arXiv subjects

Stefan Scheidl

Publications and source records attributed to Stefan Scheidl.

At least 19 recordsLinked to original sources

arXiv2006

Frustrated Heisenberg antiferromagnets: fluctuation induced first order vs deconfined quantum criticality

Recently it was argued that quantum phase transitions can be radically different from classical phase transitions with as a highlight the 'deconfined critical points' exhibiting fractionalization of quantum numbers due to Berry phase effects. Such transitions are supposed to occur in frustrated ('$J_1$-$J_2$') quantum magnets. We have developed a novel renormalization approach for such systems which is fully respecting the underlying lattice structure. According to our findings, another profound phenomenon is around the corner: a fluctuation induced (order-out-of-disorder) first order transition. This has to occur for large spin and we conjecture that it is responsible for the weakly first order behavior recently observed in numerical simulations for frustrated $S=1/2$ systems.

cond-mat.str-el↗

arXiv2005

Quantum Phase Transitions of Frustrated Heisenberg Antiferromagnets: a Comprehensive Renormalization Approach

We develop a novel renormalization approach for frustrated quantum antiferromagnets. It is designed to consistently treat spin-wave interactions all over the magnetic Brillouin zone, including high-energy modes in outer regions as well as low energy modes in the center. Focusing on the paradigmatic $J_1$-$J_2$ model, we find a unifying description of the second-order transition between the Néel phase and the paramagnetic phase and the first-order transition between the Néel phase and the columnar phase. Our approach provides explicit results for the renormalized values of the spin stiffness and spin-wave velocity characterizing the low-energy magnons in the Néel phase.

cond-mat.other↗

arXiv2004

The spin excitation spectrum in striped bilayer compounds

The spin dynamics of bilayer cuprate compounds are studied in a basic model. The magnetic spectral properties are calculated in linear spin-wave theory for several stripe configurations which differ by the relative location of the stripes in the layers. We focus on the bilayer splitting of the magnon bands near the incommensurate low energy peaks as well as near the $π$ resonance, distinguishing between the odd and even channel. We find that a x-shaped dispersion near the $π$ resonance is generic for stripes. By comparison of our results to neutron scattering data for $\mathrm{YBa_2Cu_3O_{6+x}}$ we conclude that the stripe model is consistent with characteristic features of bilayer high-$T_c$ compounds.

cond-mat.supr-con↗

arXiv2002

Spin dynamics of stripes

The spin dynamics of stripes in high-temperature superconductors and related compounds is studied in the framework of a spin-wave theory for a simple spin-only model. The magnon dispersion relation and the magnetic structure factor are calculated for diagonal and vertical stripes. Acoustical as well as optical bands are included in the analysis. The incommensurability and the $π$ resonance appear as complementary features of the band structure at different energy scales. The dependence of spin-wave velocities and resonance frequencies on the stripe spacing and coupling is calculated. At low doping, the resonance frequency is found to scale roughly inversely proportional to the stripe spacing. The favorable comparison of the results with experimental data suggests that the spin-only model provides a suitable and simple basis for calculating and understanding the spin dynamics of stripes.

cond-mat↗

arXiv2002

Delocalization in Coupled Luttinger Liquids with Impurities

We study effects of quenched disorder on coupled two-dimensional arrays of Luttinger liquids (LL) as a model for stripes in high-T_c compounds. In the framework of a renormalization-group analysis, we find that weak inter-LL charge-density-wave couplings are always irrelevant as opposed to the pure system. By varying either disorder strength, intra- or inter-LL interactions, the system can undergo a delocalization transition between an insulator and a novel strongly anisotropic metallic state with LL-like transport. This state is characterized by short-ranged charge-density-wave order, the superconducting order is quasi long-ranged along the stripes and short-ranged in the transversal direction.

cond-mat.dis-nn↗

arXiv2002

Non-universal ordering of spin and charge in stripe phases

We study the interplay of topological excitations in stripe phases: charge dislocations, charge loops, and spin vortices. In two dimensions these defects interact logarithmically on large distances. Using a renormalization-group analysis in the Coulomb gas representation of these defects, we calculate the phase diagram and the critical properties of the transitions. Depending on the interaction parameters, spin and charge order can disappear at a single transition or in a sequence of two transitions (spin-charge separation). These transitions are non-universal with continuously varying critical exponents. We also determine the nature of the points where three phases coexist.

