We investigate the magnetic properties and the phonon-induced relaxation dynamics of the first regular ${\mathrm{Cr}}_9$ antiferromagnetic (AF) ring, which represents a prototype frustrated AF ring. Geometrical frustration in ${\mathrm{Cr}}_9$ yields an energy spectrum with twofold degenerate low-lying levels and a low-spin ground state. The electronic relaxation dynamics is probed by ${}^1\mathrm{H}$-NMR through the temperature dependence of the spin-lattice relaxation rate $1/T_1$. We develop a microscopic model that reproduces $1/T_1\left(T\right)$ curves, taking also into account the wipeout effect. By interpreting these measurements we determine the spin-phonon coupling strength and we investigate the decay of the cluster magnetization due to the spin-phonon interaction. We find that at very low temperatures, the relaxation is characterized by a single dominating Arrhenius-type relaxation process, whereas several relevant processes emerge at higher temperatures. In addition, we calculate the temperature and magnetic field dependence of level lifetimes.

Self-consistent perturbation expansion up to the second order in the interaction strength is used to study a single-level quantum dot with local Coulomb repulsion attached asymmetrically to two generally different superconducting leads. At zero temperature and a wide range of other parameters, the spin-symmetric version of the expansion yields excellent results for the position of the $0\text{−}\pi$ impurity quantum phase transition boundary and Josephson current together with the energy of Andreev bound states in the 0 phase as confirmed by numerical calculations using the numerical renormalization group method. We analytically prove that the method is charge conserving as well as thermodynamically consistent. Explicit formulas for the position of the $0\text{−}\pi$ phase boundary are presented for the Hartree-Fock approximation as well as for its variant called generalized atomic limit. It is shown that the generalized atomic limit can be used as a quick estimate for the position of the phase boundary at half-filling in a broad range of parameters. We apply our second-order perturbation method to the interpretation of the existing experimental data on the phase boundary with very satisfactory outcome, suggesting that the so-far employed heavy numerical tools such as numerical renormalization group and/or quantum Monte Carlo are not necessary in a class of generic situations and can be safely replaced by a perturbative approach.

We use the proximity effect to generate effective topological superconductors by placing metals with strong spin-orbit coupling in contact with a superconductor, aiming to produce Majorana zero modes useful for topologically protected quantum computation. In recent experiments, several quintuple layers of ${\text{Bi}}_2{\text{Se}}_3$ were epitaxially grown on the high-$T_c$ material ${\text{Bi}}_2{\text{Sr}}_2{\text{CaCu}}_2{\text{O}}_{8+\delta }$, and conflicting experimental results were reported. We use the standard mean-field approach to study this heterostructure and find it is unlikely to have a large proximity-induced superconducting gap. Despite the seemingly correct temperature dependence, the $s$-wave gap claimed to be observed may not be purely superconducting in origin. Future work on the proximity-induced bulk superconducting gap and the interfacial band structure should shed light on this issue.

We investigated electronic-structure changes of tensile-strained ultrathin $\mathrm{LaNi}{\mathrm{O}}_3$ (LNO) films from ten to one unit cells (UCs) using angle-resolved photoemission spectroscopy (ARPES). We found that there is a critical thickness $t_c$ between four and three UCs below which Ni $e_g$ electrons are confined in two-dimensional space. Furthermore, the Fermi surfaces (FSs) of LNO films below $t_c$ consist of two orthogonal pairs of one-dimensional (1D) straight parallel lines. Such a feature is not accidental as observed in constant-energy surfaces at all binding energies, which is not explained by first-principles calculations or the dynamical mean-field theory. The ARPES spectra also show anomalous spectral behaviors, such as no quasiparticle peak at the Fermi momentum but fast band dispersion comparable to the bare-band one, which is typical in a 1D system. As its possible origin, we propose 1D FS nesting, which also accounts for FS superstructures observed in ARPES.

Negative compressibility is a many-body effect wherein strong correlations give rise to an enhanced gate capacitance in two-dimensional (2D) electronic systems. We observe capacitance enhancement in a newly emerged 2D layered material, atomically thin black phosphorus (BP). The encapsulation of BP by hexagonal boron nitride sheets with few-layer graphene as a terminal ensures ultraclean heterostructure interfaces, allowing us to observe negative compressibility at low hole carrier concentrations. We explain the negative compressibility based on the Coulomb correlation among in-plane charges and their image charges in a gate electrode in the framework of Debye screening.

