arXiv subjects

T. Kloss

Publications and source records attributed to T. Kloss.

8 recordsLinked to original sources

arXiv2017

Local particle-hole pair excitations by SU(2) symmetry fluctuations

Understanding the pseudo-gap phase which opens in the under-doped regime of cuprate superconductors is one of the most enduring challenges of the physics of these compounds. A depletion in the electronic density of states is observed, which is gapping out part of the Fermi surface, leading to the formation of mysterious lines of massless excitations- the Fermi arcs. Here we give a new theoretical account of the physics of the pseudo-gap phase in terms of the emergence of local patches of particle-hole pairs generated by SU(2) symmetry fluctuations. The proliferation of these local patches accounts naturally for the robustness of the pseudo-gap phase to disturbances like disorder or magnetic field and is shown to gap out part fo the Fermi surface, leading to the formation of the Fermi arcs. Most noticeably, we show that these patches induce a modulated charge distribution on the Oxygen atoms, in remarkable agreement with recent X-ray and STM observations.

cond-mat.supr-con↗

arXiv2017

Effective SU(2) theory for the pseudogap state

This paper exposes in a detailed manner the recent findings about the SU(2) scenario for the underdoped phase of the Cuprate superconductors. The SU(2) symmetry is formulated as a rotation between the $d$-wave SC phase and a $d$-wave charge order. We define the operators responsible for the SU(2) rotations and we derive the non-linear $σ$-model associated with it. In this framework, we demonstrate that SU(2) fluctuations are massless in finite portions of the Brillouin Zone corresponding to the anti-nodal regions ($0,$$π$), ($π,0$). We argue that the presence of SU(2) fluctuations in the anti-nodal region leads to the opening of Fermi arcs around the Fermi surface and to the formation of the pseudo-gap. Moreover, we show that SU(2) fluctuations lead, in turn, to the emergence of a finite momentum SC order -or Pair Density Wave (PDW)- and more importantly to a new kind of excitonic particle-hole pairs liquid, the Resonant Excitonic State (RES), which is made of patches of preformed particle-hole pairs with multiple momenta. When the RES liquid becomes critical, we demonstrate that electronic scattering through the critical modes leads to anomalous transport properties. This new finding can account for the Strange Metal (SM) phase at finite temperature, on the right hand side of the SC dome, shedding light on another notoriously mysterious part of the phase diagram of the Cuprates.

cond-mat.supr-con↗

arXiv2016

Angle Resolved Photo-Emission Spectroscopy signature of the Resonant Excitonic State

We calculate the Angle Resolved PhotoEmission Spectroscopy (ARPES) signature of the Resonant Excitonic State (RES) that was proposed as the Pseudo-Gap state of cuprate superconductors [ArXiv 1510.03038]. This new state can be described as a set of excitonic (particle-hole) patches with an internal checkerboard modulation. Here, we modelize the RES as a charge order with $\bf{2p_{F}}$ wave vectors, where $\bf{2p_{F}}$ is the ordering vector connecting two opposite sides of the Fermi surface. We calculate the spectral weight and the density of states in the RES and we find that our model correctly reproduces the opening of the PG in Bi-2201.

cond-mat.supr-con↗

arXiv2016

$η$ collective mode as A$_{1g}$ Raman resonance in cuprate superconductors

We discuss the possible existence a spin singlet excitation with charge $\pm2$ ($η$-mode) originating the $A_{1g}$ Raman resonance in cuprate superconductors. This $η$-mode relates the $d$-wave superconducting singlet pairing channel to a $d$-wave charge channel. We show that the $η$ boson forms a particle-particle bound state below the $2Δ$ threshold of the particle-hole continuum where $Δ$ is the maximum $d$-wave gap. Within a generalized random phase approximation and Bethe-Salpether approximation study, we find that this mode has energies similar to the resonance observed by Inelastic Neutron Scattering (INS) below the superconducting (SC) coherent peak at $2Δ$ in various SC cuprates compounds. We show that it is a very good candidate for the resonance observed in Raman scattering below the $2Δ$ peak in the $A_{1g}$ symmetry. Since the $η$-mode sits in the $S=0$ channel, it may be observable via Raman, X -ray or Electron Energy Loss Spectroscopy probes.

