93 research outputs found

### Topological Weyl Superconductor to Diffusive Thermal Hall Metal Crossover in the B-Phase of UPt$_3$

The recent phase sensitive measurements in the superconducting $B$-phase of
UPt$_3$ provide strong evidence for the triplet, chiral $k_z(k_x \pm ik_y)^2$
pairing symmetries, which endow the Cooper pairs with orbital angular momentum
projections $L _z= \pm 2$ along the $c$-axis. In the absence of disorder such
pairing can support both line and point nodes, and both types of nodal
quasiparticles exhibit nontrivial topology in the momentum space. The point
nodes, located at the intersections of the closed Fermi surfaces with the
$c$-axis, act as the double monopoles and the antimonopoles of the Berry
curvature, and generalize the notion of Weyl quasiparticles. Consequently, the
$B$ phase should support an anomalous thermal Hall effect, the polar Kerr
effect, in addition to the protected Fermi arcs on the (1,0,0) and the (0,1,0)
surfaces. The line node at the Fermi surface equator acts as a vortex loop in
the momentum space and gives rise to the zero energy, dispersionless Andreev
bound states on the (0,0,1) surface. At the transition from the $B$-phase to
the $A$-phase, the time reversal symmetry is restored, and only the line node
survives inside the $A$-phase. As both line and double-Weyl point nodes possess
linearly vanishing density of states, we show that weak disorder acts as a
marginally relevant perturbation. Consequently, an infinitesimal amount of
disorder destroys the ballistic quasiparticle pole, while giving rise to a
diffusive phase with a finite density of states at the zero energy. The
resulting diffusive phase exhibits $T$-linear specific heat, and an anomalous
thermal Hall effect. We predict that the low temperature thermodynamic and
transport properties display a crossover between a ballistic thermal Hall
semimetal and a diffusive thermal Hall metal.Comment: 8 pages, 1 figure; replaced by the version accepted by Phys. Rev.

### Coexistence of ferromagnetism and superconductivity near quantum phase transition: The Heisenberg- to Ising-type crossover

A microscopic mean-field theory of the phase coexistence between
ferromagnetism and superconductivity in the weakly ferromagnetic itinerant
electron system is constructed, while incorporating a realistic mechanism for
superconducting pairing due to the exchange of critical spin fluctuations. The
self-consistent solution of the resulting equations determines the
superconducting transition temperature which is shown to depend strongly on the
exchange splitting. The effect of phase crossover from isotropic
(Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum
phase transition is analysed and the generic phase diagram is obtained. This
scenario is then applied to the case of itinerant ferromagnet ZrZn2, which
sheds light on the proposed phase diagram of this compound. Possible
explanation of superconductivity in UGe2 is also discussed.Comment: 5 pages, 3 figure

### Ising-nematic order in the bilinear-biquadratic model for the iron pnictides

Motivated by the recent inelastic neutron scattering (INS) measurements in
the iron pnictides which show a strong anisotropy of spin excitations in
directions perpendicular and parallel to the ordering wave-vector even above
the magnetic transition temperature $T_N$, we study the frustrated Heisenberg
model with a biquadratic spin-spin exchange interaction. Using the Dyson-Maleev
(DM) representation, which proves appropriate for all temperature regimes, we
find that the spin-spin dynamical structure factors are in excellent agreement
with experiment, exhibiting breaking of the $C_4$ symmetry even into the
paramagnetic region $T_N<T<T_{\sigma}$ which we refer to as the Ising-nematic
phase. In addition to the Heisenberg spin interaction, we include the
biquadratic coupling $K (\mathbf{S}_i\cdot \mathbf{S}_j)^2$ and study its
effect on the dynamical temperature range $T_{\sigma}-T_N$ of the Ising-nematic
phase. We find that this range reduces dramatically when even small values of
the interlayer exchange $J_c$ and biquadratic coupling $K$ are included. To
supplement our analysis, we benchmark the results obtained using the DM method
against those from different non-linear spin-wave theories, including the
recently developed generalized spin-wave theory (GSWT), and find good
qualitative agreement among the different theoretical approaches as well as
experiment for both the spin-wave dispersions and the dynamical structure
factors

