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A Rosch

Publications and source records attributed to A Rosch.

13 recordsLinked to original sources
PubMed2006

Scaling of the low-temperature dephasing rate in Kondo systems.

We present phase coherence time measurements in quasi-one-dimensional Ag wires doped with Fe Kondo impurities of different concentrations n_{s}. Because of the relatively high Kondo temperature T_{K} approximately 4.3 K of this system, we are able to explore a temperature range from above T_{K} down to below 0.01T_{K}. We show that the magnetic contribution to the dephasing rate gamma_{m} per impurity is described by a single, universal curve when plotted as a function of T/T_{K}. For T>0.1T_{K}, the dephasing rate is remarkably well described by recent numerical results for spin S=1/2 impurities. At lower temperature, we observe deviations from this theory. Based on a comparison with theoretical calculations for S>1/2, we discuss possible explanations for the observed deviations.

Journal Article↗
PubMed2006

Universal dephasing rate due to diluted Kondo impurities.

We calculate the dephasing rate due to magnetic impurities in a weakly disordered metal as measured in a weak-localization experiment. If the density nS of magnetic impurities is sufficiently low, the dephasing rate 1/tauphi is a universal function, 1/tauphi=(nS/nu)f(T/TK), where TK is the Kondo temperature and nu is the density of states. We show that inelastic vertex corrections with a typical energy transfer DeltaE are suppressed by powers of 1/(tauphiDeltaE) proportional to nS. Therefore, the dephasing rate can be calculated from the inelastic cross section proportional to pinu ImT-/pinuT/2, where T is the T matrix which is evaluated numerically exactly using the numerical renormalization group.

Journal Article↗
PubMed2006

Transport in almost integrable models: perturbed Heisenberg chains.

The heat conductivity kappa(T) of integrable models, like the one-dimensional spin-1/2 nearest-neighbor Heisenberg model, is infinite even at finite temperatures as a consequence of the conservation laws associated with integrability. Small perturbations lead to finite but large transport coefficients which we calculate perturbatively using exact diagonalization and moment expansions. We show that there are two different classes of perturbations. While an interchain coupling of strength J(perpendicular) leads to kappa(T) proportional to 1/J(perpendicular)2 as expected from simple golden-rule arguments, we obtain a much larger kappa(T) proportional to 1/J'4 for a weak next-nearest-neighbor interaction J'. This can be explained by a new approximate conservation law of the J-J' Heisenberg chain.

Journal Article↗
PubMed2004

Climbing the entropy barrier: driving the single- towards the multichannel Kondo effect by a weak coulomb blockade of the leads.

We study a model proposed recently in which a small quantum dot is coupled symmetrically to several large quantum dots characterized by a charging energy E(c). Even if E(c) is much smaller than the Kondo temperature T(K), the long-ranged interactions destabilize the single-channel Kondo effect and induce a flow towards a multichannel Kondo fixed point associated with a rise of the impurity entropy with decreasing temperature. Such an "uphill flow" implies a negative impurity specific heat, in contrast with all systems with local interactions. An exact solution found for a large number of channels allows us to capture this physics and to predict transport properties.

Journal Article↗
PubMed2004

Relaxation of hot quasiparticles in a d-wave superconductor.

Motivated by recent pump-probe experiments we consider the processes by which "hot" quasiparticles produced near the antinodes of a d-wave superconductor can relax. We show that in a large region of momentum space processes which break Cooper pairs are forbidden by energy and momentum conservation. Equilibration then occurs by scattering with thermal quasiparticles: Umklapp scattering is exponentially suppressed at low temperatures, but small-angle scattering leads to power-law behavior. By solving the Boltzmann equation analytically we make detailed predictions for the temperature and intensity dependence of these processes, which we compare with experiment.

Journal Article↗
PubMed2004

Partial order in the non-Fermi-liquid phase of MnSi.

Only a few metallic phases have been identified in pure crystalline materials. These include normal, ferromagnetic and antiferromagnetic metals, systems with spin and charge density wave order, and superconductors. Fermi-liquid theory provides a basis for the description of all of these phases. It has been suggested that non-Fermi-liquid phases of metals may exist in some heavy-fermion compounds and oxide materials, but the discovery of a characteristic microscopic signature of such phases presents a major challenge. The transition-metal compound MnSi above a certain pressure (p(c) = 14.6 kbar) provides what may be the cleanest example of an extended non-Fermi-liquid phase in a three-dimensional metal. The bulk properties of MnSi suggest that long-range magnetic order is suppressed at p(c) (refs 7-12). Here we report neutron diffraction measurements of MnSi, revealing that sizeable quasi-static magnetic moments survive far into the non-Fermi-liquid phase. These moments are organized in an unusual pattern with partial long-range order. Our observation supports the existence of novel metallic phases with partial ordering of the conduction electrons (reminiscent of liquid crystals), as proposed for the high-temperature superconductors and heavy-fermion compounds.

