arXiv subjects

T. P. Devereaux

Publications and source records attributed to T. P. Devereaux.

At least 73 records · Page 4Linked to original sources

arXiv2014

Direct characterization of photo-induced lattice dynamics in BaFe2As2

Ultrafast light pulses can modify the electronic properties of quantum materials by perturbing the underlying, intertwined degrees of freedom. In particular, iron-based superconductors exhibit a strong coupling among electronic nematic fluctuations, spins, and the lattice, serving as a playground for ultrafast manipulation. Here we use time-resolved x-ray scattering to measure the lattice dynamics of photo-excited BaFe2As2. Upon optical excitation, no signature of an ultrafast change of the crystal symmetry is observed, but the lattice oscillates rapidly in time due to the coherent excitation of an A1g mode that modulates the Fe-As-Fe bond angle. We directly quantify the coherent lattice dynamics and show that even a small photo-induced lattice distortion can induce notable changes in the electronic and magnetic properties. Our analysis implies that transient structural modification can generally be an effective tool for manipulating the electronic properties of multi-orbital systems, where electronic instabilities are sensitive to the orbital character of bands near the Fermi level.

cond-mat.str-el↗

arXiv2014

Theory of pump-probe photoemission in graphene: Ultrafast tuning of Floquet bands and local pseudospin textures

The control of physical properties of solids with short laser pulses is an intriguing prospect of ultrafast materials science. Continuous-wave high-frequency laser driving with circular polarization was predicted to induce a light-matter coupled new state possessing a quasi-static band structure with an energy gap and a quantum Hall effect, coined "Floquet topological insulator". Whereas the envisioned Floquet topological insulator requires well separated Floquet bands and therefore high-frequency pumping, a natural follow-up question regards the creation of Floquet-like states in graphene with realistic pump laser pulses. Here we predict that with short low-frequency laser pulses attainable in pump-probe experiments, states with local spectral gaps at the Dirac points and novel pseudospin textures can be achieved in graphene using circular light polarization. We demonstrate that time- and angle-resolved photoemission spectroscopy can track these states by measuring sizeable energy gaps and quasi-Floquet energy bands that form on femtosecond time scales. By analyzing Floquet energy level crossings and snapshots of pseudospin textures near the Dirac points, we identify transitions to new states with optically induced nontrivial changes of sublattice mixing that can lead to Berry curvature corrections of electrical transport and magnetization.

cond-mat.mes-hall↗

arXiv2014

Numerical exploration of spontaneous broken symmetries in multi-orbital Hubbard models

We study three proposals for broken symmetry in the cuprate pseudogap - oxygen antiferromagnetism, $Θ_{II}$ orbital loop currents, and circulating currents involving apex oxygens - through numerical exploration of multi-orbital Hubbard models. Our numerically exact results show no evidence for the existence of oxygen antiferromagnetic order or the $Θ_{II}$ phase in the three-orbital Hubbard model. The model also fails to sustain an ordered current pattern even with the presence of additional apex oxygen orbitals. We thereby conclude that it is difficult to stabilize the aforementioned phases in the multi-orbital Hubbard models for parameters relevant to cuprate superconductors. However, the $Θ_{II}$ phase might be stabilized through explicit flux terms. We find an enhanced propensity for circulating currents with such terms in calculations simulating applied stress or strain, which skew the copper-oxygen plane to resemble a kagome lattice. We propose an experimental viewpoint to shed additional light on this problem.

cond-mat.supr-con↗

arXiv2014

A balancing act: Evidence for a strong subdominant d-wave pairing channel in ${\rm Ba_{0.6}K_{0.4}Fe_2As_2}$

