The electron energy-loss spectroscopy technique known as electron magnetic circular dichroism (EMCD) has enormous potential for quantitatively probing the magnetic behavior of materials on the nanoscale. However, the requirement for mostly parallel illumination conditions greatly complicates the extraction of EMCD signals from surface areas under a few square nanometers, because scanning probe methods are limited to around this spatial resolution by the need for higher convergence angles. Here we propose theoretically and demonstrate experimentally that EMCD detection is feasible with convergence angles that are sufficiently large even for atomic resolution spectroscopy. Utilizing scanning transmission electron microscopy we experimentally detect a clear EMCD signal from a 50-nm-thick sample of bcc iron using a convergence semiangle of 8 mrad at 300 keV acceleration voltage, resulting in a probe size of approximately 2 Å. We subsequently estimate the number of chirally scattered electrons needed for an unambiguous detection of the EMCD signal and present a method to quantify confidence in signal detection.

Controlling the relaxation of magnetization in magnetic nanostructures is key to optimizing magnetic storage device performance. This relaxation is governed by both intrinsic and extrinsic relaxation mechanisms and with the latter strongly dependent on the interactions between the nanostructures. In the present work we investigate laser induced magnetization dynamics in a broadband optical resonance type experiment revealing the role of interactions between nanostructures on the relaxation processes of granular magnetic structures. The results are corroborated by constructing a temperature dependent numerical micromagnetic model of magnetization dynamics based on the Landau-Lifshitz-Bloch equation. The model predicts a strong dependence of damping on the key material properties of coupled granular nanostructures in good agreement with the experimental data. We show that the intergranular, magnetostatic and exchange interactions provide a large extrinsic contribution to the damping. Finally we show that the mechanism can be attributed to an increase in spin-wave degeneracy with the ferromagnetic resonance mode as revealed by semianalytical spin-wave calculations.

The unconventional superconductivity in the noncentrosymmetric ${\mathrm{LaNiC}}_2$, and its evolution with pressure, is analyzed basing on the ab initio computations and the full Eliashberg formalism. First-principles calculations of the electronic structure, phonons, and the electron-phonon coupling are reported in the pressure range 0–15 GPa. The thermodynamic properties of the superconducting state were determined numerically solving the Eliashberg equations. We found that already at $p=0$ GPa, the superconducting parameters deviate from the BCS-type, and a large value of the Coulomb pseudopotential ${\mu }^{☆}=0.22$ is required to get the critical temperature $T_c=2.8$ K consistent with experiment. If such ${\mu }^{☆}$ is used, the Eliashberg formalism reproduces also the experimentally observed values of the superconducting order parameter, the electronic specific heat jump at the critical temperature, and the change of the London penetration depth with temperature. This shows, that deviation of the above-mentioned parameters from the BCS predictions do not prejudge on the triplet or multiple gap nature of the superconductivity in this compound. Under the external pressure, calculations predict continuous increase of the electron-phonon coupling constant in the whole pressure range 0–15 GPa, consistent with the experimentally observed increase in $T_c$ for the pressure range 0–4 GPa, but inconsistent with the drop of $T_c$ above 4 GPa and the disappearance of the superconductivity above 7 GPa, reported experimentally. The disappearance of superconductivity may be accounted for by increasing the ${\mu }^{☆}$ to 0.36 at 7 GPa, which supports the hypothesis of the formation of a new high-pressure electronic phase, which competes with the superconductivity.

We study possible applications of high critical temperature nodal superconductors for the search for Majorana bound states in the DIII class. We propose a microscopic analysis of the proximity effect induced by $d$-wave superconductors on a semiconductor wire with strong spin-orbit coupling. We characterize the induced superconductivity on the wire employing a numerical self-consistent tight-binding Bogoliubov–de Gennes approach, and analytical considerations on the Green's function. The order parameter induced on the wire, the pair correlation function, and the renormalization of the Fermi points are analyzed in detail, as well as the topological phase diagram in the case of weak coupling. We highlight optimal Hamiltonian parameters to access the nontrivial topological phase which could display time-reversal invariant Majorana doublets at the boundaries of the wire.

