17 research outputs found

### Coexistence of $\Theta_{II}$-loop-current order with checkerboard d-wave CDW/PDW order in a hot-spot model for cuprate superconductors

We investigate the strong influence of the $\Theta_{II}$-loop-current order
on both unidirectional and bidirectional d-wave
charge-density-wave/pair-density-wave (CDW/PDW) composite orders along axial
momenta $(\pm Q_0,0)$ and $(0,\pm Q_0)$ that emerge in an effective hot spot
model departing from the three-band Emery model relevant to the phenomenology
of the cuprate superconductors. This study is motivated by the compelling
evidence that the $\Theta_{II}$-loop-current order described by this model may
explain groundbreaking experiments such as spin-polarized neutron scattering
performed in these materials. Here, we demonstrate, within a saddle-point
approximation, that the $\Theta_{II}$-loop-current order clearly coexists with
bidirectional (i.e. checkerboard) d-wave CDW and PDW orders along axial
momenta, but is visibly detrimental to the unidirectional (i.e. stripe) case.
This result has potentially far-reaching implications for the physics of the
cuprates and agrees well with very recent x-ray experiments on YBCO that
indicate that at higher dopings the CDW order has indeed a tendency to be
bidirectional.Comment: Published in Physical Review

### Complete renormalization group calculation up to two-loop order of an effective two-band model for iron-based superconductors

We perform a renormalization group (RG) study up to two-loop order of an
effective low-energy two-band model to describe some of the recently discovered
iron-based superconductors. Our starting point is the itinerant electronic
model proposed by Chubukov \emph{et al.} [Phys. Rev. B \textbf{78}, 134512
(2008)], which displays two small, almost nested Fermi pockets with one hole
pocket centered at $(0,0)$ and one electron pocket centered at $\mathbf{Q} =
(\pi,\pi)$ in the folded Brillouin zone. We then proceed to implement a
complete two-loop RG calculation for this model of four-point vertex
corrections, quasiparticle weight and several order-parameter susceptibilities
in order to evaluate the robustness of one-loop RG results available in the
literature with respect to including self-energy effects and higher-order
quantum fluctuations.Comment: 6 pages, 6 figures; accepted for publication in Europhysics Letter

### DC resistivity near a nematic quantum critical point: Effects of weak disorder and acoustic phonons

We calculate the resistivity associated with an Ising-nematic quantum
critical point in the presence of disorder and acoustic phonons in the lattice
model. We use the memory-matrix transport theory, which has a crucial advantage
compared to other methods of not relying on the existence of well-defined
quasiparticles in the low-energy effective theory. As a result, we obtain that
by including an inevitable interaction between the nematic fluctuations and the
elastic degrees of freedom of the lattice (parametrized by the nemato-elastic
coupling $\kappa_{\text{latt}}$), the resistivity $\rho(T)$ of the system as a
function of temperature obeys a universal scaling form described by
$\rho(T)\sim T\ln (1/T)$ at high temperatures, reminiscent of the paradigmatic
strange metal regime observed in many strongly correlated compounds. For a
window of temperatures comparable with
$\kappa^{3/2}_{\text{latt}}\varepsilon_F$ (where $\varepsilon_F$ is the Fermi
energy of the microscopic model), the system displays another regime in which
the resistivity is consistent with a description in terms of $\rho(T)\sim
T^{\alpha}$, where the effective exponent roughly satisfies the inequality
$1\lesssim\alpha\lesssim 2$. However, in the low-temperature limit (i.e.,
$T\ll\kappa^{3/2}_{\text{latt}}\varepsilon_F$), the properties of the quantum
critical state change in an important way depending on the types of disorder
present in the system: It can either recover a conventional Fermi liquid
described by $\rho(T)\sim T^2$ or it could exhibit yet another non-Fermi liquid
regime characterized by the scaling form $\rho(T)-\rho_0\sim T^2\ln T$. Our
results emphasize the key role played by both phonon and disorder effects in
the scenario of nematic quantum criticality and might be fundamental for
addressing recent transport experiments in some iron-based superconductors.Comment: 18 pages, 9 figure

### Multipolar spin liquid in an exactly solvable model for $j_\mathrm{eff} = \frac{3}{2}$ moments

We study an exactly solvable model with bond-directional quadrupolar and
octupolar interactions between spin-orbital entangled $j_{\mathrm{eff}} =
\frac{3}{2}$ moments on the honeycomb lattice. We show that this model features
a multipolar spin liquid phase with gapless fermionic excitations. In the
presence of perturbations that break time-reversal and rotation symmetries, we
find Abelian and non-Abelian topological phases in which the Chern number
evaluates to $0$, $\pm 1$, and $\pm 2$. We also investigate quantum phase
transitions out of the multipolar spin liquid using a parton mean-field
approach and orbital wave theory. In the regime of strong
integrability-breaking interactions, the multipolar spin liquid gives way to
ferroquadrupolar-vortex and antiferro-octupolar ordered phases that harbor a
hidden spin-$\frac{1}{2}$ Kitaev spin liquid. Our work unveils mechanisms for
unusual multipolar orders and quantum spin liquids in Mott insulators with
strong spin-orbit coupling.Comment: 14 pages, 9 figure

