Electron self-energy near a nematic quantum critical point

We consider an isotropic Fermi liquid in two dimensions near the n=2 Pomeranchuk instability in the charge channel. The order parameter is a quadrupolar stress tensor with two polarizations, longitudinal and transverse to the quadrupolar momentum tensor. Longitudinal and transverse bosonic modes are characterized by dynamical exponents z_parallel=3 and z_perp=2, respectively. Previous studies have found that such a system exhibits multiscale quantum criticality with two different energy scales omega ~ xi^{-z_{parallel,perp}}, where xi is the correlation length. We study the impact of the multiple energy scales on the electron Green function. The interaction with the critical z_parallel =3 mode is known to give rise to a local self-energy that develops a non-Fermi liquid form, Sigma(omega) ~ omega^{2/3} for frequencies larger than the energy scale omega ~ xi^{-3}. We find that the exchange of transverse z_perp=2 fluctuations leads to a logarithmically singular renormalizations of the quasiparticle residue Z and the vertex Gamma. We derive and solve renormalization group equations for the flow of Z and Gamma and show that the system develops an anomalous dimension at the nematic quantum-critical point (QCP). As a result, the spectral function at a fixed omega and varying k has a non-Lorentzian form. Away from the QCP, we find that the flow of Z is cut at the energy scale omega_{FL} ~ xi^{-1}, associated with the z=1 dynamics of electrons. The z_perp=2 energy scale, omega ~ xi^{-2}, affects the flow of Z only if one includes into the theory self-interaction of transverse fluctuations.Comment: 14 pages, 10 figures; (v2) minor changes, published versio

History dependence of the magnetic properties of single-crystal Fe1−x_{1-x}Cox_{x}Si

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of Fe$_{1-x}$Co$_{x}$Si, $0.20 \leq x \leq 0.50$. We determine the magnetic phase diagrams for all major crystallographic directions and cooling histories. After zero-field cooling, the phase diagrams resemble that of the archetypal stoichiometric cubic chiral magnet MnSi. Besides the helical and conical state, we observe a pocket of skyrmion lattice phase just below the helimagnetic ordering temperature. At the phase boundaries between these states evidence for slow dynamics is observed. When the sample is cooled in small magnetic fields, the phase pocket of skyrmion lattice may persist metastably down to lowest temperatures. Taken together with the large variation of the transition temperatures, transition fields, and the helix wavelength as a function of composition, this hysteresis identifies Fe$_{1-x}$Co$_{x}$Si as an ideal material for future experiments exploring, for instance, the topological unwinding of the skyrmion lattice.Comment: 14 pages, 11 figure

Quantum criticality with multiple dynamics

Quantum critical systems with multiple dynamics possess not only one but several time scales, tau_i ~ xi^(z_i), which diverge with the correlation length xi. We investigate how scaling predictions are modified for the simplest case of multiple dynamics characterized by two dynamical critical exponents, z_> and z_<. We argue that one should distinguish the case of coupled and decoupled multiple dynamic scaling depending on whether there exists a scaling exponent which depends on both z_i or not. As an example, we study generalized Phi^4-theories with multiple dynamics below their upper critical dimension, d+z_<<4. We identify under which condition coupled scaling is generated. In this case the interaction of quantum and classical fluctuations leads to an emergent dynamical exponent, z_e=z_>/(nu (z_>-z_<)+1).Comment: 8 pages including supplementary material. Minor changes as compared to the previous versio

Edge instabilities and skyrmion creation in magnetic layers

We study both analytically and numerically the edge of two-dimensional ferromagnets with Dzyaloshinskii-Moriya (DM) interactions, considering both chiral magnets and magnets with interface-induced DM interactions. We show that in the field-polarized ferromagnetic phase magnon states exist which are bound to the edge, and we calculate their spectra within a continuum field theory. Upon lowering an external magnetic field, these bound magnons condense at a finite momentum and the edge becomes locally unstable. Micromagnetic simulations demonstrate that this edge instability triggers the creation of a helical phase which penetrates the field-polarized state within the bulk. A subsequent increase of the magnetic field allows to create skyrmions close to the edge in a controlled manner.Comment: 10 pages, 8 figures; (v2) minor corrections, published versio

Kondo "underscreening" cloud: spin-spin correlations around a partially screened magnetic impurity

We consider the spatial spin correlations around a partially screened spin-1 magnetic moment in a metal exhibiting the underscreened Kondo effect. We find that the underscreening of the impurity spin results in spatial spin correlations that are more pronounced as compared to the fully screened Kondo effect; their power-law decay is weaker because of characteristic logarithmic corrections at large distances. The spin correlator also changes sign as a function of distance to the impurity allowing for ferromagnetic correlations between conduction electron spin density and the local moment. The numerical findings are shown to be in agreement with the predictions deriving from an effective ferromagnetic Kondo Hamiltonian.Comment: 4+ pages, 2 figures, submitted to Phys. Rev.

Critical speeding-up near the monopole liquid-gas transition in magnetoelectric spin-ice

Competing interactions in the so-called spin-ice compounds stabilize a frustrated ground-state with finite zero-point entropy and, interestingly, emergent magnetic monopole excitations. The properties of these monopoles are at the focus of recent research with particular emphasis on their quantum dynamics. It is predicted that each monopole also possesses an electric dipole moment, which allows to investigate their dynamics via the dielectric function \epsilon(\nu). Here, we report on broadband spectroscopic measurements of \epsilon(\nu) in Dy2Ti2O7 down to temperatures of 200mK with a specific focus on the critical endpoint present for a magnetic field along the crystallographic [111] direction. Clear critical signatures are revealed in the dielectric response when, similarly as in the liquid-gas transition, the density of monopoles changes in a critical manner. Surprisingly, the dielectric relaxation time \tau\ exhibits a critical speeding-up with a significant enhancement of 1/\tau\ as the temperature is lowered towards the critical temperature. Besides demonstrating the magnetoelectric character of the emergent monopole excitations, our results reveal unique critical dynamics near the monopole condensation transition.Comment: Changes: Data shown and discussed as function of internal field H and flux density B, Figs.3&4 rearranged, references adde

Quantum phase transitions in the quasi-periodic kicked rotor

We present a microscopic theory of transport in quasi-periodically driven environments (`kicked rotors'), as realized in recent atom optic experiments. We find that the behavior of these systems depends sensitively on the value of Planck's constant $\tilde h$: for irrational values of $\tilde h/(4\pi)$ they fall into the universality class of disordered electronic systems and we derive the microscopic theory of the ensuing localization phenomena. In contrast, for rational values the rotor-Anderson insulator acquires an infinite (static) conductivity and turns into a `super-metal'. Signatures of the corresponding metal/super-metal transition are discussed.Comment: 4 pages, 1 figure, 1 tabl