The phase diagram of the spin-1 chain with bilinear-biquadratic and next-nearest-neighbor interactions, recently investigated by Pixley, Shashi, and Nevidomskyy [Phys. Rev. B 90, 214426 (2014)], has been revisited in the light of results we have recently obtained on a similar model. Combining extensive density-matrix renormalization-group simulations with conformal-field theory arguments, we confirm the presence of the three phases identified by Pixley et al., a Haldane phase, a next-nearest-neighbor (NNN) Haldane phase, and a dimerized phase, but we come to significantly different conclusions regarding the nature of the phase transitions to the dimerized phase: (i) We provide numerical evidence of a continuous Ising transition between the NNN-Haldane phase and the dimerized phase. (ii) We show that the tricritical end point, where the continuous transition between the Haldane phase and the dimerized phase turns into a first-order transition, is distinct from the triple point where the three phases meet. (iii) Finally, we demonstrate that the tricritical end point is in the same Wess-Zumino-Witten $\mathrm{SU}{\left(2\right)}_2$ universality class as the continuous transition line that ends at this point.

The mapping between a classical length and inverse temperature as imaginary time provides a direct equivalence between the Casimir force of a classical system in $D$ dimensions and internal energy of a quantum system in $d=D-1$ dimensions. The scaling functions of the critical Casimir force of the classical system with periodic boundaries thus emerge from the analysis of the symmetry related quantum critical point. We show that both nonperturbative renormalization group and quantum Monte Carlo analysis of quantum critical points provide quantitative estimates for the critical Casimir force in the corresponding classical model, giving access to widely different aspect ratios for the geometry of confined systems. In light of these results, we propose protocols for the realization of critical Casimir forces for periodic boundaries through state-of-the-art cold-atom and solid-state experiments.

In this paper, we have systematically studied the single-hole problem in two-leg Hubbard and $t-J$ ladders by large-scale density-matrix renormalization-group calculations. We found that the doped holes in both models behave similarly, while the three-site correlated hopping term is not important in determining the ground-state properties. For more insights, we have also calculated the elementary excitations, i.e., the energy gaps to the excited states of the system. In the strong-rung limit, we found that the doped hole behaves as a Bloch quasiparticle in both systems where the spin and charge of the doped hole are tightly bound together. In the isotropic limit, while the hole still behaves like a quasiparticle in the long-wavelength limit, our results show that its spin and charge components are only loosely bound together inside the quasiparticle, whose internal structure can lead to a visible residual effect which dramatically changes the local structure of the ground-state wave function.

We present exact numerical results for the effects of thermal fluctuations on the experimentally relevant thermodynamic and spectral properties of Peierls chains. To this end, a combination of classical Monte Carlo sampling and exact diagonalization is used to study adiabatic half-filled Holstein and Su-Schrieffer-Heeger models. The classical nature of the lattice displacements in combination with parallel tempering permit simulations on large system sizes and a direct calculation of spectral functions in the frequency domain. Most notably, the long-range order and the associated Peierls gap give rise to a distinct low-temperature peak in the specific heat. The closing of the gap and suppression of order by thermal fluctuations involves in-gap excitations in the form of soliton-antisoliton pairs and is also reflected in the dynamic density and bond structure factors as well as in the optical conductivity. We compare our data to the widely used mean-field approximation and highlight relations to symmetry-protected topological phases and disorder problems.

