We theoretically study the superconducting proximity effect in a quantum dot coupled to two superconducting leads when the intradot interaction between electrons is made attractive. Because of the superconducting proximity effect, the electronic states for the embedded quantum dot are either spin-polarized states with an odd occupation number or BCS-like states with an even occupation number. We show that in the presence of an external magnetic field, the system can exhibit quantum phase transitions of fermion parity associated with the occupation number. In this paper, we adopt a self-consistent theoretical method to extend our considerations beyond the so-called superconducting atomic limit in which the superconducting gap for the leads is assumed to be the largest energy scale. This method enables us to numerically investigate the electronic structure of the dot as a result of the attractive interaction. For energy phase diagrams in the regime away from the atomic limit, we find a reentrant behavior where a BCS-like phase of the dot exists in an intermediate range of the hybridization strength between the quantum dot and the leads. We also consider Josephson current-phase relations and identify a number of examples showing 0-$\pi$ phase transitions that may offer important switching effects.

We extend the two leading methods for the ab initio computational description of phonon-mediated superconductors, namely Eliashberg theory and density-functional theory for superconductors (SCDFT), to include plasmonic effects. Furthermore, we introduce a hybrid formalism in which the Eliashberg approximation for the electron-phonon coupling is combined with the SCDFT treatment of the dynamically screened Coulomb interaction. The methods have been tested on a set of well-known conventional superconductors by studying how the plasmon contribution affects the phononic mechanism in determining the critical temperature $\left(T_C\right)$. Our simulations show that plasmonic SCDFT leads to a good agreement between predicted and measured $T_C$'s, whereas Eliashberg theory considerably overestimates the plasmon-mediated pairing and, therefore, $T_C$. The hybrid approach, on the other hand, gives results close to SCDFT and overall in excellent agreement with experiments.

Motivated by the recent discovery of superconductivity in square-planar nickelates as well as by longstanding puzzling experiments in heavy-fermion superconductors, we study Cooper pairing between correlated $d$ electrons coupled to a band of weakly correlated electrons. We perform self-consistent large N calculations on an effective $t-J$ model for the $d$ electrons with additional hybridization. Unlike previous studies of mixed-valent systems, we focus on parameter regimes where both hybridized bands are relevant to determine the pairing symmetry. For sufficiently strong hybridization, we find a robust $(s+id)$ pairing which breaks time-reversal and point-group symmetries in the mixed-valent regime. Our results illustrate how intrinsically multiband systems such as heavy fermions can support a number of highly nontrivial pairing states. They also provide a putative microscopic realization of previous phenomenological proposals of ($s+id$) pairing and suggest a potential resolution to puzzling experiments in heavy-fermion superconductors such as ${\mathrm{U}}_{1-x}{\mathrm{Th}}_x{\mathrm{Be}}_{13}$ which exhibit two superconducting phase transitions and a full gap at lower temperatures.

The Edelstein effect is useful for electric control of the magnetic moment. However, Joule heating created by a dissipative current is harmful to its practical applications. In this paper, we investigate two-dimensional noncentrosymmetric superconductors (NCSs) with either $s$-wave or $d$-wave symmetry and demonstrate that the surface Edelstein effect is significantly enhanced in $d$-wave NCSs. The origin of the enhancement is attributed to surface Majorana states characteristic of gapless spin-singlet noncentrosymmetric superconductors. In the view of superconducting spintronics, this result would give a route to magnetic domain switching by a dissipationless supercurrent. We discuss a possible experimental observation in cuprate superconductor heterostructures and heavy fermion superlattices.

Anderson localization is reported in both the limits of disorder, namely, at weak disorder in a periodic system and at maximal possible disorder. However, the statistics of the transport, quantified using generalized conductance fluctuations, reveal significant differences between the two regimes. Under maximal possible disorder, the generalized conductance remains below unity for any configuration of the disorder, attesting to the arrested nature of transport. Contrarily, at near-periodic disorder, the distribution of generalized conductance is heavy tailed towards large conductance values, indicating that the overall transport is delocalized. Theoretical results from a model based on the tight-binding approximation, augmented to include open boundaries, are in excellent agreement with experiments, and provide further insights into the transport characteristics.

