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D. Belitz

Publications and source records attributed to D. Belitz.

At least 73 records · Page 4Linked 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

Criticality in inhomogeneous magnetic systems: Application to quantum ferromagnets

We consider a $ϕ^4$-theory with a position-dependent distance from the critical point. One realization of this model is a classical ferromagnet subject to non-uniform mechanical stress. We find a sharp phase transition where the envelope of the local magnetization vanishes uniformly. The first-order transition in a quantum ferromagnet also remains sharp. The universal mechanism leading to a tricritical point in an itinerant quantum ferromagnet is suppressed, and in principle one can recover a quantum critical point with mean-field exponents. Observable consequences of these results are discussed.

cond-mat.stat-mech↗
arXiv2007

Elasticity and melting of skyrmion flux lattices in p-wave superconductors

We analytically calculate the energy, magnetization curves (B(H)), and elasticity of skyrmion flux lattices in p-wave superconductors near the lower critical field H_c1, and use these results with the Lindemann criterion to predict their melting curve. In striking contrast to vortex flux lattices, which always melt at an external field H > H_c1, skyrmion flux lattices never melt near H_c1. This provides a simple and unambiguous test for the presence of skyrmions.

cond-mat.supr-con↗
arXiv2007

Phase-ordering dynamics in itinerant quantum ferromagnets

The phase-ordering dynamics that result from domain coarsening are considered for itinerant quantum ferromagnets. The fluctuation effects that invalidate the Hertz theory of the quantum phase transition also affect the phase ordering. For a quench into the ordered phase a transient regime appears, where the domain growth follows a different power law than in the classical case, and for asymptotically long times the prefactor of the t^{1/2} growth law has an anomalous magnetization dependence. A quench to the quantum critical point results in a growth law that is not a power-law function of time. Both phenomenological scaling arguments and renormalization-group arguments are given to derive these results, and estimates of experimentally relevant length and time scales are presented.

cond-mat.stat-mech↗
arXiv2006

Breakdown of Hydrodynamic Transport Theory in the Ordered Phase of Helimagnets

It is shown that strong fluctuations preclude a hydrodynamic description of transport phenomena in helimagnets, such as MnSi, at T>0. This breakdown of hydrodynamics is analogous to the one in chiral liquid crystals. Mode-mode coupling effects lead to infinite renormalizations of various transport coefficients, and the actual macroscopic description is nonlocal. At T=0 these effects are weakened due to the fluctuation-dissipation theorem, and the renormalizations remain finite. Observable consequences of these results, as manifested in the neutron scattering cross-section, are discussed

cond-mat.stat-mech↗
arXiv2006

Nearly Ferromagnetic Superconductors

The electromagnetic properties of superconductors near a ferromagnetic instability are investigated by means of a generalized Ginzburg-Landau theory. It is found that the magnetic flux expulsion capability of the superconductor gets stronger, in a well-defined sense, as the normal-state magnetic susceptibility increases. The temperature dependencies of the London penetration depth, the critical fields, and the critical current are all strongly affected by ferromagnetic fluctuations. In particular, for the critical current we find a temperature exponent alpha ~ 2 over an appreciable temperature range. The extent to which proximity to magnetic criticality may be a viable explanation for recent observations in MgCNi microfibers, which find alpha ~ 2, is discussed.

cond-mat.supr-con↗
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↗
arXiv2005

Nonanalytic Corrections to Fermi-Liquid Behavior in Helimagnets

The Goldstone mode in the ordered phase of itinerant helimagnets, such as MnSi or FeGe, is determined and shown to have a strongly anisotropic dispersion relation. The softness of this mode is, in a well-defined sense, in between that of ferromagnetic and antiferromagnetic magnons, respectively. It is shown that this soft mode leads to nonanalytic corrections to Fermi-liquid behavior, with a T-contribution to the specific heat coefficient, and a T^{5/2}-contribution to the resistivity. The quasi-particle inelastic lifetime shows anisotropic behavior in momentum space.

cond-mat.str-el↗
arXiv2005

Blue Quantum Fog: Chiral Condensation in Quantum Helimagnets

It is shown that a condensation transition involving a chiral order parameter can occur in itinerant helimagnets, in analogy to the transition between the isotropic phase and the phase known as blue fog or blue phase III in cholesteric liquid crystals. It is proposed that such a transition is the explanation for recent neutron scattering results in MnSi. Predictions are made that will allow to experimentally test this proposal.

