We report on ultrastrong coupling between a superconducting flux qubit and a resonant mode of a system comprised of two superconducting coplanar stripline resonators coupled galvanically to the qubit. With a coupling strength as high as $17.5\%$ of the mode frequency, exceeding that of previous circuit quantum electrodynamics experiments, we observe a pronounced Bloch-Siegert shift. The spectroscopic response of our multimode system reveals a clear breakdown of the Jaynes-Cummings approximation. In contrast to earlier experiments, the high coupling strength is achieved without making use of an additional inductance provided by a Josephson junction.

We study supercurrent and proximity vortices in a Josephson junction made of disordered surface states of a three-dimensional topological insulator with a proximity induced in-plane helical magnetization. In a regime where the rotation period of helical magnetization is larger than the junction width, we find supercurrent $0-\pi$ crossovers as a function of junction thickness, magnetization strength, and parameters inherent to the helical modulation and surface states. The supercurrent reversals are associated with proximity induced vortices, nucleated along the junction width, where the number of vortices and their locations can be manipulated by means of the superconducting phase difference and the parameters mentioned above.

We have measured the lifetime of spin imbalances in the quasiparticle population of a superconductor $\left({\tau }_s\right)$ in the frequency domain. A time-dependent spin imbalance is created by injecting spin-polarized electrons at finite excitation frequencies into a thin-film mesoscopic superconductor (Al) in an in-plane magnetic field (in the Pauli limit). The time-averaged value of the spin-imbalance signal as a function of excitation frequency $f_{\text{rf}}$ shows a cutoff at $f_{\text{rf}}\approx 1/\left(2\pi {\tau }_s\right)$. The spin-imbalance lifetime is relatively constant in the accessible ranges of temperatures, with perhaps a slight increase with increasing magnetic field. Taking into account sample thickness effects, ${\tau }_s$ is consistent with previous measurements and of the order of the electron-electron scattering time ${\tau }_{ee}$. Our data are qualitatively well described by a theoretical model taking into account all quasiparticle tunneling processes from a normal metal into a superconductor.

We present a simplified strong-randomness renormalization group (RG) that captures some aspects of the many-body localization (MBL) phase transition in generic disordered one-dimensional systems. This RG can be formulated analytically and is mathematically equivalent to a domain coarsening model that has been previously solved. The critical fixed-point distribution and critical exponents (that satisfy the Chayes inequality) are thus obtained analytically or to numerical precision. This reproduces some, but not all, of the qualitative features of the MBL phase transition that are indicated by previous numerical work and approximate RG studies: our RG might serve as a “zeroth-order” approximation for future RG studies. One interesting feature that we highlight is that the rare Griffiths regions are fractal. For thermal Griffiths regions within the MBL phase, this feature might be qualitatively correctly captured by our RG. If this is correct beyond our approximations, then these Griffiths effects are stronger than has been previously assumed.

We report our investigation of the electronic and magnetic excitations of ${\mathrm{Sr}}_3{\mathrm{NiIrO}}_6$ by resonant inelastic x-ray scattering at the Ir $L_3$ edge. The intra-$t_{2g}$ electronic transitions are analyzed using an atomic model, including spin-orbit coupling and trigonal distortion of the ${\mathrm{IrO}}_6$ octahedron, confronted with ab initio quantum chemistry calculations. The Ir spin-orbital entanglement is quantified and its implications for the magnetic properties, in particular in inducing highly anisotropic magnetic interactions, are highlighted. These are included in the spin-wave model proposed to account for the dispersionless magnetic excitation that we observe at $90\mathrm{m}\mathrm{e}\mathrm{V}$. By counterbalancing the strong ${\mathrm{Ni}}^{2+}$ easy-plane anisotropy that manifests itself at high temperature, the anisotropy of the interactions finally leads to the remarkable easy-axis magnetism reported in this material at low temperature.