Serial double quantum dots created in semiconductor nanostructures provide a versatile platform for investigating two-electron spin quantum states, which can be tuned by electrostatic gating and an external magnetic field. In this Rapid Communication, we directly measure the supercurrent reversal between adjacent charge states of an InAs nanowire double quantum dot with superconducting leads, in good agreement with theoretical models. In the even charge parity sector, we observe a supercurrent blockade with increasing magnetic field, corresponding to the spin singlet to triplet transition. Our results demonstrate a direct spin to supercurrent conversion, the superconducting equivalent of the Pauli spin blockade. This effect can be exploited in hybrid quantum architectures coupling the quantum states of spin systems and superconducting circuits.

Fundamental flaws in the Heisenberg Hamiltonian are highlighted in the context of its application to bcc Fe, including the particular issues arising when modeling lattice defects using spin dynamics. Exchange integrals are evaluated using the magnetic force theorem. The bilinear exchange coupling constants are calculated for all the interacting pairs of atomic magnetic moments in large simulation cells containing defects, enabling a direct mapping of the magnetic energy onto the Heisenberg Hamiltonian and revealing its limitations. We provide a simple procedure for extracting the Landau parameters from density-functional theory calculations to construct a Heisenberg-Landau Hamiltonian. We quantitatively show how the Landau terms correct the exchange-energy hypersurface, which is essential for the accurate evaluation of energies and migration barriers of defects.

We examine the metallization of fluid helium with molecular dynamics simulations based on density functional theory. The insulator-to-metal transition is studied at densities between 1 and $22\mathrm{g}/{\mathrm{cm}}^3$ and temperatures between 10 000 and 50 000 K. We calculate the equation of state, the band gap dependent on density and temperature by using different definitions, the DC conductivity, the reflectivity, and the ionization degree for which a novel method has been proposed recently [see M. Bethkenhagen et al., Phys. Rev. Research 2, 023260 (2020)]. We find no indication of a first-order phase transition in any of the properties studied here and therefore conclude that the metallization of fluid helium is continuous. For instance, we do not observe jumps in the DC conductivity and/or the reflectivity when the band gap closes. However, the ionization degree increases continuously from below 10% at the lowest to over 99% at the highest densities which reflects the continuous insulator-to-metal transition. The increase is almost exclusively driven by pressure ionization and shows only a weak temperature dependency. We discuss the high-pressure phase diagram of helium and the implications of our results on the structure of astrophysical objects like gas giant planets and brown dwarfs.

We study the dynamics of one- and two-dimensional disordered lattice bosons/fermions initialized to a Fock state with 1 particle on a set of lattice sites ($A$) and 0 particles on the rest of the sites ($\overline{A}$). Such states have been considered in recent ultracold atomic experiments to detect many body localization. For noninteracting systems we establish a universal relation between the long time density imbalance between $A$ and $\overline{A}$ sites, $I\left(\infty \right)$, the localization length ${\xi }_l$, and the geometry of the initial pattern. For the alternating initial pattern of 1 and 0 particles in one dimension, $I\left(\infty \right)=tanh[a/{\xi }_l]$, where $a$ is the lattice spacing. For systems with mobility edge, we find analytic relations between $I\left(\infty \right)$, the effective localization length ${\overline{\xi }}_l$, and the fraction of localized states $f_l$. The imbalance as a function of disorder shows nonanalytic behavior when the mobility edge passes through a band edge. For interacting bosonic systems, we show that there is a mechanism to retain a finite long-time imbalance in the system even in presence of dissipative and stochastic processes coming from interparticle scattering. The scattering of particles, which lead to a decay of the memory of initial conditions through dissipative processes, also creates excitations in the system. For strong disorder, the excitations act as a local bath, whose noise correlators retain information of the initial pattern. This sustains a finite imbalance at long times in strongly disordered interacting systems.