cond-mat.str-el↗
arXiv2004

Influence of Generic Scale Invariance on Classical and Quantum Phase Transitions

This review discusses a paradigm that has become of increasing importance in the theory of quantum phase transitions; namely, the coupling of the order-parameter fluctuations to other soft modes, and the resulting impossibility of constructing a simple Landau-Ginzburg-Wilson theory in terms of the order parameter only. The soft modes in question are manifestations of generic scale invariance, i.e., the appearance of long-range order in whole regions in the phase diagram. The concept of generic scale invariance, and its influence on critical behavior, is explained using various examples, both classical and quantum mechanical. The peculiarities of quantum phase transitions are discussed, with emphasis on the fact that they are more susceptible to the effects of generic scale invariance than their classical counterparts. Explicit examples include: the quantum ferromagnetic transition in metals, with or without quenched disorder; the metal-superconductor transition at zero temperature; and the quantum antiferromagnetic transition. Analogies with classical phase transitions in liquid crystals and classical fluids are pointed out, and a unifying conceptual framework is developed for all transitions that are influenced by generic scale invariance.

cond-mat.stat-mech↗
arXiv2004

Tricritical behavior in itinerant quantum ferromagnets

It is shown that the peculiar features observed in the low-temperature phase diagrams of ZrZn_2, UGe_2, and MnSi can be understood in terms of a simple mean-field theory. The nature of the ferromagnetic transition changes from second order to first order at a tricritical point, and in a small external magnetic field surfaces of first-order transitions emerge which terminate in quantum critical points. This field dependence of the phase diagram follows directly from the existence of the tricritical point. The quantum critical behavior in a nonzero field is calculated exactly.

cond-mat.stat-mech↗
arXiv2004

Breakdown of the perturbative renormalization group at certain quantum critical points

It is shown that the presence of multiple time scales at a quantum critical point can lead to a breakdown of the loop expansion for critical exponents, since coefficients in the expansion diverge. Consequently, results obtained from finite-order perturbative renormalization-group treatments may be not be an approximation in any sense to the true asymptotic critical behavior. This problem manifests itself as a non-renormalizable field theory, or, equivalently, as the presence of a dangerous irrelevant variable. The quantum ferromagnetic transition in disordered metals provides an example.

cond-mat.stat-mech↗
arXiv2004

Softer than normal, but not as soft as one might think: Spontaneous flux lattices in ferromagnetic spin-triplet superconductors

A theory is developed for the spontaneous vortex lattice that is expected to occur in the ferromagnetic superconductors ZrZn_2, UGe_2, and URhGe, where the superconductivity is likely of spin-triplet nature. The long-wavelength fluctuations of this spontaneous flux lattice are predicted to be huge compared to those of a conventional flux lattice, and to be the same as those for spin-singlet ferromagnetic superconductors. It is shown that these fluctuations lead to unambiguous experimental signatures which may provide the easiest way to observe the spontaneous flux lattice.

cond-mat.supr-con↗
arXiv2004

Electron localization in an external electric field

The impact of a weak electric field on the weak-localization corrections is studied within the framework of a nonlinear sigma-model. Two scaling regimes are obtained. In one, the scaling is dominated by temperature; in the other, by the electric field. An explicit expression is derived for the crossover temperature between the two regimes.

cond-mat.stat-mech↗
arXiv2003

Split transition in ferromagnetic superconductors

The split superconducting transition of up-spin and down-spin electrons on the background of ferromagnetism is studied within the framework of a recent model that describes the coexistence of ferromagnetism and superconductivity induced by magnetic fluctuations. It is shown that one generically expects the two transitions to be close to one another. This conclusion is discussed in relation to experimental results on URhGe. It is also shown that the magnetic Goldstone modes acquire an interesting structure in the superconducting phase, which can be used as an experimental tool to probe the origin of the superconductivity.

cond-mat.supr-con↗
arXiv2003

Localization in an external electric field

The impact of an electric field on the electron localization problem is studied within the framework of a field-theoretic formulation. The investigation shows that the impact of the electric field on the localization corrections is governed by the interplay between two time scales, one set by the electric field, and the other by the phase relaxation rate. At very low temperatures the scaling of the conductivity is governed by the electric field. In this regime the conductivity depends logarithmically on the field, and an arbitrarily small electric field delocalizes the electron states. At higher temperatures the behavior of the conductivity is governed by the temperature scaling. In this regime the field has no impact on the observable leading localization corrections.

cond-mat.stat-mech↗