The quantum triangular XXZ model has recently enjoyed a wealth of new theoretical results, especially in relation to the modeling of the ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ compound. In particular, it has been understood that in a longitudinal magnetic field the umbrella (cone) phase, classically stable in all the easy-plane region of the ground-state phase diagram, is considerably reduced by the effect of quantum fluctuations. We provide more quantitative information for this phenomenon at arbitrary value of the site spin $S$, by employing the dilute Bose gas expansion, valid in the high-field regime; our results improve the available estimates of the $1/S$ expansion. We quantify the extent to which a higher spin suppresses the effect of quantum fluctuations. Besides, we show how in three-dimensional layered systems a relatively small antiferromagnetic interlayer coupling has a similar consequence of bringing back the umbrella phase in some part of the phase diagram.

We examine the superconducting solution in the Holstein model, where the conduction electrons couple to the dispersionless boson fields, using the Migdal-Eliashberg theory and dynamical mean field theory. Although different in numerical values, both methods imply the existence of an optimal boson energy for superconductivity at a given electron-boson coupling. This nonmonotonous behavior can be understood as an interplay between the polaron and superconducting physics, as the electron-boson coupling is the origin of the superconductor, but at the same time traps the conduction electrons making the system more insulating. Our calculation provides a simple explanation of the recent experiment on sulfur hydride, where an optimal pressure for the superconductivity was observed. The validities of both methods are discussed.

Quantum impurity problems can be solved using the numerical renormalization group (NRG), which involves discretizing the free conduction electron system and mapping to a “Wilson chain.” It was shown recently that Wilson chains for different electronic species can be interleaved by use of a modified discretization, dramatically increasing the numerical efficiency of the RG scheme [Phys. Rev. B 89, 121105(R) (2014)]. Here we systematically examine the accuracy and efficiency of the “interleaved” NRG (iNRG) method in the context of the single impurity Anderson model, the two-channel Kondo model, and a three-channel Anderson-Hund model. The performance of iNRG is explicitly compared with “standard” NRG (sNRG): when the average number of states kept per iteration is the same in both calculations, the accuracy of iNRG is equivalent to that of sNRG but the computational costs are significantly lower in iNRG when the same symmetries are exploited. Although iNRG weakly breaks SU($N$) channel symmetry (if present), both accuracy and numerical cost are entirely competitive with sNRG exploiting full symmetries. iNRG is therefore shown to be a viable and technically simple alternative to sNRG for high-symmetry models. Moreover, iNRG can be used to solve a range of lower-symmetry multiband problems that are inaccessible to sNRG.

We investigate the film thickness dependent metal-to-insulator transition temperature $\left(T_{\mathrm{MIT}}\right)$ of $\mathrm{NdNi}{\mathrm{O}}_3$ films under tensile and compressive strain states. For the films exceeding the critical thickness for strain relaxation, $T_{\mathrm{MIT}}$ varies gradually with the film thickness caused by strain relaxation. The variation tendency differs dramatically for the films below the critical thickness: an increase (decrease) of $T_{\mathrm{MIT}}$ with increasing the film thickness for the case of tensile (compressive) strain, which is attributed to the decaying of orbital polarization. As the overlap of $\mathrm{O}2p_{x,y}$ orbits with $\mathrm{Ni}3d{x}^2-y^2$ orbits determines $T_{\mathrm{MIT}}$, a decrease of $x^2-y^2$ orbital occupation with increasing film thickness would reduce the orbital overlap and resultant enhanced $T_{\mathrm{MIT}}$ for tensile strained films, while their compressive counterparts do the opposite. Our findings identify the importance of orbital polarization in regulating the metal-to-insulator transitions, opening up a new perspective for orbital physics in transition metal oxides.

Working within the stochastic series expansion framework, we introduce and characterize a plaquette-based quantum cluster algorithm for quantum Monte Carlo simulations of transverse field Ising models with frustrated Ising exchange interactions. As a demonstration of the capabilities of this algorithm, we show that a relatively small ferromagnetic next-nearest-neighbor coupling drives the transverse field Ising antiferromagnet on the triangular lattice from an antiferromagnetic three-sublattice ordered state at low temperature to a ferrimagnetic three-sublattice ordered state.