We report on results of nonequilibrium transport measurements made on thin films of germanium-telluride $\left({\mathrm{Ge}}_x\mathrm{Te}\right)$ at cryogenic temperatures. Owing to a rather large deviation from stoichiometry $(\approx 10\%$ of Ge vacancies), these films exhibit $p\text{−}\mathrm{type}$ conductivity with carrier concentration $N\ge {10}^{\text{20}}{\mathrm{cm}}^{-3}$ and can be made either in the diffusive or strongly localized regime by a judicious choice of preparation and post-treatment conditions. In both regimes, the system shows persistent photoconductivity following excitation by a brief exposure to infrared radiation. Persistent photoconductivity is also observed in ${\mathrm{Ge}}_x\mathrm{Te}$ samples alloyed with Mn. However, in both ${\mathrm{Ge}}_x\mathrm{Te}$ and ${\mathrm{GeMn}}_x{\mathrm{Te}}_y$, the effect is much weaker than that observable in ${\mathrm{GeSb}}_x{\mathrm{Te}}_y$ alloys, suggesting that antimony plays an important role in the phenomenon. Structural studies of these films reveal an unusual degree of texture that is rarely realized in strongly disordered systems with high carrier concentrations. Anderson-localized samples of ${\mathrm{Ge}}_x\mathrm{Te}$ exhibit nonergodic transport which is characteristic of intrinsic electron glasses, including a well-developed memory dip and slow relaxation of the excess conductance created in the excited state. These results support the conjecture that electron-glass effects with inherently long relaxation times is a generic property of all Anderson-localized systems with large carrier concentration.

Motivated by the recent experimental report of a possible light-induced superconductivity in ${\mathrm{K}}_3{\mathrm{C}}_{60}$ at high temperature [Mitrano et al., Nature 530, 451 (2016)], we investigate theoretical mechanisms for enhanced superconductivity in $A_3{\mathrm{C}}_{60}$ fullerenes. We find that an “interaction imbalance” corresponding to a smaller value of the Coulomb matrix element for two of the molecular orbitals in comparison to the third one, efficiently enhances superconductivity. Furthermore, we perform first-principle calculations of the changes in the electronic structure and in the screened Coulomb matrix elements of ${\mathrm{K}}_3{\mathrm{C}}_{60}$, brought in by the deformation associated with the pumped $T_{1u}$ intramolecular mode. We find that an interaction imbalance is indeed induced, with a favorable sign and magnitude for superconductivity enhancement. The physical mechanism responsible for this enhancement consists of a stabilization of the intramolecular states containing a singlet pair, while preserving the orbital fluctuations allowing for a coherent interorbital delocalization of the pair. Other perturbations have also been considered and found to be detrimental to superconductivity. The light-induced deformation and ensuing interaction imbalance is shown to bring superconductivity further into the strong-coupling regime.

Combining Raman scattering measurements with mean-field calculations of the Raman response, we show that Kitaev-like magnetic exchange is dominant in the hyperkagome iridate ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$. In the measurements we observe a broad Raman band at $\sim 3500{\mathrm{cm}}^{-1}$ with a bandwidth $\sim 1700{\mathrm{cm}}^{-1}$. Calculations of the Raman response of the Kitaev-Heisenberg model on the hyperkagome lattice shows that the experimental observations are consistent with the calculated Raman response where the Kitaev exchange interaction ($J_K$) is much larger than the Heisenberg term ${\mathrm{J}}_1$ ($J_1/J_K\sim 0.1$). A comparison with the theoretical model gives an estimate of the Kitaev exchange interaction parameter.

We study theoretically the circularly polarized light-induced Floquet state in line-node semimetals with time-reversal symmetry and inversion symmetry. It is found that the Floquet state can show the photovoltaic anomalous Hall effect when an applied circularly polarized light creates a gap in the line node in the bulk and leaves Weyl point nodes. The Hall conductivity is sensitive to the location of the Fermi level: When the Fermi level is located at the node, the Hall conductivity depends on the radius of the line node and is nearly independent of the intensity of the light. Far from the line node, the Hall conductivity is dependent on the intensity of the light. The sensitive Fermi-level dependence of the Hall conductivity in the presence of a laser of weak intensity can have applications in phototransistors based on thin films of line-node semimetals.

