edge physics of graphene based systems is well known to be highly sensitive to the atomic structure at the boundary, with localized zero mode edge states found only on the zigzag-type termination of the lattice. Here we demonstrate that the graphene twist bilayer supports an additional class of edge states, that (i) are found for all edge geometries and thus are robust against edge roughness, (ii) occur at energies coinciding with twist induced Van Hove singularities in the bulk and (iii) possess an electron density strongly modulated by the moiré lattice. Interestingly, these “moiré edge states” exist only for certain lattice commensurations and thus the edge physics of the twist bilayer is, in dramatic contrast to that of the bulk, not uniquely determined by the twist angle.

Charge transport in channel-shaped 2D Dirac systems is studied employing the Boltzmann equation. The dependence of the resistivity on temperature and chemical potential is investigated. An accurate understanding of the influence of electron-electron interaction and material disorder allows us to identify a parameter regime, where the system reveals hydrodynamic transport behavior. We point out the conditions for three Dirac fermion specific features: heat flow hydrodynamics, pseudodiffusive transport, and the electron-hole scattering dominated regime. It is demonstrated that for clean samples the relativistic Gurzhi effect, a definite indicator of hydrodynamic transport, can be observed.

The pressure dependence of the magnetic and superconducting transitions and that of the superconducting upper critical field are reported for $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$, the first example of an Fe-based superconductor with spin-vortex-crystal-type magnetic ordering. Resistance measurements were performed on single crystals with two substitution levels $\left(x=0.033,0.050\right)$ under hydrostatic pressures up to 5.12 GPa and in magnetic fields up to 9 T. Our results show that, for both compositions, magnetic transition temperatures $T_{\mathrm{N}}$ are suppressed upon applying pressure; the superconducting transition temperatures $T_{\mathrm{c}}$ are suppressed by pressure as well, except for $x=0.050$ in the pressure region where $T_{\mathrm{N}}$ and $T_{\mathrm{c}}$ cross. Furthermore, the pressure associated with the crossing of the $T_{\mathrm{N}}$ and $T_{\mathrm{c}}$ lines also coincides with a minimum in the normalized slope of the superconducting upper critical field, consistent with a likely Fermi-surface reconstruction associated with the loss of magnetic ordering. Finally, at $p\sim 4$ GPa, both Ni-substituted $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$ samples likely go through a half-collapsed-tetragonal phase transition, similar to the parent compound ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$.

We study the spectrum of Andreev bound states and Josephson currents across a junction of $N$ superconducting wires which may have $s$- or $p$-wave pairing symmetries and develop a scattering matrix based formalism which allows us to address transport across such junctions. For $N\ge 3$, it is well known that Berry curvature terms contribute to the Josephson currents; we chart out situations where such terms can have relatively large effects. For a system of three $s$-wave or three $p$-wave superconductors, we provide analytic expressions for the Andreev bound-state energies and study the Josephson currents in response to a constant voltage applied across one of the wires; we find that the integrated transconductance at zero temperature is quantized to integer multiples of $4e^2/h$, where $e$ is the electron charge and $h=2\pi \hslash$ is Planck's constant. For a sinusoidal current with frequency $\omega$ applied across one of the wires in the junction, we find that Shapiro plateaus appear in the time-averaged voltage $\langle V_1\rangle$ across that wire for any rational fractional multiple (in contrast to only integer multiples in junctions of two wires) of $2e\langle V_1\rangle /\left(\hslash \omega \right)$. We also use our formalism to study junctions of two $p$-wave and one $s$-wave wires. We find that the corresponding Andreev bound-state energies depend on the spin of the Bogoliubov quasiparticles; this produces a net magnetic moment in such junctions. The time variation of these magnetic moments may be controlled by an external voltage applied across the junction. We discuss experiments which may test our theory.

We fabricate devices in which a magnetic nanopillar spin valve makes contact to a Co/Pt bilayer thin film with perpendicular magnetic anisotropy, to achieve local control of domains in the Co/Pt bilayer underneath the nanopillar. The goal is to develop the ability to nucleate, detect, and annihilate magnetic skyrmions in the Co/Pt using spin-polarized currents from the nanopillar. We demonstrate the ability to distinguish the local behavior of the Co/Pt film beneath the nanopillar from the extended film and show that the two can switch independently of each other. This allows us to isolate a localized domain under the pillar that can be controlled separately from the rest of the Co/Pt film using applied currents and magnetic fields. Micromagnetic simulations indicate that this localized domain has skyrmion symmetry. Our results represent a first step toward controlling room-temperature skyrmions using localized spin-transfer torque.

We investigate the behavior of the collective excitations of adsorbed ${}^4\mathrm{He}$ in an ordered hexagonal mesopore, examining the crossover from a thin film to a confined fluid. Here, we present the inelastic scattering results as a function of filling at constant temperature. We find a monotonic transition of the maxon excitation as a function of filling. This has been interpreted as corresponding to an increasing density of the adsorbed helium, which approaches the bulk value as filling increases. The roton minimum exhibits a more complicated behavior that does not monotonically approach bulk values as filling increases. The full pore scattering resembles the bulk liquid accompanied by a layer mode. The maxon and roton scattering, taken together, at intermediate fillings does not correspond to a single bulk liquid dispersion at negative, low, or high pressure.