arXiv subjects

A. Rosch

Publications and source records attributed to A. Rosch.

At least 55 records · Page 3Linked to original sources

arXiv2007

Theory of helimagnons in itinerant quantum systems II: Nonanalytic corrections to Fermi-liquid behavior

A recent theory for the ordered phase of helical or chiral magnets such as MnSi is used to calculate observable consequences of the helical Goldstone modes or helimagnons. In systems with no quenched disorder, the helimagnon contributions to the specific heat coefficient is shown to have a linear temperature dependence, while the quasi-particle inelastic scattering rate is anisotropic in momentum space and depends on the electronic dispersion relation. For cubic lattices the generic temperature dependence is given by a non-Fermi-liquid T^(3/2) behavior. The contribution to the temperature dependence of the resistivity is shown to be T^(5/2) in a Boltzmann approximation. The helimagnon thus leads to nonanalytic corrections to Fermi-liquid behavior. Implications for experiments, and for transport theories beyond the Boltzmann level, are discussed.

cond-mat.str-el↗

arXiv2007

Fragility of String Orders

One-dimensional gapped systems are often characterized by a 'hidden' non-local order parameter, the so-called string order. Due to the gap, thermodynamic properties are robust against a weak higher-dimensional coupling between such chains or ladders. To the contrary, we find that the string order is not stable and decays for arbitrary weak inter-chain or inter-ladder coupling. We investigate the vanishing of the order for three different systems: spin-one Haldane chains, band insulators, and the transverse-field Ising model. Using perturbation theory and bosonization, we show that the fragility of the string order arises from non-local commutation relations between the non-local order parameter and the perturbation.

cond-mat.str-el↗

arXiv2007

String order and adiabatic continuity of Haldane chains and band insulators

The ground state of spin-1 Haldane chains is characterized by the so-called string order. We show that the same hidden order is also present in ordinary one-dimensional band insulators. We construct a family of Hamiltonians which connects adiabatically band insulators to two topologically non-equivalent spin models, the Haldane chain and the antiferromagnetic spin-1/2 ladder. We observe that the localized spin-1/2 edge-state characteristic of spin-1 chains is smoothly connected to a surface-bound state of band insulators and its emergence is not related to any bulk phase transition. Furthermore, we show that the string order is absent in any dimensions higher than one.

cond-mat.str-el↗

arXiv2007

Magnetothermal transport in the spin-1/2 chains of copper pyrazine dinitrate

We present experiments on the thermal transport in the spin-1/2 chain compound copper pyrazine dinitrate Cu(C_4 H_4 N_2)(NO_3)_2. The heat conductivity shows a surprisingly strong dependence on the applied magnetic field B, characterized at low temperatures by two main features. The first one appearing at low B is a characteristic dip located at mu_B B ~ k_B T, that may arise from Umklapp scattering. The second one is a plateau-like feature in the quantum critical regime, mu_B |B-B_c| < k_B T, where B_c is the saturation field at T=0. The latter feature clearly points towards a momentum and field independent mean free path of the spin excitations, contrary to theoretical expectations.

cond-mat.str-el↗

arXiv2007

Magnetic Field Dependence of Dephasing Rate due to Diluted Kondo Impurities

We investigate the dephasing rate, 1/tau_phi, of weakly disordered electrons due to scattering from diluted dynamical impurities. Our previous result for the weak-localization dephasing rate is generalized from diluted Kondo impurities to arbitrary dynamical defects with typical energy transfer larger than 1/tau_phi. For magnetic impurities, we study the influence of magnetic fields on the dephasing of Aharonov-Bohm oscillations and universal conductance fluctuations both analytically and using the numerical renormalization group. These results are compared to recent experiments.

cond-mat.mes-hall↗

arXiv2006

Fermi liquid near a quantum critical point

We investigate the approach to the quantum critical point of a Pomeranchuk instability from the symmetric, disordered side of the phase diagram. In the low-temperature limit, a Fermi liquid description of the metal is possible and becomes exact for T -> 0. We discuss in detail which features of the approach to quantum criticality can be captured within Fermi liquid theory and which are outside of its scope.