Coherent control is a method used to manipulate the state of matter using oscillatory electromagnetic radiation which interacts with the state in a nonequilibrium manner before the steady precessional state is reached. It is commonly applied in quantum processing applications. This technique is interesting in the context of ferromagnetic materials because of the ability to combine it with spintronics for the purpose of fundamental spin transport research, low-power information processing, and potentially future quantum bit (Qubit) applications. In this work we address the theoretical grounds of coherent manipulation in practical ferromagnetic systems. We study the electromagnetic radiation-driven interaction that is enhanced in the presence of spin-polarized current and map the conditions that allow coherent manipulation for which Rabi oscillations take place. The role of the magnetic anisotropy field is shown to act as an additional oscillatory driving field. We discuss the Gilbert losses in the context of effective coherence decay rates and show that it is possible to control these rates by application of a static spin current. The case of coherent manipulation using oscillatory spin current that is free of radiation is discussed as well. Our work paves the way towards spin current amplification as well as radiation-free coherent control schemes that may potentially lead to novel Qubits that are robust and scalable.

The interplay between magnetism, band topology, and electronic correlation in low dimensions has been a fascinating subject of research. Here, we propose two-dimensional (2D) material systems which demonstrate such an interesting interplay. Based on first-principles calculations and structural search algorithms, we identify three lowest energy 2D CoSe structures, termed as the $\alpha$-, $\beta$-, and $\gamma$-CoSe. We show that $\alpha$- and $\beta$-CoSe are two rare examples of 2D antiferromagnetic metals, which are related to their Fermi surfaces nesting features, and meanwhile, $\gamma$-CoSe is a ferromagnetic metal. They possess a range of interesting physical properties, including anisotropic valleys connected by crystalline symmetries, strain-tunable valley polarization, strain-induced metal-semiconductor and/or magnetic phase transitions, as well as topological band features such as the magnetic Weyl point and the magnetic Weyl loop. Remarkably, all the topological features here are robust against spin-orbit coupling. Some experimental aspects of our predictions have been discussed.

We establish a way to handle main collective fluctuations in correlated quantum systems based on a fluctuating local field concept. This technique goes beyond standard mean-field approaches, such as Hartree-Fock and dynamical mean-field theories (DMFT), as it includes a fluctuating classical field that acts on the leading order parameter of the system. Effective model parameters of this theory are determined from the variational principle, which allows one to resolve the Fierz ambiguity in decoupling of the local interaction term. In the saddle-point approximation for the fluctuating field our method reproduces the mean-field result. The exact numerical integration over this field allows one to consider nonlinear fluctuations of the global order parameter of the system while local correlations can be accounted for by solving the DMFT impurity problem. We apply our method to the magnetic susceptibility of finite Hubbard systems at half-filling and demonstrate that the introduced technique leads to a superior improvement of results with respect to parental mean-field approaches, without significant numerical complications. We show that the fluctuating local field method can be used in a very broad range of temperatures substantially below the Néel temperature of DMFT, which remains a major challenge for all existing theoretical approaches.

$\mathrm{SrCu}{\mathrm{Te}}_2{\mathrm{O}}_6$ consists of a three-dimensional arrangement of spin-$\frac{1}{2}{\mathrm{Cu}}^{2+}$ ions. The first-, second-, and third-neighbor interactions, respectively, couple ${\mathrm{Cu}}^{2+}$ moments into a network of isolated triangles, a highly frustrated hyperkagome lattice consisting of corner-sharing triangles and antiferromagnetic chains. Of these, the chain interaction dominates in $\mathrm{SrCu}{\mathrm{Te}}_2{\mathrm{O}}_6$ while the other two interactions lead to frustrated interchain coupling giving rise to long-range magnetic order at suppressed temperatures. In this paper, we investigate the magnetic properties in $\mathrm{SrCu}{\mathrm{Te}}_2{\mathrm{O}}_6$ using muon relaxation spectroscopy and neutron diffraction and present the low-temperature magnetic structure as well as the directional-dependent magnetic phase diagram as a function of field.