We present an analysis of the Raman spectra of optimally doped ${\rm Ba_{0.6}K_{0.4}Fe_2As_2}$ based on LDA band structure calculations and the subsequent estimation of effective Raman vertices. Experimentally a narrow, emergent mode appears in the $B_{1g}$ ($d_{x^2-y^2}$) Raman spectra only below $T_c$, well into the superconducting state and at an energy below twice the energy gap on the electron Fermi surface sheets. The Raman spectra can be reproduced quantitatively with estimates for the magnitude and momentum space structure of the s$_{+-}$ pairing gap on different Fermi surface sheets, as well as the identification of the emergent sharp feature as a Bardasis-Schrieffer exciton, formed as a Cooper pair bound state in a subdominant $d_{x^2-y^2}$ channel. The binding energy of the exciton relative to the gap edge shows that the coupling strength in this subdominant $d_{x^2-y^2}$ channel is as strong as 60% of that in the dominant $s_{+-}$ channel. This result suggests that $d_{x^2-y^2}$ may be the dominant pairing symmetry in Fe-based sperconductors which lack central hole bands.

cond-mat.str-el↗

arXiv2014

Effect of dynamical spectral weight redistribution on effective interactions in time-resolved spectroscopy

The redistribution of electrons in an ultrafast pump-probe experiment causes significant changes to the effective interaction between electrons and bosonic modes. We study the influence of these changes on pump-probe photoemission spectroscopy for a model electron-phonon coupled system using the nonequilibrium Keldysh formalism. We show that spectral rearrangement due to the driving field preserves an overall sum rule for the electronic self-energy, but modifies the effective electron-phonon scattering as a function of energy. Experimentally, this pump-modified scattering can be tracked by analyzing the fluence or excitation energy dependence of population decay rates and transient changes in dispersion kinks.

cond-mat.mes-hall↗

arXiv2014

Direct observation of bulk charge modulations in optimally-doped Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+δ}$

Bulk charge density modulations, recently observed in high critical-temperature ($T_\mathrm{c}$) cuprate superconductors, coexist with the so-called pseudogap and compete with superconductivity. However, its direct observation has been limited to a narrow doping region in the underdoped regime. Using energy-resolved resonant x-ray scattering we have found evidence for such bulk charge modulations, or soft collective charge modes (soft CCMs), in optimally doped Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+δ}$ (Pb-Bi2212) around the summit of the superconducting dome with momentum transfer $q_{\parallel}\sim0.28$ reciprocal lattice units (r.l.u.) along the Cu-O bond direction. The signal is stronger at $T\simeq T_\mathrm{c}$ than at lower temperatures, thereby confirming a competition between soft CCMs and superconductivity. These results demonstrate that soft CCMs are not constrained to the underdoped regime, suggesting that soft CCMs appear across a large part of the phase diagram of cuprates and are intimately entangled with high-$T_\mathrm{c}$ superconductivity.

cond-mat.supr-con↗

arXiv2014

Asymmetry of collective excitations in electron and hole doped cuprate superconductors

High-temperature superconductivity (HTSC) mysteriously emerges upon doping holes or electrons into insulating copper oxides with antiferromagnetic (AFM) order. It has been thought that the large energy scale of magnetic excitations, compared to phonon energies for example, lies at the heart of an electronically-driven superconducting phase at high temperatures. However, despite extensive studies, little information is available for comparison of high-energy magnetic excitations of hole- and electron-doped superconductors to assess a possible correlation with the respective superconducting transition temperatures. Here, we use resonant inelastic x-ray scattering (RIXS) at the Cu L3-edge to reveal high-energy collective excitations in the archetype electron-doped cuprate Nd2-xCexCuO4 (NCCO). Surprisingly, despite the fact that the spin stiffness is zero and the AFM correlations are short-ranged, magnetic excitations harden significantly across the AFM-HTSC phase boundary, in stark contrast with the hole-doped cuprates. Furthermore, we find an unexpected and highly dispersive mode in superconducting NCCO that is undetected in the hole-doped compounds, which emanates from the zone center with a characteristic energy comparable to the pseudogap, and may signal a quantum phase distinct from superconductivity. The uncovered asymmetry in the high-energy collective excitations with respect to hole and electron doping provides additional constraints for modeling the HTSC cuprates.