cond-mat↗

arXiv2002

Quantum Brownian motion in ratchet potentials

We investigate the dynamics of quantum particles in a ratchet potential subject to an ac force field. We develop a perturbative approach for weak ratchet potentials and force fields. Within this approach, we obtain an analytic description of dc current rectification and current reversals. Transport characteristics for various limiting cases -- such as the classical limit, limit of high or low frequencies, and/or high temperatures -- are derived explicitly. To gain insight into the intricate dependence of the rectified current on the relevant parameters, we identify characteristic scales and obtain the response of the ratchet system in terms of scaling functions. We pay a special attention to inertial effects and show that they are often relevant, for example, at high temperatures. We find that the high temperature decay of the rectified current follows an algebraic law with a non-trivial exponent, $j\propto T^{-17/6}$.

cond-mat.stat-mech↗

arXiv2001

Bond-disordered spin systems: Theory and application to doped high-Tc compounds

We examine the stability of magnetic order in a classical Heisenberg model with quenched random exchange couplings. This system represents the spin degrees of freedom in high-$T_\textrm{c}$ compounds with immobile dopants. Starting from a replica representation of the nonlinear $σ$-model, we perform a renormalization-group analysis. The importance of cumulants of the disorder distribution to arbitrarily high orders necessitates a functional renormalization scheme. From the renormalization flow equations we determine the magnetic correlation length numerically as a function of the impurity concentration and of temperature. From our analysis follows that two-dimensional layers can be magnetically ordered for arbitrarily strong but sufficiently diluted defects. We further consider the dimensional crossover in a stack of weakly coupled layers. The resulting phase diagram is compared with experimental data for La$_{2-x}$Sr$_x$CuO$_4$.

cond-mat.dis-nn↗

arXiv2001

Chern-Simons Theory for Quantum Hall Stripes

We develop a Chern-Simons theory to describe a two-dimensional electron gas in intermediate magnetic fields. Within this approach, inhomogeneous states emerge in analogy to the intermediate state of a superconductor. At half filling of the highest Landau level we find unidirectional charge-density-wave (CDW) solutions. With a semiclassical calculation we give an intuitive explanation of the change of CDW orientation in the presence of an in-plane magnetic field. An anisotropy in the electron band mass is suggested as a possible source of the reproducible orientation of the CDW.

cond-mat.mes-hall↗

arXiv2001

Pinning of stripes in cuprate superconductors

We examine the effects of disorder on striped phases in high-temperature superconductors and related materials. In the presence of quenched disorder, pinning by the atomic lattice - which might give rise to commensuration effects - is irrelevant for the stripe array on large length scales. As a consequence, the stripes have divergent displacement fluctuations and topological defects are present at all temperatures. Therefore the positional order of the stripe array is short ranged, with a finite correlation length even at zero temperature. Thus lock-in phenomena can exist only as crossovers but not as transitions. In addition, this implies the glassy nature of stripes observed in recent experiments.

cond-mat.supr-con↗

arXiv2000

Elastic theory of quantum Hall smectics: effects of disorder

We study the effect of disorder on quantum Hall smectics within the framework of an elastic theory. Based on a renormalization group calculation, we derive detailed results for the degrees of translational and orientational order of the stripe pattern at zero temperature and carefully map out the disorder and length-scale regimes in which the system effectively exhibits smectic, nematic, or isotropic behavior. We show that disorder always leads to a finite density of free dislocations and estimate the scale on which they begin to appear.

cond-mat.str-el↗

arXiv2000

Interface fluctuations in disordered systems: Universality and non-Gaussian statistics

We employ a functional renormalization group to study interfaces in the presence of a pinning potential in $d=4-ε$ dimensions. In contrast to a previous approach [D.S. Fisher, Phys. Rev. Lett. {\bf 56}, 1964 (1986)] we use a soft-cutoff scheme. With the method developed here we confirm the value of the roughness exponent $ζ\approx 0.2083 ε$ in order $ε$. Going beyond previous work, we demonstrate that this exponent is universal. In addition, we analyze the generation of higher cumulants in the disorder distribution and the role of temperature as a dangerously irrelevant variable.

