21 research outputs found
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
Optical conductivity and resistivity in a four-band model for ZrTe from ab-initio calculations
ZrTe is considered a potential candidate for either a Dirac semimetal or
a topological insulator in close proximity to a topological phase transition.
Recent optical conductivity results motivated a two-band model with a conical
dispersion in 2D, in contrast to density functional theory calculations. Here,
we reconcile the two by deriving a four-band model for ZrTe using
theory, and fitting its parameters to the
ab-initio band structure. The optical conductivity with an adjusted electronic
structure matches the key features of experimental data. The chemical potential
varies strongly with temperature, to the point that it may cross the gap
entirely between zero and room temperature. The temperature-dependent
resistivity displays a broad peak, and confirms theoretically the conclusions
of recent experiments attributing the origin of the resistivity peak to the
large shift of the chemical potential with temperature
Synthetic gravitational horizons in low-dimensional quantum matter
We propose a class of lattice models realizable in a wide range of setups
whose low-energy dynamics exactly reduces to Dirac fields subjected to
(1+1)-dimensional gravitational backgrounds, including (anti-)de Sitter
spacetime. Wave-packets propagating on the lattice exhibit an eternal slowdown
for power-law position-dependent hopping integrals when
, signalling the formation of black hole event horizons. For
instead the wave-packets behave radically different and bounce off
the horizon. We show that the eternal slowdown relates to a zero-energy
spectral singularity of the lattice model and that the semiclassical wave
packets trajectories coincide with the geodesics on (1+1)D dilaton gravity,
paving the way for new and experimentally feasible routes to mimic black hole
horizons and realize (1+1)D spacetimes as they appear in certain gravity
theories.Comment: 6 pages, 5 figure
Quantum dynamics in 1D lattice models with synthetic horizons
We investigate the wave packet dynamics and eigenstate localization in
recently proposed generalized lattice models whose low-energy dynamics mimics a
quantum field theory in (1+1)D curved spacetime with the aim of creating
systems analogous to black holes. We identify a critical slowdown of
zero-energy wave packets in a family of 1D tight-binding models with power-law
variation of the hopping parameter, indicating the presence of a horizon.
Remarkably, wave packets with non-zero energies bounce back and reverse
direction before reaching the horizon. We additionally observe a power-law
localization of all eigenstates, each bordering a region of exponential
suppression. These forbidden regions dictate the closest possible approach to
the horizon of states with any given energy. These numerical findings are
supported by a semiclassical description of the wave packet trajectories, which
are shown to coincide with the geodesics expected for the effective metric
emerging from the considered lattice models in the continuum limit.Comment: 13 pages, 8 figure
Effects of stoichiometric doping in superconducting Bi-O-S compounds.
Newly discovered Bi-O-S compounds remain an enigma in attempts to understand their electronic properties. A recent study of Bi4O4S3 has shown it to be a mixture of two phases, Bi2OS2 and Bi3O2S3, the latter being superconducting (Phelan et al 2013 J. Am. Chem. Soc. 135 5372-4). Using density functional theory, we explore the electronic structure of both the phases and the effect of the introduction of extra BiS2 bilayers. Our results demonstrate that the S2 layers dope the bismuth-sulphur bands and this causes metallisation. The bands at the Fermi level are of clear two-dimensional character. One band manifold is confined to the two adjacent, square-lattice bismuth-sulphur planes, a second manifold is confined to the square lattice of sulphur dimers. We show that the introduction of extra BiS2 bilayers does not influence the electronic structure. Finally, we also show that spin-orbit coupling does not have any significant effect on the states close to the Fermi level at the energy scale considered
Thermalization by a synthetic horizon
Synthetic horizons in models for quantum matter provide an alternative route
to explore fundamental questions of modern gravitational theory. Here, we apply