We put forward nonadiabatic charge pumping as a method for accessing the different charge relaxation rates as well as the relaxation rates of excited orbital states in double-quantum-dot setups, based on extremely size-limited quantum dots and dopant systems. The rates are obtained in a well-separated manner from plateaus, occurring when comparing the steady-state current for reversed driving cycles. This yields a reliable readout independent of any fitting parameters. Importantly, the nonadiabatic pumping spectroscopy essentially exploits the same driving scheme that the operation of these devices generally employs. We provide a detailed analysis of the working principle of the readout scheme as well as of possible errors, thereby demonstrating its broad applicability. The precise knowledge of relaxation rates is highly relevant for the implementation of time-dependently operated devices, such as electron pumps for metrology or qubits in quantum information.

A low-temperature scanning tunneling microscope was used to fabricate atomic contacts on Pb(111). Conductance characteristics of the junctions were simultaneously recorded with forming and subsequent breaking of the contacts. A pronounced hysteresis effect in conductance traces was observed from junctions comprising the clean Pb(111) surface. The hysteretic behavior was less profound in contacts to single Pb atoms adsorbed to Pb(111). Density-functional calculations reproduced the experimental results by performing a full ab initio modeling of plastic junction deformations. A comprehensive description of the experimental findings was achieved by considering different atomic tip apex geometries.

We study the bulk entanglement of a series of gapped ground states of spin ladders, representative of the Haldane phase. These ground states of spin $S/2$ ladders generalize the valence bond solid ground state. In the case of spin 1/2 ladders, we study a generalization of the Affleck-Kennedy-Lieb-Tasaki and Nersesyan-Tsvelik states and fully characterize the bulk entanglement Hamiltonian. In the case of general spin $S$, we argue that in the Haldane phase the bulk entanglement spectrum of a half-integer ladder is either gapless or possess a degenerate ground state. For ladders with integer valued spin particles, the generic bulk entanglement spectrum should have an entanglement gap. Finally, we give an example of a series of trivial states of higher spin $S>1$ in which the bulk entanglement Hamiltonian is critical, signaling that the relation between topological states and a critical bulk entanglement Hamiltonian is not unique to topological systems.

The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications and are the subject of intense study in many disciplines. We here apply experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) and $\mathrm{GdTi}{\mathrm{O}}_3$ (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form ${[6\mathrm{unit}\mathrm{cells}(\mathrm{u}.\mathrm{c}.)\mathrm{STO}/3\mathrm{u}.\mathrm{c}.\mathrm{of}\mathrm{GTO}]}_{20}$ using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been exploited as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG in detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG and suggesting that oxygen vacancies are not the origin of it. Similar multitechnique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.

The presence of an exchange field in topological insulators reveals novel spin related phenomena derived from the combination of topology and magnetism. In the present work we show the controlled occurrence of either metallic or gapped topological Dirac states at the interface between ultrathin Cr films and the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ surface. The opening and closing of the gap at the Dirac point is caused by the spin reorientation transitions arising in the Cr films. We find that atom-thin layers of Cr adhered to ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ surfaces present a magnetic ground state with ferromagnetic planes coupled antiferromagnetically. As the thickness of the Cr film increases stepwise from one to three atomic layers, the direction of the magnetization changes twice from out of plane to in plane and to out of plane again. The out-of-plane magnetization drives the gap opening and the topological surface states acquire a circular meron spin structure. Therefore, the Cr spin reorientation leads to the metal-insulator transition in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ surface and to the correlated modification of the surface-state spin texture. Consequently, the thickness of the Cr film provides an effective and controllable mechanism to modify the metallic or gapped nature, as well as the spin texture of the topological Dirac states.

