We study the combined effect of interactions and disorder on topological order in one dimension. Toward that end, we consider a generalized Kitaev chain including fermion-fermion interactions and disorder in the chemical potential. We determine the phase diagram by performing density-matrix renormalization-group calculations on the corresponding spin-1/2 chain. We find that moderate disorder or repulsive interactions individually stabilize the topological order, which remains valid for their combined effect. However, both repulsive and attractive interactions lead to a suppression of the topological phase at strong disorder.

We show that charge fluctuation processes are crucial for the nonlinear heat conductance through an interacting nanostructure, even far from a resonance. We illustrate this for an Anderson quantum dot accounting for the first two leading orders of the tunneling in a master equation. The often made assumption that off-resonant transport proceeds entirely by virtual occupation of charge states, underlying exchange-scattering models, can fail dramatically for heat transport. The identified energy-transport resonances in the Coulomb blockade regime provide qualitative information about relaxation processes, for instance, by a strong negative differential heat conductance relative to the heat current. These can go unnoticed in the charge current, making nonlinear heat-transport spectroscopy with energy-level control a promising experimental tool. [6]. Here, by analyzing the generic effects of Coulomb interactions on the nonlinear heat transport in nanoscale systems, we will show that this is very promising.Interaction effects have long been probed using gate controlled charge-current spectroscopy, a well-developed experimental tool to access the discrete quantum levels of nanostructures. Two prominent features in the charge current driven by a source-drain voltage underpin this successful method. The first is resonant or single-electron tunneling (SET), which depends on the level position relative to the electrochemical potential, μ R in Fig. 1(a): An electron jumps into or out of an orbital level, directly leading to a real change of its occupancy. The current shows sharp steps as new resonant transport processes are switched on with increasing bias. These processes are routinely identified in a three-terminal setup by plotting the charge conductance as function of the applied bias V and the gate voltage, as exemplified in Fig. 2(a). Two-terminal measurements, e.g., using a scanning probe, correspond to line traces through such a plot. The second type of resonance is independent of the level position and appears as a horizontal line at V = since it originates in the inelastic excitation by an energy at fixed local electron number on the nanostructure. This off-resonant feature requires a second-order tunneling process in which an electron "scatters through," other charge states being only visited virtually [see Fig. 1(b)]. This is known as inelastic electron tunneling (IETS) [7,8] or inelastic cotunneling (ICOT) [9][10][11][12].This inelastic tunneling resonance develops into a nonequilibrium Kondo resonance for low and low temperatures [13][14][15], which is much sharper [11,12] Thermoelectric transport has also been investigated within the two above-mentioned physical transport pictures. Theory mostly focused on the thermopower in the linear-response regime. This includes the study of resonant tunneling [26], inelastic tunneling [27][28][29], and Kondo processes [30][31][32][33]. Works addressing the nonlinear regime have either applied effective single-particle descriptions [34][35][36][37][38] or focused on th...

We show that the coupling of homogeneous Heisenberg spin-1/2 ladders in different phases leads to the formation of interfacial zero energy Majorana bound states. Unlike Majorana bound states at the interfaces of topological quantum wires, these states are void of topological protection and generally susceptible to local perturbations of the host spin system. However, a key message of our work is that in practice they show a high degree of resilience over wide parameter ranges which may make them interesting candidates for applications. Introduction:The Majorana fermion has become one of the most important fundamental quasi particles of condensed matter physics. Besides its key role as a building block in correlated quantum matter, much of this interest is motivated by perspectives in quantum information. 1-3 Majorana qubits have unique properties which make them ideal candidates for applications in, e.g., stabilizer code quantum computation. 4 Current experimental attempts to isolate and manipulate Majorana bound states (MBSs) focus on interfaces between distinct phases of symmetry protected topological (SPT) quantum matter. These material platforms have the appealing property that MBSs are protected against local perturbations by principles of topology. In practice, however, topological protection may play a lesser role than one might hope, and various obtrusive aspects of realistic quantum materials appear to challenge the isolation and manipulation of MBSs. Specifically, in topological quantum wires based on the hybrid semiconductor-superconductor platform 5 or on coupled ferromagnetic atoms, 6 all relevant scales are confined to narrow windows in energy. In this regard, proposals to realize MBSs in topological insulator nanowires 7 may offer superior solutions. However, these realizations require a high level of tuning of external parameters, notably of magnetic fields, and may be met with their own difficulties.In this Letter, we suggest an alternative hardware platform for the isolation of zero-energy MBSs. Our proposal does not engage topology. Specifically, local perturbations of the microscopic Hamiltonian may induce nonlocal correlations between the emergent Majorana quantum particles. However, we argue below that in practice this problem is less drastic than one might fear, and that the current architecture may grant a high level of effective protection. The numerical evidence provided below certainly points in this direction.The material platform we suggest is based on spin ladder materials. Their phases can be classified by combining standard Landau-Ginzburg symmetry breaking with the presence of SPT order. 8,9 We show here that combining ladders in different phases provides a systematic means to generating interface MBSs. The formal bridge between the physics of spin ladders and that of Majorana fermions is provided by a two-step mapping, first representing the spin degrees of freedom by bosons, followed by refermionization of the latter into an effective Majorana theory. 10 We will discuss how nume...

We develop a theory for spin transport and magnetization dynamics in a quantum dot spin valve, i.e., two magnetic reservoirs coupled to a quantum dot. Our theory is able to take into account effects of strong correlations. We demonstrate that, as a result of these strong correlations, the dot gate voltage enables control over the current-induced torques on the magnets and, in particular, enables voltage-controlled magnetic switching. The electrical resistance of the structure can be used to read out the magnetic state. Our model may be realized by a number of experimental systems, including magnetic scanning-tunneling microscope tips and artificial quantum dot systems.

We study the time-evolving currents flowing in an interacting ring-shaped nanostructure after a bias voltage has been switched on. The source-to-drain current exhibits the expected relaxation towards its quasistatic equilibrium value at a rate 0 reflecting the lead-induced broadening of the ring states. In contrast, the current circulating within the ring decays with a different rate , which is a rapidly decaying function of the interaction strength and thus can take values orders of magnitude below 0 . This implies the existence of a regime in which the nanostructure is far from equilibrium even though the transmitted current is already stationary. We discuss experimental setups to observe the long-lived ring transients.