Pseudomagnetic fields, which can result from nonuniform strain distributions, have received much attention in graphene systems due to the possibility of mimicking real magnetic fields with magnitudes of greater than 100 T. We examine systems with such strains confined to finite regions (“pseudomagnetic dots”) and provide a transparent explanation for the characteristic sublattice polarization occurring in the presence of a pseudomagnetic field. In particular, we focus on a triaxial strain leading to a constant field in the central region of the dot. This field causes the formation of pseudo-Landau levels, where the zeroth order level shows significant differences compared to the corresponding level in a real magnetic field. Analytic arguments based on the Dirac model are employed to predict the sublattice and valley dependencies of the density of states in these systems. Numerical tight-binding calculations of single pseudomagnetic dots in extended graphene sheets confirm these predictions, and are also used to study the effect of rotating the strain direction with respect to the underlying graphene lattice, and varying the size of the pseudomagnetic dot.

Electrostatic interactions between thin films of two different ferroelectric materials lead to a layer of free charges localized at the interface between ferroelectrics according to a computational analysis presented in this work. The free charges at the interface between $\mathrm{BaTi}{\mathrm{O}}_3$ and $\mathrm{PbZ}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}{\mathrm{O}}_3$ reduce polarization coupling strength significantly. This helps the layers to retain most of their single-layer polarizations and enables a layer-by-layer polarization switching, which has been observed in recent experimental studies. The sheet carrier density at the interface decreases by at least eight orders of magnitude when the film thickness is reduced from 30 to 10 nm. This drop in the carrier density is accompanied by a strong increase of polarization coupling between ferroelectric layers. This polarization coupling transition can explain the difference between strong coupling in thin ferroelectric superlattices and weak coupling in thicker multilayer films.

The many-body localization transition (MBLT) between ergodic and many-body localized phases in disordered interacting systems is a subject of much recent interest. The statistics of eigenenergies is known to be a powerful probe of crossovers between ergodic and integrable systems in simpler examples of quantum chaos. We consider the evolution of the spectral statistics across the MBLT, starting with mapping to a Brownian motion process that analytically relates the spectral properties to the statistics of matrix elements. We demonstrate that the flow from Wigner-Dyson to Poisson statistics is a two-stage process. First, a fractal enhancement of matrix elements upon approaching the MBLT from the delocalized side produces an effective power-law interaction between energy levels, and leads to a plasma model for level statistics. At the second stage, the gas of eigenvalues has local interactions and the level statistics belongs to a semi-Poisson universality class. We verify our findings numerically on the XXZ spin chain. We provide a microscopic understanding of the level statistics across the MBLT and discuss implications for the transition that are strong constraints on possible theories.

Triplet-triplet annihilation allows two low-energy photons to be upconverted into a single high-energy photon. By essentially engineering the solar spectrum, this allows solar cells to be made more efficient and even exceed the Shockley-Quiesser limit. Unfortunately, optimizing the reaction pathway is difficult, especially with limited access to the microscopic time scales and states involved in the process. Optical measurements can provide detailed information: triplet-triplet annihilation is intrinsically spin dependent and exhibits substantial magnetoluminescence in the presence of a static magnetic field. Pulsed optically detected magnetic resonance is especially suitable, since it combines high spin sensitivity with coherent manipulation. In this paper, we develop a time-domain theory of triplet-triplet annihilation for complexes with arbitrary spin-spin coupling. We identify unique “Rabi fingerprints” for each coupling regime and show that this can be used to characterize the microscopic Hamiltonian.

Employing first-principles calculations, we study the effect of an external electric field on the magnetic properties of a single Fe adatom and a Fe dimer hosted on a bilayer graphene surface grown on a ${\mathrm{SiO}}_2$ substrate. We find that an electric field perpendicular to the bilayer graphene modulates the charge and spin state of the single Fe adatom over a wide range. States ranging from $3d^6,S=2$ to $3d^{10},S=0$ have been observed for the Fe adatom, which are inaccessible under normal conditions. This would be of interest in the context of the orbitally controlled Kondo effect. Further, for a Fe dimer, we find that a small electric field is able to tune the magnetic exchange coupling. Interestingly, we also observe an unusual magnetostructural coupling for the Fe dimer, which stabilizes a ferrimagnetic state over a fully compensated antiferromagnetic spin configuration.

Fluctuation and dissipation are byproducts of coupling to the “environment.” The Caldeira-Leggett model, a successful paradigm of quantum Brownian motion, views the environment as a collection of harmonic oscillators linearly coupled to the system. However, symmetry considerations may forbid a linear coupling, e.g., for a neutral particle in quantum electrodynamics. We argue that the absence of linear couplings can lead to a fundamentally different behavior. Specifically, we consider a heavy particle quadratically coupled to quantum fluctuations of the bath. In one dimension the particle undergoes anomalous diffusion, unfolding as a power-law distribution in space, reminiscent of Lévy flights. We suggest condensed matter analogs where similar effects may arise.