cond-mat.supr-con↗

arXiv2015

Charge orders, magnetism and pairings in the cuprate superconductors

We review the recent developments in the field of cuprate superconductors with the special focus on the recently observed charge order in the underdoped compounds. We introduce new theoretical developments following the study of the antiferromagnetic (AF) quantum critical point (QCP) in two dimensions, in which preemptive orders in the charge and superconducting (SC) sectors emerged, that are in turn related by an SU(2) symmetry. We consider the implications of this proliferation of orders in the underdoped region, and provide a study of the type of fluctuations which characterize the SU(2) symmetry. We identify an intermediate energy scale where the SU(2) pairing fluctuations are dominant and argue that they are unstable towards the formation of a Resonant Peierls Excitonic (RPE) state at the pseudogap (PG) temperature $T^{*}$. We discuss the implications of this scenario for a few key experiments.

cond-mat.supr-con↗

arXiv2015

SU(2)-symmetry in a realistic spin-fermion model for cuprate superconductors

We consider the Pseudo-Gap (PG) state of high-$T_c$ superconductors in form of a composite order parameter fluctuating between 2$p_F$-charge ordering and superconducting (SC) pairing. In the limit of linear dispersion and at the hotspots, both order parameters are related by a SU(2) symmetry and the eight hotspot model of Efetov $\textit{et al.}$ [Nat. Phys. $\bf{9}$, 442 (2013)] is recovered. In the general case however, curvature terms of the dispersion will break this symmetry and the degeneracy between both states is lifted. Taking the full momentum dependence of the order parameter into account, we measure the strength of this SU(2) symmetry breaking over the full Brillouin zone. For realistic dispersion relations including curvature we find generically that the SU(2) symmetry breaking is small and robust to the fermiology and that the symmetric situation is restored in the large paramagnon mass and coupling limit. Comparing the level splitting for different materials we propose a scenario that could account for the competition between the PG and the SC states in the phase diagram of high-$T_c$ superconductors.

cond-mat.supr-con↗

arXiv2014

Pseudogap, charge order, and pairing density wave at the hot spots in cuprate superconductors

We address the timely issue of the presence of charge ordering at the hot-spots in the pseudo-gap phase of cuprate superconductors in the context of an emergent SU(2)-symmetry which relates the charge and pairing sectors. Performing the Hubbard-Stratonovich decoupling such that the free energy stays always real and physically meaningful we exhibit three solutions of the spin-fermion model at the hot spots. A careful examination of their stability and free energy shows that, at low temperature, the system tends towards a co-existence of charge density wave (CDW) and the composite order parameter made of diagonal quadrupolar density wave and pairing fluctuations of Ref. [Nat. Phys. $\bf{9}$, 1745 (2013)].The CDW is sensitive to the shape of the Fermi surface in contrast to the diagonal quadrupolar order, which is immune to it. SU(2) symmetry within the pseudo-gap phase also applies to the CDW state, which therefore admits a pairing density pave counterpart breaking time reversal symmetry.

cond-mat.supr-con↗

arXiv2005

The nature of iron-oxygen vacancy defect centers in PbTiO3

The iron(III) center in ferroelectric PbTiO3 together with an oxygen vacancy forms a charged defect associate, oriented along the crystallographic c-axis. Its microscopic structure has been analyzed in detail comparing results from a semi-empirical Newman superposition model analysis based on finestructure data and from calculations using density functional theory. Both methods give evidence for a substitution of Fe3+ for Ti4+ as an acceptor center. The position of the iron ion in the ferroelectric phase is found to be similar to the B-site in the paraelectric phase. Partial charge compensation is locally provided by a directly coordinated oxygen vacancy. Using high-resolution synchrotron powder diffraction, it was verified that lead titanate remains tetragonal down to 12 K, exhibiting a c/a-ratio of 1.0721.

cond-mat.mtrl-sci↗