Hexagonally patterned two-dimensional $p$-type semiconductor systems are quantum simulators of graphene with strong and highly tunable spin-orbit interactions. We show that application of purely in-plane magnetic fields, in combination with the crystallographic anisotropy present in low-symmetry semiconductor interfaces, allows Chern insulating phases to emerge from an originally topologically insulating state after a quantum phase transition. These phases are characterized by pairs of co-propagating edge modes and Hall conductivities ${\sigma }_{xy}=+\frac{2e^2}{h},-\frac{2e^2}{h}$ in the absence of Landau levels or cyclotron motion. With current lithographic technology, the Chern insulating transitions are predicted to occur in GaAs heterostructures at magnetic fields of $\sim 5\text{T}$.

We report a theoretical study on the spin Josephson effect arising from the exchange coupling of the two ferromagnets (Fs), which are deposited on a two-dimensional (2D) time-reversal-invariant topological insulator. An anomalous spin supercurrent $J_{sz}\sim sin(\alpha +{\alpha }_0)$ is found to flow in between the two Fs and the ground state of the system is not limited to the magnetically collinear configuration $(\alpha =n\pi ,n$ is an integer) but determined by a controllable angle ${\alpha }_0$, where $\alpha$ is the crossed angle between the two F magnetizations. The angle ${\alpha }_0$ is the dynamic phase of the electrons traveling in between the two Fs and can be controlled electrically by a gate voltage. This anomalous spin Josephson effect, similar to the conventional ${\phi }_0$ superconductor junction, originates from the definite electron chirality of the helical edge states in the 2D topological insulator. These results indicate that the magnetic coupling in a topological system is different from the usual one in conventional materials.

A monolayer of molecules or quantum dots sandwiched between electrodes can be driven out of equilibrium by the application of a bias voltage between the electrodes. We study charge ordering, i.e., the spontaneous formation of a charge density wave, and the perpendicular current in such a system within a master-equation approach augmented by mean-field and classical Monte Carlo methods. Our approach is suitable for weak tunneling between the monolayer and the electrodes. For a square lattice with nearest-neighbor Coulomb repulsion, we present a comprehensive study of the zero-temperature phases controlled by the onsite energy, the bias voltage, and the degeneracy of the occupied single-site state. One of the most interesting results is the prediction of a conducting charge-density-wave phase that only occurs at a finite-bias voltage. We also study the universality classes of the phase transitions towards charge-ordered states at zero and nonzero temperatures. While all transitions at $T>0$ and some at $T=0$ belong to the two-dimensional Ising universality class, we also find an absorbing-to-active phase transition in the ${\mathbb{Z}}_2$ symmetric directed percolation (DP2) class at $T=0$.

We study the critical Josephson current flowing through a double quantum dot weakly coupled to two superconducting leads. We use analytical as well as numerical methods to investigate this setup in the limit of small and large bandwidth leads in all possible charging states, where we account for on-site interactions exactly. Our results provide clear signatures of nonlocal spin-entangled pairs, which support interpretations of recent experiments [R. S. Deacon, A. Oiwa, J. Sailer, S. Baba, Y. Kanai, K. Shibata, K. Hirakawa, and S. Tarucha, Nat. Commun. 6, 7446 (2015)]. In addition, we find that the ground state with one electron on each quantum dot can undergo a tunable singlet-triplet phase transition in the regime where the superconducting gap in the leads is not too large, which gives rise to an additional new signature of nonlocal Cooper-pair transport.

Quantum Hall effects offer a formidable playground for the investigation of quantum transport phenomena. Edge modes can be deflected, branched, and mixed by designing a suitable potential landscape in a two-dimensional conducting system subject to a strong magnetic field. In the present work, we demonstrate a buried split-gate architecture and use it to control electron conduction in large-scale single-crystal monolayer graphene grown by chemical vapor deposition. The control of the edge trajectories is demonstrated by the observation of various fractional quantum resistances, as a result of a controllable interedge scattering. Experimental data are successfully modeled both numerically and analytically within the Landauer-Büttiker formalism. Our architecture is particularly promising and unique in view of the investigation of quantum transport via scanning probe microscopy, since graphene constitutes the topmost layer of the device. For this reason, it can be approached and perturbed by a scanning probe down to the limit of mechanical contact.

