90 research outputs found
Fractionalization and Fermi surface volume in heavy fermion compounds: the case of YbRh Si
We establish an effective theory for heavy fermion compounds close to a zero
temperature Anti-Ferromagnetic (AF) transition. Coming from the heavy Fermi
liquid phase across to the AF phase, the heavy electron fractionalizes into a
light electron, a bosonic spinon and a {\it new} excitation: a spinless
fermionic field. Assuming this field acquires dynamics and dispersion when one
integrates out the high energy degrees of freedom, we give a scenario for the
volume of its Fermi surface through the phase diagram. We apply our theory to
the special case of YbRh(Si Ge) where we recover, within
experimental resolution, several low temperature exponents for transport and
thermodynamics.Comment: 4 pages, 5 figure
The phase diagram of the underdoped cuprates at high magnetic field
The experimentally measured phase diagram of cuprate superconductors in the
temperature-applied magnetic field plane illuminates key issues in
understanding the physics of these materials. At low temperature, the
superconducting state gives way to a long-range charge order with increasing
magnetic field; both the orders coexist in a small intermediate region. The
charge order transition is strikingly insensitive to temperature, and quickly
reaches a transition temperature close to the zero-field superconducting .
We argue that such a transition along with the presence of the coexisting phase
cannot be described simply by a competing orders formalism. We demonstrate that
for some range of parameters there is an enlarged symmetry of the strongly
coupled charge and superconducting orders in the system depending on their
relative masses and the coupling strength of the two orders. We establish that
this sharp switch from the superconducting phase to the charge order phase can
be understood in the framework of a composite SU(2) order parameter comprising
the charge and superconducting orders. Finally, we illustrate that there is a
possibility of the coexisting phase of the competing charge and superconducting
orders only when the SU(2) symmetry between them is weakly broken due to
biquadratic terms in the free energy. The relation of this sharp transition to
the proximity to the pseudogap quantum critical doping is also discussed
Collective mode in the SU(2) theory of cuprates
Recent advances in momentum-resolved electron energy-loss spectroscopy
(MEELS) and resonant inelastic X-ray scattering (RIXS) now allow one to access
the charge response function with unprecedented versatility and accuracy. This
allows for the study of excitations which were inaccessible recently, such as
low-energy and finite momentum collective modes. The SU(2) theory of the
cuprates is based on a composite order parameter with SU(2) symmetry
fluctuating between superconductivity and charge order. The phase where it
fluctuates is a candidate for the pseudogap phase of the cuprates. This theory
has a signature, enabling its strict experimental test, which is the
fluctuation between these two orders, corresponding to a charge 2 spin 0 mode
at the charge ordering wave-vector. Here we derive the influence of this SU(2)
collective mode on the charge susceptibility in both strong and weak coupling
limits, and discuss its relation to MEELS, RIXS and Raman experiments. We find
two peaks in the charge susceptibility at finite energy, whose middle is the
charge ordering wave-vector, and discuss their evolution in the phase diagram
Coexistence of -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 -loop-current order
on both unidirectional and bidirectional d-wave
charge-density-wave/pair-density-wave (CDW/PDW) composite orders along axial
momenta and 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 -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 -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
Pairing gaps near ferromagnetic quantum critical points
We address the quantum-critical behavior of a two-dimensional itinerant
ferromagnetic systems described by a spin-fermion model in which fermions
interact with close to critical bosonic modes. We consider Heisenberg
ferromagnets, Ising ferromagnets, and the Ising nematic transition. Mean-field
theory close to the quantum critical point predicts a superconducting gap with
spin-triplet symmetry for the ferromagnetic systems and a singlet gap for the
nematic scenario. Studying fluctuations in this ordered phase using a nonlinear
sigma model, we find that these fluctuations are not suppressed by any small
parameter. As a result, we find that a superconducting quasi-long-range order
is still possible in the Ising-like models but long-range order is destroyed in
Heisenberg ferromagnets.Comment: 13 pages, 7 figure
3D Modulated Spin Liquid model applied to URuSi
We have developed a 3D version for the Modulated Spin Liquid in a
body-centered tetragonal lattice structure to describe the hidden order
observed in URuSi at K. This second order transition
is well described by our model confirming our earlier hypothesis. The symmetry
of the modulation is minimized for . We assume a linear
variation of the interaction parameters with the lattice spacing and our
results show good agreement with uniaxial and pressure experiments.Comment: 5 pages, 4 figure
Incipient loop current order in the under-doped cuprate superconductors
There are growing experimental evidence which indicate discrete symmetry
breaking like time-reversal (), parity () and C
lattice rotation in the pseudo-gap state of the under-doped copper-oxide based
(cuprate) superconductors. The discrete symmetry breaking manifests a true
phase transition to an ordered state. A detailed thermodynamic understanding of
these orders can answer various puzzles related to the nature of the transition
at the pseudo-gap temperature T. In this work, we investigate thermodynamic
signature of symmetry breaking considering superconductivity
(SC) and bond-density wave (BDW) as two primary orders. The BDW can generate
both modulating charge and current densities. This framework takes into account
an intricate competition between the ubiquitous charge density wave and SC,
which is prominent in various cuprates in the under-doped regime. We
demonstrate that within mean-field approach of competing BDW and SC orders, a
breaking ground state of coexisting BDW and SC can be
stabilized, provided the BDW itself breaks . But this
ground state ceases to occur at higher temperatures. However, we show that
fluctuations in SC and BDW can drive emergence of a new unusual translational
symmetry preserving order due to a preemptive phase transition by spontaneously
breaking at a higher temperature before the primary orders set
in. We refer this order to be magneto-electric loop current (MELC) order. We
present possible nature of phase transition for this new incipient MELC order
and discuss some experimental relevance.Comment: To appear in Physical Review
Strong competition between -loop-current order and -wave charge order along the diagonal direction in a two-dimensional hot spot model
We study the fate of the so-called -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 plane
that includes two hopping parameters and , local on-site
Coulomb interactions and and nearest-neighbor
couplings between the fermions in the copper [Cu] and
oxygen [O and O] orbitals. By focusing on the lowest-energy
band, we proceed to decouple the local interaction 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 to introduce the order parameter of the
-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 -wave
pairing fluctuations proposed in Efetov \emph{et al.} [Nat. Phys. \textbf{9},
442 (2013)] with the -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
-loop-current-phase is calculated and compared with experimental
results. Accepted for publication in Physical Review
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