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Register a new userPositive magnetoresistance induced by hydrodynamic fluctuations in chiral media
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We analyze the combined effects of hydrodynamic fluctuations and chiral magnetic effect (CME) for a chiral medium in the presence of a background magnetic field. Based on the recently developed non-equilibrium effective field theory, we show fluctuations give rise to a CME-related positive contribution to magnetoresistance, while the early studies without accounting for the fluctuations find a CME-related negative magnetoresistance. At zero axial relaxation rate, the fluctuations contribute to the transverse conductivity in addition to the longitudinal one.
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We investigate the time-dependent perturbations of strongly coupled $mathcal{N} = 4$ SYM theory at finite temperature and finite chemical potential with a second order phase transition. This theory is modelled by a top-down Einstein-Maxwell-dilaton description which is a consistent truncation of the dimensional reduction of type IIB string theory on AdS$_5times$S$^5$. We focus on spin-1 and spin-2 sectors of perturbations and compute the linearized hydrodynamic transport coefficients up to the third order in gradient expansion. We also determine the radius of convergence of the hydrodynamic mode in spin-1 sector and the lowest non-hydrodynamic modes in spin-2 sector. Analytically, we find that all the hydrodynamic quantities have the same critical exponent near the critical point $theta = 1/2$. Moreover, we establish a relation between symmetry enhancement of the underlying theory and vanishing the only third order hydrodynamic transport coefficient $theta_1$, which appears in the shear dispersion relation of a conformal theory on a flat background.
Layered van der Waals semimetallic $T_mathrm{d}$-WTe$_{2}$, exhibiting intriguing properties which include non-saturating extreme positive magnetoresistance (MR) and tunable chiral anomaly, has emerged as model topological type-II Weyl semimetal system. Here, $sim$45 nm thick mechanically exfoliated flakes of $T_mathrm{d}$-WTe$_{2}$ are studied $via$ atomic force microscopy, Raman spectroscopy, low-$T$/high-$mu_{0}H$ magnetotransport measurements and optical reflectivity. The contribution of anisotropy of the Fermi liquid state to the origin of the large positive transverse $mathrm{MR}_perp$ and the signature of chiral anomaly of the type-II Weyl fermions are reported. The samples are found to be stable in air and no oxidation or degradation of the electronic properties are observed. A transverse $mathrm{MR}_perp$ $sim$1200,% and an average carrier mobility of $5000$, cm$^{2}$V$^{-1}$s$^{-1}$ at $T=5,mathrm{K}$ for an applied perpendicular field $mu_{0}H_{perp} = 7,mathrm{T}$ are established. The system follows a Fermi liquid model for $Tleq50,mathrm{K}$ and the anisotropy of the Fermi surface is concluded to be at the origin of the observed positive MR. The anisotropy of the electronic behaviour is also confirmed by optical reflectivity measurements. The relative orientation of the crystal axes and of the applied electric and magnetic fields is proven to give rise to the observed chiral anomaly in the in-plane magnetotransport.
In principle, there is no obstacle to gapping fermions preserving any global symmetry that does not suffer a t Hooft anomaly. In practice, preserving a symmetry that is realised on fermions in a chiral manner necessitates some dynamics beyond simple quadratic mass terms. We show how this can be achieved using familiar results about supersymmetric gauge theories and, in particular, the phenomenon of confinement without chiral symmetry breaking. We present simple models that gap fermions while preserving a symmetry group under which they transform in chiral representations. For example, we show how to gap a collection of 4d fermions that carry the quantum numbers of one generation of the Standard Model, but without breaking electroweak symmetry. We further show how to gap fermions in groups of 16 while preserving certain discrete symmetries that exhibit a mod 16 anomaly.
From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T_Curie ~ 2.5 K and T_SC ~ 0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H || c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic field region where the longitudinal FM spin fluctuations are active. These results combined with model calculations strongly suggest that the longitudinal FM spin fluctuations tuned by H || c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.
Axial anomaly and nesting is elucidated in the context of the inhomogeneous chiral phase. Using the Gross-Neveu models in 1+1 dimensions, we shall discuss axial anomaly and nesting from two different points of view: one is homogeneous chiral transition and the other is the Ferrel-Fulde-Larkin-Ovchinnikov (FFLO) state in superconductivity, which are closely related to each other by way of duality. It is shown that axial anomaly leads to a particular kind of the FFLO state within the two dimensional Nambu-Jona Lasinio model, where axial anomaly is manifested in a different mode. Nesting is a driving mechanism for both phenomena, but its realization has different features. We reconsider the effect of nesting in the context of duality.
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