cond-mat.dis-nn↗

arXiv2000

Nonequilibrium dislocation dynamics and instability of driven vortex lattices in two dimensions

We consider dislocations in a vortex lattice that is driven in a two-dimensional superconductor with random impurities. The structure and dynamics of dislocations is studied in this genuine nonequilibrium situation on the basis of a coarse-grained equation of motion for the displacement field. The presence of dislocations leads to a characteristic anisotropic distortion of the vortex density that is controlled by a Kardar-Parisi-Zhang nonlinearity in the coarse-grained equation of motion. This nonlinearity also implies a screening of the interaction between dislocations and thereby an instability of the vortex lattice to the proliferation of free dislocations.

cond-mat.stat-mech↗

arXiv1998

XY models with disorder and symmetry-breaking fields in two dimensions

The combined effect of disorder and symmetry-breaking fields on the two-dimensional XY model is examined. The study includes disorder in the interaction among spins in the form of random phase shifts as well as disorder in the local orientation of the field. The phase diagrams are determined and the properties of the various phases and phase transitions are calculated. We use a renormalization group approach in the Coulomb gas representation of the model. Our results differ from those obtained for special cases in previous works. In particular, we find a changed topology of the phase diagram that is composed of phases with long-range order, quasi-long-range order, and short-range order. The discrepancies can be ascribed to a breakdown of the fugacity expansion in the Coulomb gas representation. Implications for physical systems such as planar Josephson junctions and the faceting of crystal surfaces are discussed.

cond-mat.dis-nn↗

arXiv1997

Dynamic Melting and Decoupling of the Vortex Lattice in Layered Superconductors

The dynamic phase diagram of vortex lattices driven in disorder is calculated in two and three dimensions. A modified Lindemann criterion for the fluctuations of the distance of neighboring vortices is used, which unifies previous analytic approaches to the equilibrium and non-equilibrium phase transitions. The temperature shifts of the dynamic melting and decoupling transitions are found to scale inversely proportional to large driving currents. A comparison with two-dimensional simulations shows that this phenomenological approach can provide quantitative estimate for the location of these transitions.

cond-mat.supr-con↗

arXiv1997

Charge Modulation at the Surface of High-T_c Superconductors

It is shown here that surfaces of high-temperature superconductors are covered by dipole layers. The charge density modulation is induced by the local suppression of the gap function at the surface. This effect is studied in the framework of the Ginzburg-Landau theory and crucially depends on the appropriate boundary conditions. Those are derived from Gor'kov's equations for a d-wave pairing symmetry. Within this framework the structure of the surface dipole layer is determined. The contribution of this charging to a lens-effect of superconducting films with holes, which has been studied in recent experiments, is discussed.

cond-mat.supr-con↗

arXiv1997

Driven Dynamics of Periodic Elastic Media in Disorder

We analyze the large-scale dynamics of vortex lattices and charge density waves driven in a disordered potential. Using a perturbative coarse-graining procedure we present an explicit derivation of non-equilibrium terms in the renormalized equation of motion, in particular Kardar-Parisi-Zhang non-linearities and dynamic strain terms. We demonstrate the absence of glassy features like diverging linear friction coefficients and transverse critical currents in the drifting state. We discuss the structure of the dynamical phase diagram containing different elastic phases very small and very large drive and plastic phases at intermediate velocity.

cond-mat.stat-mech↗

arXiv1997

Gliding dislocations in a driven vortex lattice

The dynamics of dislocations in a two-dimensional vortex lattice is studied in the presence of a pinning potential and a transport current. In a vortex lattice drifting with velocity $v$ a glide velocity $V_d$ of the dislocation with respect to the vortex lattice is found to decay like $V_d \sim v^{-4}$ for large drive. From this result the velocity for the crossover between a regime of coherent elastic motion and a regime of incoherent plastic motion of vortices is estimated.

cond-mat.supr-con↗