We report on the growth of a monolayer-thick ${\mathrm{BiAg}}_2$ surface alloy on thin Ag films grown on Pt(111) and Cu(111). Using low energy electron diffraction (LEED), angle resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) we show that the surface structure of the $\frac{1}{3}$ ML Bi/$x$-ML Ag/Pt(111) system ($x\ge 2$) is strongly affected by the annealing temperature required to form the alloy. As judged from the characteristic ($\sqrt{3}\times \sqrt{3}$)$R{30}^{\circ }$ LEED pattern, the ${\mathrm{BiAg}}_2$ alloy is partially formed at room temperature. A gentle, gradual increase in the annealing temperatures successively results in the formation of a pure ${\mathrm{BiAg}}_2$ phase, a combination of that phase with a ($2\times 2$) superstructure, and finally the pure ($2\times 2$) phase, which persists at higher annealing temperatures. These results complement recent work reporting the ($2\times 2$) as a predominant phase, and attributing the absence of ${\mathrm{BiAg}}_2$ alloy to the strained Ag/Pt interface. Likewise, we show that the growth of the ${\mathrm{BiAg}}_2$ alloy on similarly lattice-mismatched 1 and 2 ML Ag-Cu(111) interfaces also requires a low annealing temperature, whilst higher temperatures result in ${\mathrm{BiAg}}_2$ clustering and the formation of a ${\mathrm{BiCu}}_2$ alloy. The demonstration that the ${\mathrm{BiAg}}_2$ alloy can be formed on thin Ag films on different substrates presenting a strained interface has the prospect of serving as bases for technologically relevant systems, such as Rashba alloys interfaced with magnetic and semiconductor substrates.

Measuring transmission between four antennas in microwave cavities, we investigate directly the channel cross correlations $C$ of the cross sections ${\sigma }^{ab}$ from antenna at ${\vec{r}}_a$ to antenna ${\vec{r}}_b$. Specifically we look for the $C_{\mathrm{\Sigma }}$ and $C_{\mathrm{\Lambda }}$, where the only difference is that $C_{\mathrm{\Lambda }}$ has none of the four channels in common, whereas $C_{\mathrm{\Sigma }}$ has exactly one channel in common. We find experimentally that these two channel cross correlations are antiphased as a function of the channel coupling strength, as predicted by theory. This anticorrelation is essential to obtain the correct values for the universal conductance fluctuations. To obtain good agreement between experiment and predictions from random matrix theory the effect of absorption has to be included.

A low-temperature magnetic phase diagram under magnetic fields along the orthorhombic $a$ axis of an isosceles-triangular-lattice antiferromagnet ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ was investigated through single-crystal neutron diffraction measurements made at temperatures down to $T=0.5\mathrm{K}$. We produced a phase diagram that consists of three magnetically ordered phases, i.e., the antiferromagnetic (AF), the incommensurate sinusoidal magnetic (IC), and the induced ferromagnetic (IFM) phases, which were characterized by the propagation wave vectors $\mathbf{Q}=\left(0\frac{1}{2}0\right)$, (0 $q$ 0), and (0 0 0), respectively. We found that a field-induced ferrimagnetic phase with $\mathbf{Q}=\left(0\frac{1}{4}0\right)$ that had been observed by previous neutron diffraction studies down to $T=1.8\mathrm{K}$ [H. Weitzel et al., Phys. Rev. B 62, 12146 (2000)] does not exist as a single equilibrium phase, but rather it always coexists with the other ordered phases near the triple point where the AF, IC, and IFM phases meet. We also found that the relaxation time of the system becomes extremely long below $T=0.6\mathrm{K}$ in comparison with our observation time; this was considered to possibly be the reason for magnetization plateaus appearing at $T=0.5\mathrm{K}$. These plateaus have a half-saturation magnetization, from which another field-induced state was inferred in previous magnetization measurements [J. Phys. Soc. Jpn. 63, 2706 (1994)].