these concepts to the problem of emergence of thermal quantum states in the
presence of a horizon, by studying ground-state thermalization due to
instantaneous horizon creation in a gravitational setting and its condensed
matter analogue. By a sudden quench to position-dependent hopping amplitudes in
a one-dimensional lattice model, we establish the emergence of a thermal state
accompanying the formation of a synthetic horizon. The resulting temperature
for long chains is shown to be identical to the corresponding Unruh
temperature, provided that the post-quench Hamiltonian matches the entanglement
Hamiltonian of the pre-quench system. Based on detailed analysis of the
outgoing radiation we formulate the conditions required for the synthetic
horizon to behave as a purely thermal source, paving a way to explore this
interplay of quantum-mechanical and gravitational aspects experimentally
Recommended from our members
Horizon physics of quasi-one-dimensional tilted Weyl cones on a lattice
To simulate the dynamics of massless Dirac fermions in curved space-times with one, two, and three spatial dimensions, we construct tight-binding Hamiltonians with spatially varying hoppings. These models represent tilted Weyl semimetals where the tilting varies with position, in a manner similar to the light cones near the horizon of a black hole. We illustrate the gravitational analogies in these models by numerically evaluating the propagation of wave packets on the lattice and then comparing them to the geodesics of the corresponding curved space-time. We also show that the motion of electrons in these spatially varying systems can be understood through the conservation of energy and the quasiconservation of quasimomentum. This picture is confirmed by calculations of the scattering matrix, which indicate an exponential suppression of any noncontinuous change in the quasimomentum. Finally, we show that horizons in the lattice models can be constructed also at finite energies using specially designed tilting profiles
Electronic and magnetic properties of superconducting OFBiS ( = La, Ce, Pr, and Nd) from first principles
A density functional theory study of the BiS2 superconductors containing rare-earths: LnO1-x F x BiS2 (Ln = La, Ce, Pr, and Nd) is presented. We find that CeO0.5F0.5BiS2 has competing ferromagnetic and weak antiferromagnetic tendencies, the first one corresponding to experimental results. We show that PrO0.5F0.5BiS2 has a strong tendency for magnetic order, which can be ferromagnetic or antiferromagnetic depending on subtle differences in 4f orbital occupations. We demonstrate that NdO0.5F0.5BiS2 has a stable magnetic ground state with weak tendency to order. Finally, we show that the change of rare earth does not affect the Fermi surface, and predict that CeOBiS2 should display a pressure induced phase transition to a metallic, if not superconducting, phase under pressure.This is the final version of the article. It first appeared from IOP Science via http://dx.doi.org/10.1088/0953-8984/28/34/34550
Efficient extreme-ultraviolet high-order wave mixing from laser-dressed silica
The emission of high-order harmonics from solids
\cite{ghimire11a,schubert14a,luu15a,golde08a} under intense laser-pulse
irradiation is revolutionizing our understanding of strong-field solid-light
interactions
\cite{ghimire11a,schubert14a,luu15a,vampa15b,yoshikawa17a,hafez18a,jurgens20a},
while simultaneously opening avenues towards novel, all-solid, coherent,
short-wavelength table-top sources with tailored emission profiles and
nanoscale light-field control\cite{franz19a,roscamCLEO21}. To date, broadband
spectra have been generated well into the extreme-ultraviolet (XUV)
\cite{luu15a,luu18b,han19a,uzan20a}, but the comparatively low conversion
efficiency still lags behind gas-based high-harmonic generation (HHG) sources
\cite{luu15a,luu18b}, and have hindered wider-spread applications. Here, we
overcome the low conversion efficiency by two-color wave mixing. A quantum
theory reveals that our experiments follow a novel generation mechanism where
the conventional interband and intraband nonlinear dynamics are boosted by
Floquet-Bloch dressed states, that make solid HHG in the XUV more efficient by
at least one order of magnitude. Emission intensity scalings that follow
perturbative optical wave mixing, combined with the angular separation of the
emitted frequencies, make our approach a decisive step for all-solid coherent
XUV sources and for studying light-engineered materials