cond-mat.str-el↗

arXiv2006

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 $n_s$. Due to the relatively high Kondo temperature $T_{K}\approx 4.3K$ of this system, we are able to explore a temperature range from above $T_{K}$ down to below $0.01 T_{K}$. We show that the magnetic contribution to the dephasing rate $γ_m$ per impurity is described by a single, universal curve when plotted as a function of $(T/T_K)$. For $T>0.1 T_K$, 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.

cond-mat.mes-hall↗

arXiv2006

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 n_S of magnetic impurities is sufficiently low, the dephasing rate 1/tau_phi is a universal function, 1/tau_phi = (n_S/nu) f(T/T_K), where T_K is the Kondo temperature and nu the density of states. We show that inelastic vertex corrections with a typical energy transfer Delta E are suppressed by powers of 1/(tau_phi Delta E) proportional to n_S. Therefore the dephasing rate can be calculated from the inelastic cross section proportional to pi nu Im T- |pi nu T|^2, where T is the T-matrix which is evaluated numerically exactly using the numerical renormalization group.

cond-mat.mes-hall↗

arXiv2006

Theory of helimagnons in itinerant quantum systems

The nature and effects of the Goldstone mode in the ordered phase of helical or chiral itinerant magnets such as MnSi are investigated theoretically. It is shown that the Goldstone mode, or helimagnon, is a propagating mode with a highly anisotropic dispersion relation, in analogy to the Goldstone mode in chiral liquid crystals. Starting from a microscopic theory, a comprehensive effective theory is developed that allows for an explicit description of the helically ordered phase, including the helimagnons, for both classical and quantum helimagnets. The directly observable dynamical spin susceptibility, which reflects the properties of the helimagnon, is calculated.

cond-mat.str-el↗

arXiv2006

Nonequilibrium Singlet-Triplet Kondo Effect in Carbon Nanotubes

The Kondo-effect is a many-body phenomenon arising due to conduction electrons scattering off a localized spin. Coherent spin-flip scattering off such a quantum impurity correlates the conduction electrons and at low temperature this leads to a zero-bias conductance anomaly. This has become a common signature in bias-spectroscopy of single-electron transistors, observed in GaAs quantum dots as well as in various single-molecule transistors. While the zero-bias Kondo effect is well established it remains uncertain to what extent Kondo correlations persist in non-equilibrium situations where inelastic processes induce decoherence. Here we report on a pronounced conductance peak observed at finite bias-voltage in a carbon nanotube quantum dot in the spin singlet ground state. We explain this finite-bias conductance anomaly by a nonequilibrium Kondo-effect involving excitations into a spin triplet state. Excellent agreement between calculated and measured nonlinear conductance is obtained, thus strongly supporting the correlated nature of this nonequilibrium resonance.

cond-mat.str-el↗

arXiv2006

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_perp leads to kappa(T) propto 1/J_perp^2 as expected from simple golden-rule arguments, we obtain a much larger kappa(T) propto 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.

cond-mat.str-el↗

arXiv2005

Optical Conductivity of Clean Metals

We briefly review some basic aspects of transport in clean metals focusing on the role of electron-electron interactions and neglecting the effects of impurities, phonons and interband transitions. Both for small Fermi surfaces of two and three-dimensional metals and open Fermi surfaces of quasi one-dimensional metals the dc conductivity sigma is largely dominated by momentum and pseudo-momentum conservation, respectively. In general, the frequency and temperature dependencies of sigma(w,T) have very little in common. For small Fermi surfaces in three dimensions we find for example that the scattering rate is quadratic in frequency, Gamma \propto w^2, even in the absence of a T^2 contribution.

cond-mat.str-el↗

arXiv2005

Non-equilibrium conductance of a three-terminal quantum dot in the Kondo regime: Perturbative Renormalization Group

Motivated by recent experiments, we consider a single-electron transistor in the Kondo regime which is coupled to three leads in the presence of large bias voltages. Such a steady-state non-equilibrium system is to a large extent governed by a decoherence rate induced by the current through the dot. As the two-terminal conductance turns out to be rather insensitive to the decoherence rate, we study the conductance in a three-terminal device using perturbative renormalization group and calculate the characteristic splitting of the Kondo resonance. The interplay between potential biases and anisotropy in coupling to the three leads determines the decoherence rate and the conditions for strong coupling.