Journal Article↗
PubMed2003

Nonequilibrium transport through a Kondo dot in a magnetic field: perturbation theory and poor man's scaling.

We consider electron transport through a quantum dot described by the Kondo model in the regime of large transport voltage V in the presence of a magnetic field B with max((V,B)>>T(K). The electric current I and the local magnetization M are found to be universal functions of V/T(K) and B/T(K), where T(K) is the equilibrium Kondo temperature. We present a generalization of the perturbative renormali-zation group to frequency dependent coupling functions, as necessitated by the structure of bare perturbation theory. We calculate I and M within a poor man's scaling approach and find excellent agreement with experiment.

Journal Article↗
PubMed2002

Pseudogaps in an incoherent metal.

How are the properties of a metal changed by strong inelastic scattering? We investigate this question within the two-dimensional t-J model using extended dynamical mean field theory and a generalized noncrossing approximation. Short-ranged antiferromagnetic fluctuations lead to a strongly incoherent single particle dynamics, large entropy, and resistance. Close to the Mott transition at low hole doping a pseudogap opens, accompanied by a drop in resistivity and an increase in the Hall constant for both lower temperatures T and doping levels. The behavior obtained bears surprising similarity to properties of the cuprates.

Journal Article↗
PubMed2001

Kondo effect in quantum dots at high voltage: universality and scaling.

We examine the properties of a dc-biased quantum dot in the Coulomb blockade regime. For voltages V that are large compared to the Kondo temperature T(K), the physics is governed by the scales V and gamma, where gamma approximately V/ln(2)(V/T(K)) is the nonequilibrium decoherence rate induced by the voltage-driven current. Based on scaling arguments, self-consistent perturbation theory, and perturbative renormalization group, we argue that due to the large gamma the system can be described by renormalized perturbation theory in 1/ln(V/T(K))<<1. However, in certain variants of the Kondo problem, two-channel Kondo physics is induced by a large voltage V.

Journal Article↗
PubMed2000

Conductivity of a clean one-dimensional wire

We study the low-temperature low-frequency conductivity sigma of an interacting one-dimensional electron system in the presence of a periodic potential. The conductivity is strongly influenced by conservation laws, which, we argue, need to be violated by at least two noncommuting umklapp processes to render sigma finite. The resulting dynamics of the slow modes is studied within a memory matrix approach, and we find an exponential increase as the temperature is lowered, sigma approximately (Deltan)(2)e(T0/(NT)) close to commensurate filling M/N, Deltan = n-M/N<<1, and sigma approximately e((T(')(0)/T)(2/3)) elsewhere.

Journal Article↗
PubMed1995

Populations with constant immigration.

"We consider a Leslie-type model of a one-sex (female) population of natives with constant immigration. The fertility and mortality schedule of the natives may be below or above replacement level. Immigrants retain their fertility and mortality, their children adopt the fertility and mortality of the natives. It is shown how this model may be written in a homogeneous form (without additive term) with a Leslie-type matrix. Reproductive values of individuals in each age group are discussed in terms of a left eigenvector of this matrix. The homogeneous form of our projection model permits the transformation into a Markov chain with transient and recurrent states. The Markov chain is the basis for the definition of genealogies, which incorporate immigration. It is shown that genealogies describe the life histories of individuals in a population with immigration. We calculate absorption times of the Markov chain and relate them to genealogies. This extends the theory originally designed for closed populations to populations with immigration." (SUMMARY IN FRE)

Demography↗
PubMed1993

[The biofragmentable Valtrac ring--a reliable concept for enteral anastomosis?].

In a retrospective investigation, 54 patients with 55 enteric anastomoses performed with the new biofragmentable Valtrac ring were examined. In 45 patients with disease of the colon partial resection of the large bowel had been performed. In 9 patients undergoing urinary diversion an end-to-end ileostomy was performed. The most important advantage of this device is that its use is achieved by way of a safe and quick technique which is easy to learn. Bowel stenosis, bleeding and other complications independent of anastomotic technique, such as wound infection, were no more frequent than after conventional suture anastomosis. In 3 patients an anastomotic leakage appeared. In 1 of these cases conservative management for a colocutaneous fistula was successful, but the other 2 patients had to undergo a second operation. In our opinion, the Valtrac system offers a convincing and reproducible alternative to conventional anastomotic techniques and it is easy to learn how to apply it.

Adult↗