We simulate the improved $(q+1)$-state clock model on the simple-cubic lattice at the critical point on lattices of a linear size up to $L=960$. We compute operator product expansion coefficients for the three-dimensional $XY$ universality class. These are compared with highly accurate estimates obtained by using the conformal bootstrap method. We find that the results are consistent.

Two-dimensional (2D) topological superconductors (TS) host chiral Majorana modes (MMs) localized at the boundaries. In this work, we study the effect of disorder on the localization length of MMs in two-dimensional spin-orbit (SO) coupled superconductors within quasiclassical approximation. We find nonmonotonic behavior of the Majorana localization length as a function of disorder strength. At weak disorder, the Majorana localization length decreases with an increasing disorder strength. Decreasing the disorder scattering time below a crossover value ${\tau }_c$, the Majorana localization length starts to increase. The crossover scattering time depends on the relative magnitudes of the two ingredients behind TS: SO coupling and exchange field. For dominating SO coupling, ${\tau }_c$ is small, and vice versa for the dominating exchange field.

We explore the impact of commensurate multifrequency driving protocols on the stability of topological edge modes in topological one-dimensional systems of spinless fermions. Using Floquet theory, we show that all the topological phase transitions can be mapped to their single-frequency counterparts in terms of effective driving parameters. This drastic simplification can be explained by considering the gap closures in the quasienergy dispersion. These gap closures are pinned to high-symmetry points, where all the effective Floquet Hamiltonians collapse to the same form. While for all protocols all topological phase transitions coincide, the gap size and the number of edge modes in the quasienergy spectra vary considerably depending on the chosen driving pattern. This gives rise to a full range of edge states with different degrees of localization, from highly localized to completely delocalized. Switching between different driving protocols then suggests a dynamical control of the localization and stability of topological edge modes, with possible applications in quantum computation. We illustrate our findings on three paradigmatic fermionic systems, namely the Kitaev chain, the Su-Schrieffer-Heeger model, and the Creutz ladder, and demonstrate how to control the localization length of the corresponding edge states.

We use the momentum average approximation together with perturbative approaches, in the appropriate limits, to study the single polaron physics on a perovskite lattice inspired by $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$. We investigate electron-phonon coupling of the Peierls type whereby the motion of ions modulates the values of the hopping integrals between sites and show that it cannot be mapped onto the simpler one-band Holstein model in the whole parameter space. This is because the dispersion of the Peierls polaron has sharp transitions where the ground-state momentum jumps between high-symmetry points in the Brillouin zone, whereas the Holstein polaron always has the same ground-state momentum. These results imply that careful consideration is required to choose the appropriate model for carrier-lattice coupling in such complex lattices.

We investigate the strong field nuclear spin relaxation rate in $n$-type GaAs for doping densities from the quasi-insulating over the metal-to-insulator up to the quasimetallic regime. The rate measured at 6.5 K increases in the quasi-insulating regime with doping density due to nuclear spin diffusion to the donor electrons and shows a distinct maximum at the critical density of the Mott metal-to-insulator transition. The density dependence of the nuclear spin relaxation rate can be quantitatively calculated over the whole density regime taking into account the effective number of localized electrons and the interaction of free electrons via the Korringa mechanism. Only the nuclear spin relaxation rate of the very lowest doped sample shows a significant deviation from these calculations. Temperature-dependent measurements suggest in this case an additional nuclear spin relaxation channel which is negligible at higher doping densities and is linked to the electron spin relaxation time.