We examine the Si(111) multivalley quantum Hall system and show that it exhibits an exceptionally rich interplay of broken symmetries and quantum Hall ordering already near integer fillings $\nu$ in the range $\nu =0–6$. This six-valley system has a large ${\left[\text{SU}\left(2\right)\right]}^3⋊D_3$ symmetry in the limit where the magnetic length is much larger than the lattice constant. We find that the discrete $D_3$ factor breaks over a broad range of fillings at a finite-temperature transition to a discrete nematic phase. As $T\to 0$, the ${\left[\text{SU}\left(2\right)\right]}^3$ continuous symmetry also breaks: completely near $\nu =3$, to a residual ${\left[\text{U}\left(1\right)\right]}^2\times \text{SU}\left(2\right)$ near $\nu =2$ and 4, and to a residual $\text{U}\left(1\right)\times {\left[\text{SU}\left(2\right)\right]}^2$ near $\nu =1$ and 5. Interestingly, the symmetry breaking near $\nu =2,4$ and $\nu =3$ involves a combination of selection by thermal fluctuations known as “order by disorder” and a selection by the energetics of Skyrme lattices induced by moving away from the commensurate fillings, a mechanism we term “order by doping.”

We report on the high-pressure syntheses and detailed characterizations of two effective pseudospin $S=1/2$ XY pyrochlores $\mathrm{E}{\mathrm{r}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$ and $\mathrm{Y}{\mathrm{b}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$ via x-ray/neutron powder diffraction, dc and ac magnetic susceptibility, and specific-heat measurements down to 70 mK. We found that both compounds undergo long-range magnetic transitions at $T_{N,C}\approx 0.3\mathrm{K}$, which are ascribed to an antiferromagnetic- and ferromagnetic-type order for $\mathrm{E}{\mathrm{r}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$ and $\mathrm{Y}{\mathrm{b}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$, respectively, based on the field dependence of their transition temperatures as well as the systematic comparisons with other similar pyrochlores $R_2{B}_2{\mathrm{O}}_7(R=\mathrm{Er},\mathrm{Yb};B=\mathrm{Sn},\mathrm{Ti},\mathrm{Ge})$. The observed $T_N$ of $\mathrm{E}{\mathrm{r}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$ is much lower than that expected from the relationship of $T_N$ versus the ionic radius of $B^{4+}$ derived from the series of $\mathrm{E}{\mathrm{r}}_2{\mathrm{B}}_2{\mathrm{O}}_7$, while the $T_C$ of $\mathrm{Y}{\mathrm{b}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$ is the highest among the series of ferromagnetic compounds $\mathrm{Y}{\mathrm{b}}_2{\mathrm{B}}_2{\mathrm{O}}_7(B=\mathrm{Sn},\mathrm{Pt},\mathrm{Ti})$. Given the monotonic variation of the lattice constant as a function of the $B$-cation size across these two series of $R_2{B}_2{\mathrm{O}}_7(R=\mathrm{Er},\mathrm{Yb})$, the observed anomalous values of $T_{N,C}$ in the Pt-based XY pyrochlores imply that another important factor beyond the nearest-neighbor $R-R$ distance is playing a role. In light of the anisotropic exchange interactions $J_{\mathrm{ex}}=\left\{J_{zz},J_{\pm },J_{\pm \pm },J_{z\pm }\right\}$ for the $S=1/2$ XY pyrochlores, we have rationalized these observations by considering a weakened (enhanced) antiferromagnetic planar $J_{\pm }$ (ferromagnetic Ising-like $J_{zz}$) due to strong Pt $5d-O2p$ hybridization within the plane perpendicular to the local [111] direction.

We investigate the superconducting states that arise as a consequence of mesoscopic confinement and a multicomponent order parameter in the Ginzburg-Landau model for $p$-wave superconductivity. Conventional vortices, but also half-quantum vortices and skyrmions, are found as the applied magnetic field and the anisotropy parameters of the Fermi surface are varied. The solutions are well differentiated by a topological charge that for skyrmions is given by the Hopf invariant and for vortices by the circulation of the superconducting velocity. We revealed several unique states combining vortices and skyrmions, their possible reconfiguration with varied magnetic field, as well as temporal and field-induced transitions between vortical and skyrmionic states.