cond-mat.mes-hall↗

arXiv2005

Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors

We calculate the low-frequency optical conductivity sigma(w) of clean metals and superconductors at zero temperature neglecting the effects of impurities and phonons. In general, the frequency and temperature dependences of sigma have very little in common. For small Fermi surfaces in three dimensions (but not in 2D) we find for example that Re sigma(w>0)=const. for low w which corresponds to a scattering rate Gamma proportional to w^2 even in the absence of Umklapp scattering when there is no T^2 contribution to Gamma. In the main part of the paper we discuss in detail the optical conductivity of d-wave superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest frequencies and the Umklapp processes typically set in smoothly above a finite threshold w_0 smaller than twice the maximal gap Delta. In cases where the nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among them is possible, one obtains Re sigma(w) \propto w^2.

cond-mat.str-el↗

arXiv2004

Field-tuned quantum critical point of antiferromagnetic metals

A magnetic field applied to a three-dimensional antiferromagnetic metal can destroy the long-range order and thereby induce a quantum critical point. Such field-induced quantum critical behavior is the focus of many recent experiments. We investigate theoretically the quantum critical behavior of clean antiferromagnetic metals subject to a static, spatially uniform external magnetic field. The external field does not only suppress (or induce in some systems) antiferromagnetism but also influences the dynamics of the order parameter by inducing spin precession. This leads to an exactly marginal correction to spin-fluctuation theory. We investigate how the interplay of precession and damping determines the specific heat, magnetization, magnetocaloric effect, susceptibility and scattering rates. We point out that the precession can change the sign of the leading \sqrt{T} correction to the specific heat coefficient c(T)/T and can induce a characteristic maximum in c(T)/T for certain parameters. We argue that the susceptibility χ=\partial M/\partial B is the thermodynamic quantity which shows the most significant change upon approaching the quantum critical point and which gives experimental access to the (dangerously irrelevant) spin-spin interactions.

cond-mat.str-el↗

arXiv2004

The Kondo Effect in Non-Equilibrium Quantum Dots: Perturbative Renormalization Group

While the properties of the Kondo model in equilibrium are very well understood, much less is known for Kondo systems out of equilibrium. We study the properties of a quantum dot in the Kondo regime, when a large bias voltage V and/or a large magnetic field B is applied. Using the perturbative renormalization group generalized to stationary nonequilibrium situations, we calculate renormalized couplings, keeping their important energy dependence. We show that in a magnetic field the spin occupation of the quantum dot is non-thermal, being controlled by V and B in a complex way to be calculated by solving a quantum Boltzmann equation. We find that the well-known suppression of the Kondo effect at finite V>>T_K (Kondo temperature) is caused by inelastic dephasing processes induced by the current through the dot. We calculate the corresponding decoherence rate, which serves to cut off the RG flow usually well inside the perturbative regime (with possible exceptions). As a consequence, the differential conductance, the local magnetization, the spin relaxation rates and the local spectral function may be calculated for large V,B >> T_K in a controlled way.

cond-mat.str-el↗

arXiv2004

Weak spin-orbit interactions induce exponentially flat mini-bands in magnetic metals without inversion symmetry

In metallic magnets like MnSi the interplay of two very weak spin-orbit coupling effects can strongly modify the Fermi surface. In the absence of inversion symmetry even a very small Dzyaloshinsky-Moriya interaction of strength delta<<1 distorts a ferromagnetic state into a chiral helix with a long pitch of order 1/delta. We show that additional small spin-orbit coupling terms of order delta in the band structure lead to the formation of exponentially flat minibands with a bandwidth of order exp(-1/sqrt(delta)) parallel to the direction of the helix. These flat minibands cover a rather broad belt of width sqrt(delta) on the Fermi surface where electron motion parallel to the helix practically stops. We argue that these peculiar band-structure effects lead to pronounced features in the anomalous skin effect.

cond-mat.str-el↗

arXiv2004

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 multi-channel 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 to 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.

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