We study the structural evolution of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ as a function of pressure using x-ray diffraction. At a pressure of 54 GPa at room temperature, we observe a first-order structural phase transition, associated with a change from tetragonal to monoclinic symmetry and accompanied by a 4% volume collapse. Rietveld refinement of the high-pressure phase reveals a novel modification of the Ruddlesden-Popper structure, which adopts an altered stacking sequence of the perovskite bilayers. As the positions of the oxygen atoms could not be reliably refined from the data, we use density functional theory (local-density $\text{approximation}+U+\text{spin}$ orbit) to optimize the crystal structure and to elucidate the electronic and magnetic properties of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ at high pressure. In the low-pressure tetragonal phase, we find that the in-plane rotation of the ${\mathrm{IrO}}_6$ octahedra increases with pressure. The calculations further indicate that a bandwidth-driven insulator-metal transition occurs at $\sim 20$ GPa, along with a quenching of the magnetic moment. In the high-pressure monoclinic phase, structural optimization resulted in complex tilting and rotation of the oxygen octahedra and strongly overlapping $t_{2g}$ and $e_g$ bands. The $t_{2g}$ bandwidth renders both the spin-orbit coupling and electronic correlations ineffectual in opening an electronic gap, resulting in a robust metallic state for the high-pressure phase of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$.

$\mathrm{LaOBi}{\mathrm{S}}_2$-type materials have drawn much attention recently because of various interesting physical properties, such as low-temperature superconductivity, hidden spin polarization, and electrically tunable Dirac cones. However, it was generally assumed that each $\mathrm{LaOBi}{\mathrm{S}}_2$-type compound has a unique and specific crystallographic structure (with a space group $P4$/nmm) separated from other phases. Using first-principles total energy and stability calculations we confirm that the previous assignment of the $P4$/nmm structure to $\mathrm{LaOBi}{\mathrm{S}}_2$ is incorrect. Furthermore, we find that the unstable structure is replaced by a family of energetically closely spaced modifications (polytypes) differing by the layer sequences and orientations. We find that the local Bi-S distortion leads to three polytypes of $\mathrm{LaOBi}{\mathrm{S}}_2$ with different stacking patterns of the distorted $\mathrm{Bi}{\mathrm{S}}_2$ layers. The energy difference between the polytypes of $\mathrm{LaOBi}{\mathrm{S}}_2$ is merely ∼1 meV/u.c., indicating the possible coexistence of all polytypes in the real sample and that the particular distribution of polytypes may be growth induced. The in-plane distortion can be suppressed by pressure, leading to a phase transition from polytypes to the high-symmetry $P4$/nmm structure with a pressure larger than 2.5 GPa. In addition, different choices of the intermediate atoms (replacing La) or active atoms ($\mathrm{Bi}{\mathrm{S}}_2$) could also manifest different ground-state structures. One can thus tune the distortion and the ground state by pressure or by substituting covalence atoms in the $\mathrm{LaOBi}{\mathrm{S}}_2$ family.

Ferroelectrics form domain patterns that minimize their energy subject to imposed boundary conditions. In a linear, constrained theory, that neglects domain-wall energy, periodic domain patterns in the form of multirank laminates can be identified as minimum-energy states. However, when these laminates (formed in a macroscopic crystal) comprise domains that are a few nanometers in size, the domain-wall energy becomes significant, and the behavior of laminate patterns at this scale is not known. Here, a phase-field model, which accounts for gradient energy and strain energy contributions, is employed to explore the stability and evolution of the nanoscale multirank laminates. The stress, electric field, and domain-wall energies in the laminates are computed. The effect of scaling is also discussed. In the absence of external loading, stripe domain patterns are found to be lower-energy states than the more complex, multirank laminates, which mostly collapse into simpler patterns. However, complex laminates can be stabilized by imposing external loads such as electric field, average strain, and polarization. The study provides insight into the domain patterns that may form on a macroscopic single crystal but comprising nanoscale periodic patterns, and on the effect of external loads on these patterns.