In two-dimensional systems with a continuous symmetry, the Mermin-Wagner-Hohenberg theorem precludes spontaneous symmetry breaking and condensation at finite temperature. The Berezinskii-Kosterlitz-Thouless critical temperature marks the transition from a superfluid phase characterized by quasicondensation and algebraic long-range order, to a normal phase in which vortex proliferation completely destroys superfluidity. As opposed to conventional off-diagonal long-range order typical of three-dimensional superfluid systems, algebraic long-range order is driven by quantum and thermal fluctuations strongly enhanced in reduced dimensionality. Motivated by this unique scenario and by the very recent experimental realization of trapped quasi-two-dimensional fermionic clouds, we include one-loop Gaussian fluctuations in the theoretical description of resonant Fermi superfluids in two dimensions demonstrating that first sound, second sound, and also critical temperature are strongly renormalized, away from their mean-field values. In particular, we prove that in the intermediate- and strong-coupling regimes, these quantities are radically different when Gaussian fluctuations are taken into account. Our one-loop theory shows good agreement with very recent experimental data on the Berezinskii-Kosterlitz-Thouless critical temperature [Phys. Rev. Lett. 115, 010401 (2015)] and on the first sound velocity, giving predictions for the second sound as a function of interaction strength and temperature that are open for experimental verification.

The lattice thermal conductivities of silicon clathrate frameworks II and VIII are investigated by using ab initio lattice dynamics and an iterative solution of the linearized Boltzmann transport equation (BTE) for phonons. Within the temperature range 100–350 K, the clathrate structures II and VIII were found to have lower lattice thermal conductivity values than the silicon diamond structure ($d$-Si) by factors of 1/2 and 1/3, respectively. The main reason for the lower lattice thermal conductivity of the clathrate structure II in comparison to $d$-Si was found to be the harmonic phonon spectra, while in the case of the clathrate structure VIII, the difference is mainly due to the harmonic phonon spectra and partly due to the shorter relaxation times of phonons. In the studied clathrate frameworks, the anharmonic effects have larger impact on the lattice thermal conductivity than the size of the unit cell. For the structure II, the predicted lattice thermal conductivity differs approximately by a factor of 20 from the previous experimental results obtained for a polycrystalline sample at room temperature.

The temperature dependent stability of the magnetic phases of FeRh were investigated by means of total energy calculations with magnetic disorder treated within the uncompensated disordered local moment approach. In addition, Monte Carlo simulations based on the extended Heisenberg model have been performed, using exchange coupling parameters obtained from first principles. The crucial role and interplay of two factors in the metamagnetic transition in FeRh has been revealed, namely the dependence of the Fe-Fe exchange coupling parameters on the temperature-governed degree of magnetic disorder in the system and the stabilizing nature of the induced magnetic moment on Rh-sites. An important observation is the temperature dependence of these two competing factors.

Pyrochlore ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$ has been investigated under high pressure by x-ray diffraction, resistivity and, mid-IR reflectance measurements. At $\sim 14$ GPa, the high-temperature cubic pyrochlore phase [space group (SG): $Fd\overline{3}m$] undergoes a second-order transition to a rhombohedral structure (SG: $R\overline{3}m$) with trigonal distortion in its pyrochlore lattice. Temperature-dependent resistivity $\rho \left(T\right)$ reveals a large number of exotic electronic phases (including superconducting and unconventional metallic phases) in its $P\text{−}T$ phase diagram. At the metal-nonmetal boundary (at 21 GPa) the resistivity displays quadratic power-law behavior over a wide $T$ range with possible formation of heavy mass coherent quasiparticles. In the nonmetallic state $\left(d\rho /dT<0\right)$, the nondivergent and quadratic power-law resistivity as $T\to 0$ reveals the novel character of the ground state. High-pressure optical conductivity measurement evidences electronic correlation playing the vital role in the emergence of the nonmetallic state as the electron-lattice coupling exceeds spin-orbit interaction due to large structural distortion.

Charge carrier mobility in van der Waals bonded organic crystals is strongly dependent on the transfer integral between neighboring molecules, and therefore the anisotropy of charge transport is determined by the molecular arrangement within the crystal lattice. Here we report on temperature dependent transport measurements along all three principal crystal directions of the same rubrene single crystals of high purity. Hole mobilities are obtained from the carrier transit time measured with high-frequency admittance spectroscopy perpendicular to the molecular layers $\left({\mu }_c\right)$ and from the transfer characteristics of two field-effect transistor (FET) structures oriented perpendicularly to each other in the layers $({\mu }_a$ and ${\mu }_b)$. While the measurements of the field-effect channels confirm the previously reported high mobility and anisotropy within the $ab$ plane, we find the mobility perpendicular to the molecular layers in the same crystals to be lower by about two orders of magnitude $({\mu }_c\sim 0.2{\mathrm{cm}}^2/\mathrm{Vs}$ at $300\mathrm{K})$. Although the bandwidth is vanishingly small along the $c$ direction and the transport cannot be coherent, we find ${\mu }_c$ to increase upon cooling. We show that the delocalization within the high mobility $ab$ plane prevents the formation of small polarons and leads to the observed “bandlike” temperature dependence also in the direction perpendicular to the molecular layers, despite the incoherent transport mechanism.