### Frustration and Multicriticality in the Antiferromagnetic Spin-1 Chain

We study the spin $S=1$ Heisenberg chain, with nearest neighbor, next nearest
neighbor ($\alpha$) and biquadratic ($\beta$) interactions using a combination
of the density matrix renormalization group (DMRG), an analytic variational
matrix product state wavefunction, and non-Abelian bosonization. We study the
effect of frustration ($\alpha>0$) on the Haldane phase with $-1\leq \beta < 1$
which reveals a rich phase diagram. For $-1<\beta<\beta^\ast$, we establish the
existence of a spontaneously dimerized phase for large $\alpha>\alpha_c$,
separated from the Haldane phase by the critical line $\alpha_c(\beta)$ of
second-order phase transitions connected to the Takhtajan--Babudjian integrable
point $\alpha_c(\beta=-1)=0$. In the opposite regime, $\beta>\beta^\ast$, the
transition from the Haldane phase becomes first-order into the next nearest
neighbor (NNN) AKLT phase. Based on field theoretical arguments and DMRG
calculations, we conjecture that these two regimes are separated by a
multicritical point ($\beta^\ast, \alpha^\ast$) of a different universality
class, described by the $SU(2)_4$ Wess--Zumino--Witten critical theory. From
the DMRG calculations we estimate this multicritical point to lie in the range
$-0.2<\beta^\ast<-0.15$ and $0.47<\alpha^\ast < 0.53$. We find that the
dimerized and NNN-AKLT phases are separated by a line of first-order phase
transitions that terminates at the multicritical point. Inside the Haldane
phase, we show the existence of two incommensurate crossovers: the Lifshitz
transition and the disorder transition of the first kind, marking
incommensurate correlations in momentum and real space, respectively. We show
these crossover lines stretch across the entire $(\beta,\alpha)$ phase diagram,
merging into a single incommensurate-to-commensurate transition line for
negative $\beta\lesssim \beta^\ast$ outside the Haldane phase.Comment: 25 pages, 24 figures, updated with published versio

### Composite pairing in a mixed valent two channel Anderson model

Using a two-channel Anderson model, we develop a theory of composite pairing
in the 115 family of heavy fermion superconductors that incorporates the
effects of f-electron valence fluctuations. Our calculations introduce
"symplectic Hubbard operators": an extension of the slave boson Hubbard
operators that preserves both spin rotation and time-reversal symmetry in a
large N expansion, permitting a unified treatment of anisotropic singlet
pairing and valence fluctuations. We find that the development of composite
pairing in the presence of valence fluctuations manifests itself as a
phase-coherent mixing of the empty and doubly occupied configurations of the
mixed valent ion. This effect redistributes the f-electron charge within the
unit cell. Our theory predicts a sharp superconducting shift in the nuclear
quadrupole resonance frequency associated with this redistribution. We
calculate the magnitude and sign of the predicted shift expected in CeCoIn_5.Comment: 13 pages, 5 figure

### Spin Ferroquadrupolar Order in the Nematic Phase of FeSe

We provide evidence that spin ferroquadrupolar (FQ) order is the likely
ground state in the nonmagnetic nematic phase of stoichiometric FeSe. By
studying the variational mean-field phase diagram of a bilinear-biquadratic
Heisenberg model up to the 2nd nearest neighbor, we find the FQ phase in close
proximity to the columnar antiferromagnet commonly realized in iron-based
superconductors; the stability of FQ phase is further verified by the density
matrix renormalization group. The dynamical spin structure factor in the FQ
state is calculated with flavor-wave theory, which yields a qualitatively
consistent result with inelastic neutron scattering experiments on FeSe at both
low and high energies. We verify that FQ can coexist with $C_4$ breaking
environments in the mean-field calculation, and further discuss the possibility
that quantum fluctuations in FQ act as a source of nematicity.Comment: 8 pages, 7 figures, Erratum adde

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