### Strong competition between $\Theta_{II}$-loop-current order and $d$-wave charge order along the diagonal direction in a two-dimensional hot spot model

We study the fate of the so-called $\Theta_{II}$-loop-current order that
breaks both time-reversal and parity symmetries in a two-dimensional hot spot
model with antiferromagnetically mediated interactions, using Fermi surfaces
relevant to the phenomenology of the cuprate superconductors. We start from a
three-band Emery model describing the hopping of holes in the CuO$_{2}$ plane
that includes two hopping parameters $t_{pp}$ and $t_{pd}$, local on-site
Coulomb interactions $U_{d}$ and $U_{p}$ and nearest-neighbor $V_{pd}$
couplings between the fermions in the copper [Cu$(3d_{x^{2}-y^{2}})$] and
oxygen [O$(2p_{x})$ and O$(2p_{y})$] orbitals. By focusing on the lowest-energy
band, we proceed to decouple the local interaction $U_{d}$ of the Cu orbital in
the spin channel using a Hubbard-Stratonovich transformation to arrive at the
interacting part of the so-called spin-fermion model. We also decouple the
nearest-neighbor interaction $V_{pd}$ to introduce the order parameter of the
$\Theta_{II}$-loop-current order. In this way, we are able to construct a
consistent mean-field theory that describes the strong competition between the
composite order parameter made of a quadrupole-density-wave and $d$-wave
pairing fluctuations proposed in Efetov \emph{et al.} [Nat. Phys. \textbf{9},
442 (2013)] with the $\Theta_{II}$-loop-current order parameter that is argued
to be relevant for explaining important aspects of the physics of the pseudogap
phase displayed in the underdoped cuprates.Comment: 16 pages, 5 figures. v2: minor revisions, references added. The
magnetic moment per unit-cell associated with the
$\Theta_{II}$-loop-current-phase is calculated and compared with experimental
results. Accepted for publication in Physical Review

### Topological transition from nodal to nodeless Zeeman splitting in altermagnets

In an altermagnet, the symmetry that relates configurations with flipped
magnetic moments is a rotation. This makes it qualitatively different from a
ferromagnet, where no such symmetry exists, or a collinear antiferromagnet,
where this symmetry is a lattice translation. In this paper, we investigate the
impact of the crystalline environment on the magnetic and electronic properties
of an altermagnet. We find that, because each component of the magnetization
acquires its own angular dependence, the Zeeman splitting of the bands has
symmetry-protected nodal lines residing on mirror planes of the crystal. Upon
crossing the Fermi surface, these nodal lines give rise to pinch points that
behave as single or double type-II Weyl nodes. We show that an external
magnetic field perpendicular to these mirror planes can only move the nodal
lines, such that a critical field value is necessary to collapse the nodes and
make the Weyl pinch points annihilate. This unveils the topological nature of
the transition from a nodal to a nodeless Zeeman splitting of the bands. We
also classify the altermagnetic states of common crystallographic point groups
in the presence of spin-orbit coupling, revealing that a broad family of
magnetic orthorhombic perovskites can realize altermagnetism.Comment: manuscript + supplementary materia

### Superconductivity Mediated by Nematic Fluctuations in Tetragonal $\textrm{Fe}\textrm{Se}_{1-x}\textrm{S}_{x}$

Nematic phases, where electrons in a solid spontaneously break rotational
symmetry while preserving the translational symmetry, exist in several families
of unconventional superconductors [1, 2]. Although superconductivity mediated
by nematic fluctuations is well established theoretically [3-7], it has yet to
be unambiguously identified experimentally [8, 9]. A major challenge is that
nematicity is often intertwined with other degrees of freedom, such as
magnetism and charge order. The FeSe$_{1-x}$S$_x$ family of iron based
superconductors provides a unique opportunity to explore this concept, as it
features an isolated nematic phase that can be suppressed by sulfur
substitution at a quantum critical point (QCP) near $x_c = 0.17$, where nematic
fluctuations are the largest [10-12]. Here, we performed scanning tunneling
spectroscopy measurements to visualize Boguliubov quasiparticle interference
patterns, from which we determined the momentum structure of the
superconducting gap near the Brillouin zone $\Gamma$ point of
FeSe$_{0.81}$S$_{0.19}$. The results reveal an anisotropic, near nodal gap with
minima that are $45^\circ$ rotated with respect to the Fe-Fe direction,
characteristic of a nematic pairing interaction, contrary to the usual
isotropic gaps due to spin mediated pairing in other tetragonal Fe-based
superconductors. The results are also in contrast with pristine FeSe, where the
pairing is mediated by spin fluctuations and the gap minima are aligned with
the Fe-Fe direction. Therefore, the measured gap structure demonstrates not
only a fundamental change of the pairing mechanism across the phase diagram of
FeSe$_{1-x}$S$_x$, but it also indicates the existence of superconductivity
mediated by nematic fluctuations in FeSe$_{0.81}$S$_{0.19}$