cond-mat.supr-con↗

arXiv2014

Dynamic competition between spin density wave order and superconductivity in underdoped $Ba_{1-x}K_xFe_2As_2$

An intriguing aspect of unconventional superconductivity is that it always appears in the vicinity of other competing phases, whose suppression brings the full emergence of superconductivity. In the iron-pnictides, these competing phases are marked by a tetragonal-to-orthorhombic structural transition and a collinear spin-density-wave (SDW) transition. There has been macroscopic evidence for competition between these phases and superconductivity as the magnitude of both the orthorhombicity and magnetic moment are suppressed in the superconducting state. Here, using angle-resolved photoemission spectroscopy on detwinned underdoped $Ba_{1-x}K_xFe_2As_2$, we observe a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials.

cond-mat.supr-con↗

arXiv2014

Gauge invariance in the theoretical description of time-resolved angle-resolved pump/probe photoemission spectroscopy

Nonequilibrium calculations in the presence of an electric field are usually performed in a gauge, and need to be transformed to reveal the gauge-invariant observables. In this work, we discuss the issue of gauge invariance in the context of time-resolved angle-resolved pump/probe photoemission. If the probe is applied while the pump is still on, one must ensure that the calculations of the observed photocurrent are gauge invariant. We also discuss the requirement of the photoemission signal to be positive and the relationship of this constraint to gauge invariance. We end by discussing some technical details related to the perturbative derivation of the photoemission spectra, which involve processes where the pump pulse photoexcites electrons due to nonequilibrium effects.

cond-mat.str-el↗

arXiv2014

Nonequilibrium sum rules for the Holstein model

We derive frequency moment sum rules for the retarded electronic Green's function and self-energy of the Holstein model for both equilibrium and nonequilibrium cases. We also derive sum rules for the phonon propagator in equilibrium and nonequilibrium. These sum rules allow one to benchmark nonequilibrium calculations and help with interpreting the behavior of electrons driven out of equilibrium by an applied electric field. We exactly evaluate the sum rules when the system is in the atomic limit. We also discuss the application of these sum rules to pump/probe experiments like time-resolved angle-resolved photoemission spectroscopy.

cond-mat.str-el↗

arXiv2014

Angle-resolved photoemission spectroscopy study of HgBa$_{2}$CuO$_{4+δ}$

HgBa$_{2}$CuO$_{4+δ}$ (Hg1201) has been shown to be a model cuprate for scattering, optical, and transport experiments, but angle-resolved photoemission spectroscopy (ARPES) data are still lacking owing to the absence of a charge-neutral cleavage plane. We report on progress in achieving the experimental conditions for which quasiparticles can be observed in the near-nodal region of the Fermi surface. The d-wave superconducting gap is measured and found to have a maximum of 39 meV. At low temperature, a kink is detected in the nodal dispersion at approximately 51 meV below the Fermi level, an energy that is different from other cuprates with comparable T$_c$. The superconducting gap, Fermi surface, and nodal band renormalization measured here provide a crucial momentum-space complement to other experimental probes.

cond-mat.supr-con↗

arXiv2013

Charge-orbital-lattice coupling effects in the dd-excitation profile of one dimensional cuprates

We identify dd-excitations in the quasi-one dimensional compound Ca$_2$Y$_2$Cu$_5$O$_{10}$ using resonant inelastic x-ray scattering. By tuning across the Cu L$_3$-edge, we observe abrupt shifts in the dd-peak positions as a function of incident photon energy. This observation demonstrates orbital-specific coupling of the high-energy excited states of the system to the low-energy degrees of freedom. A Franck-Condon treatment of electron-lattice coupling, consistent with other measurements in this compound, reproduces these shifts, explains the Gaussian lineshapes, and highlights charge-orbital-lattice renormalization in the high energy d-manifold.

