Logarithmic divergence of the block entanglement entropy for the ferromagnetic Heisenberg model

Recent studies have shown that logarithmic divergence of entanglement entropy as function of size of a subsystem is a signature of criticality in quantum models. We demonstrate that the ground state entanglement entropy of $n$ sites for ferromagnetic Heisenberg spin-1/2 chain of the length $L$ in a sector with fixed magnetization $y$ per site grows as ${1/2}\log_{2} \frac{n(L-n)}{L}C(y)$, where $C(y)=2\pi e({1/4}-y^{2})$Comment: 4 pages, 2 fig

Manipulating energy and spin currents in nonequilibrium systems of interacting qubits

We consider generic interacting chain of qubits, which are coupled at the edges to baths of fixed polarizations. We can determine the nonequilibrium steady states, described by the fixed point of the Lindblad Master Equation. Under rather general assumptions about local pumping and interactions, symmetries of the reduced density matrix are revealed. The symmetries drastically restrict the form of the steady density matrices in such a way that an exponentially large subset of one--point and many--point correlation functions are found to vanish. As an example we show how in a Heisenberg spin chain a suitable choice of the baths can completely switch off either the spin or the energy current, or both of them, despite the presence of large boundary gradients.Comment: 8 pages, 3 Figure

Obtaining pure steady states in nonequilibrium quantum systems with strong dissipative couplings

Dissipative preparation of a pure steady state usually involves a commutative action of a coherent and a dissipative dynamics on the target state. Namely, the target pure state is an eigenstate of both the coherent and dissipative parts of the dynamics. We show that working in the Zeno regime, i.e. for infinitely large dissipative coupling, one can generate a pure state by a non commutative action, in the above sense, of the coherent and dissipative dynamics. A corresponding Zeno regime pureness criterion is derived. We illustrate the approach, looking at both its theoretical and applicative aspects, in the example case of an open $XXZ$ spin-$1/2$ chain, driven out of equilibrium by boundary reservoirs targeting different spin orientations. Using our criterion, we find two families of pure nonequilibrium steady states, in the Zeno regime, and calculate the dissipative strengths effectively needed to generate steady states which are almost indistinguishable from the target pure states.Comment: 8 pages, 6 figure

Infinitely dimensional Lax structure for one-dimensional Hubbard model

We report a two-parametric irreducible infinitely dimensional representation of the Lax integrability condition for the fermi Hubbard chain. Besides being of fundamental interest, hinting on possible novel quantum symmetry of the model, our construction allows for an explicit representation of an exact steady state many-body density operator for non-equilibrium boundary-driven Hubbard chain with arbitrary (asymmetric) particle source/sink rates at the letf/right end of the chain and with arbitrary boundary values of chemical potentials.Comment: 5 pages in RevTex, 1 figure, version as accepted by Phys. Rev. Let

Solution of the Lindblad equation for spin helix states

Using Lindblad dynamics we study quantum spin systems with dissipative boundary dynamics that generate a stationary nonequilibrium state with a non-vanishing spin current that is locally conserved except at the boundaries. We demonstrate that with suitably chosen boundary target states one can solve the many-body Lindblad equation exactly in any dimension. As solution we obtain pure states at any finite value of the dissipation strength and any system size. They are characterized by a helical stationary magnetization profile and a superdiffusive ballistic current of order one, independent of system size even when the quantum spin system is not integrable. These results are derived in explicit form for the one-dimensional spin-1/2 Heisenberg chain and its higher-spin generalizations (which include for spin-1 the integrable Zamolodchikov-Fateev model and the bi-quadratic Heisenberg chain). The extension of the results to higher dimensions is straightforward.Comment: 23 pages, 2 figure

Inhomogeneous MPA and exact steady states of boundary driven spin chains at large dissipation

We find novel site-dependent Lax operators in terms of which we demonstrate exact solvability of a dissipatively driven XYZ spin-1/2 chain in the Zeno limit of strong dissipation, with jump operators polarizing the boundary spins in arbitrary directions. We write the corresponding nonequilibrium steady state using an inhomogeneous MPA, where the constituent matrices satisfy a simple set of linear recurrence relations. Although these matrices can be embedded into an infinite-dimensional auxiliary space, we have verified that they cannot be simultaneously put into a tridiagonal form, not even in the case of axially symmetric (XXZ) bulk interactions and general nonlongitudinal boundary dissipation. We expect our results to have further fundamental applications for the construction of nonlocal integrals of motion for the open XYZ model with arbitrary boundary fields, or the eight-vertex model.Comment: 12 pages, 3 figure

Non-KPZ modes in two-species driven diffusive systems

Using mode coupling theory and dynamical Monte-Carlo simulations we investigate the scaling behaviour of the dynamical structure function of a two-species asymmetric simple exclusion process, consisting of two coupled single-lane asymmetric simple exclusion processes. We demonstrate the appearence of a superdiffusive mode with dynamical exponent $z=5/3$ in the density fluctuations, along with a KPZ mode with $z=3/2$ and argue that this phenomenon is generic for short-ranged driven diffusive systems with more than one conserved density. When the dynamics is symmetric under the interchange of the two lanes a diffusive mode with $z=2$ appears instead of the non-KPZ superdiffusive mode.Comment: 5 pages, 7 figure

Dynamic phase transitions in electromigration-induced step bunching

Electromigration-induced step bunching in the presence of sublimation or deposition is studied theoretically in the attachment-limited regime. We predict a phase transition as a function of the relative strength of kinetic asymmetry and step drift. For weak asymmetry the number of steps between bunches grows logarithmically with bunch size, whereas for strong asymmetry at most a single step crosses between two bunches. In the latter phase the emission and absorption of steps is a collective process which sets in only above a critical bunch size and/or step interaction strength.Comment: 4 pages, 4 figure