cond-mat.str-el↗

arXiv2013

Tunneling spectroscopy for probing orbital anisotropy in iron pnictides

Using realistic multi-orbital tight-binding Hamiltonians and the T-matrix formalism, we explore the effects of a non-magnetic impurity on the local density of states in Fe-based compounds. We show that scanning tunneling spectroscopy (STS) has very specific anisotropic signatures that track the evolution of orbital splitting (OS) and antiferromagnetic gaps. Both anisotropies exhibit two patterns that split in energy with decreasing temperature, but for OS these two patterns map onto each other under 90 degree rotation. STS experiments that observe these signatures should expose the underlying magnetic and orbital order as a function of temperature across various phase transitions.

cond-mat.supr-con↗

arXiv2013

Exact solution for high harmonic generation and the response to an ac driving field for a charge density wave insulator

We develop and exactly solve a model for electrons driven by pulsed or continuous ac fields. The theory includes both the photoexcitation process as well as the subsequent acceleration of the electrons. In the case of an ac response, we examine both the nonequilibrium density of states and the current. In the case of pulsed light for high harmonic generation, we find the radiated light assumes a nearly universal behavior, with only limited dependence on the parameters of the system, except for the amplitude of the driving field, which determines the range of high harmonics generated and a tendency toward a narrowing of the peaks in a charge density wave versus a metal. This type of high harmonic generation can potentially be used for the creation of solid-state-based ultrafast light sources.

cond-mat.str-el↗

arXiv2013

Beyond Planck-Einstein quanta: amplitude driven quantum excitation

Linear-response quantum excitation is proportional to the amplitude of the field, with the energy of the excitation given by the driving frequency. As the amplitude is increased, there is a crossover, where the excitation energy is governed by the amplitude of the driving field, not its frequency. As the amplitude is increased even further, then complex quantum oscillations develop, whose origin is related to Wannier-Stark physics, but has no simple explanation. We illustrate this phenomena with the exact solution of the simplest model of a charge-density-wave insulator driven by a uniform time-dependent electric field.

cond-mat.str-el↗

arXiv2013

Time-Dependent Charge-Order and Spin-Order Recovery in Striped Systems

Using time-dependent Ginzburg-Landau theory, we study the role of amplitude and phase fluctuations in the recovery of charge and spin stripe phases in response to a pump pulse that melts the orders. For parameters relevant to the case where charge order precedes spin order thermodynamically, amplitude recovery governs the initial time scales, while phase recovery controls behavior at longer times. In addition to these intrinsic effects, there is a longer spin re-orientation time scale related to the scattering geometry that dominates the recovery of the spin phase. Coupling between the charge and spin orders locks the amplitude and similarly the phase recovery, reducing the number of distinct time scales. Our results well reproduce the major experimental features of pump-probe x-ray diffraction measurements on the striped nickelate La$_{1.75}$Sr$_{0.25}$NiO$_4$. They highlight the main idea of this work, which is the use of time-dependent Ginzburg-Landau theory to study systems with multiple coexisting order parameters.

cond-mat.str-el↗

arXiv2013

Exact solution for Bloch oscillations of a simple charge-density-wave insulator

Charge-density-wave systems have a static modulation of the electronic charge at low temperatures when they enter an ordered state. While they have been studied for decades in equilibrium, it is only recently that they have been examined in nonequilibrium with time-resolved studies. Here, we present the exact solution for the nonequilibrium response of electrons (in the simplest model for a charge density wave) when the system is placed under a strong DC electric field. This allows us to examine the formation of driven Bloch oscillations and how the presence of a current modifies the nonequilibrium density of states.

cond-mat.str-el↗

arXiv2013

Nonequilibrium "melting" of a charge density wave insulator via an ultrafast laser pulse

We employ an exact solution of the simplest model for pump-probe time-resolved photoemission spectroscopy in charge-density-wave systems to show how, in nonequilibrium the gap in the density of states disappears while the charge density remains modulated, and then the gap reforms after the pulse has passed. This nonequilibrium scenario qualitatively describes the common short-time experimental features in TaS2 and TbTe3 indicating a quasiuniversality for nonequilibrium "melting" with qualitative features that can be easily understood within a simple picture.

cond-mat.str-el↗