Anshul Kogar

Signatures of exciton condensation in a transition metal dichalcogenide

Probing an excitonic condensate Excitons—bound states of electrons and holes in solids—are expected to form a Bose condensate at sufficiently low temperatures. Excitonic condensation has been studied in systems such as quantum Hall bilayers where physical separation between electrons and holes enables a longer lifetime for their bound states. Kogar et al. observed excitons condensing in the three-dimensional semimetal 1T-TiSe2. In such systems, distinguishing exciton condensation from other types of order is tricky. To do so, the authors used momentum-resolved electron energy-loss spectroscopy, a technique developed to probe electronic collective excitations. The energy needed to excite an electronic mode became negligible at a finite momentum, signifying the formation of a condensate. Science, this issue p. 1314 The softening of an electronic mode of 1T‐TiSe2 is seen with momentum‐resolved electron energy‐loss spectroscopy. Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. Using the recently developed technique of momentum‐resolved electron energy‐loss spectroscopy (M-EELS), we studied electronic collective modes in the transition metal dichalcogenide semimetal 1T‐TiSe2. Near the phase-transition temperature (190 kelvin), the energy of the electronic mode fell to zero at nonzero momentum, indicating dynamical slowing of plasma fluctuations and crystallization of the valence electrons into an exciton condensate. Our study provides compelling evidence for exciton condensation in a three-dimensional solid and establishes M-EELS as a versatile technique sensitive to valence band excitations in quantum materials.

Observation of a Charge Density Wave Incommensuration Near the Superconducting Dome in CuxTiSe2

X-ray diffraction was employed to study the evolution of the charge density wave (CDW) in Cu_{x}TiSe_{2} as a function of copper intercalation in order to clarify the relationship between the CDW and superconductivity. The results show a CDW incommensuration arising at an intercalation value coincident with the onset of superconductivity at around x=0.055(5). Additionally, it was found that the charge density wave persists to higher intercalant concentrations than previously assumed, demonstrating that the CDW does not terminate inside the superconducting dome. A charge density wave peak was observed in samples up to x=0.091(6), the highest copper concentration examined in this study. The phase diagram established in this work suggests that charge density wave incommensuration may play a role in the formation of the superconducting state.

Temperature-resolution anomalies in the reconstruction of time dynamics from energy-loss experiments

Inelastic scattering techniques provide a powerful approach to studying electron and nuclear dynamics, via reconstruction of a propagator that quantifies the time evolution of a system. There is now growing interest in applying such methods to very low energy excitations, such as lattice vibrations, but in this limit the cross section is no longer proportional to a propagator. Significant deviations occur due to the finite temperature Bose statistics of the excitations. Here we consider this issue in the context of high-resolution electron energy-loss experiments on the copper-oxide superconductor Bi2Sr2CaCu2O8. We find that simple division of a Bose factor yields an accurate propagator on energy scales greater than the resolution width. However, at low energy scales, the effects of resolution and finite temperature conspire to create anomalies in the dynamics at long times. We compare two practical ways for dealing with such anomalies, and discuss the range of validity of the technique in light of this comparison.

Anomalous density fluctuations in a strange metal

Significance The strange metal is a poorly understood state of matter found in a variety of quantum materials, notably both Cu- and Fe-based high-temperature superconductors. Strange metals exhibit a nonsaturating, T-linear electrical resistivity, seemingly indicating the absence of electron quasiparticles. Using inelastic electron scattering, we report a momentum-resolved measurement of the dynamic charge susceptibility of a strange metal, optimally doped Bi2.1Sr1.9CaCu2O8+x. We find that it does not exhibit propagating collective modes, such as the plasmon excitation of normal metals, but instead exhibits a featureless continuum lacking either temperature or momentum dependence. Our study suggests the defining characteristic of the strange metal is a singular type of charge dynamics of a new kind for which there is no generally accepted theory. A central mystery in high-temperature superconductivity is the origin of the so-called strange metal (i.e., the anomalous conductor from which superconductivity emerges at low temperature). Measuring the dynamic charge response of the copper oxides, χ″(q,ω), would directly reveal the collective properties of the strange metal, but it has never been possible to measure this quantity with millielectronvolt resolution. Here, we present a measurement of χ″(q,ω) for a cuprate, optimally doped Bi2.1Sr1.9CaCu2O8+x (Tc = 91 K), using momentum-resolved inelastic electron scattering. In the medium energy range 0.1–2 eV relevant to the strange metal, the spectra are dominated by a featureless, temperature- and momentum-independent continuum persisting to the electronvolt energy scale. This continuum displays a simple power-law form, exhibiting q2 behavior at low energy and q2/ω2 behavior at high energy. Measurements of an overdoped crystal (Tc = 50 K) showed the emergence of a gap-like feature at low temperature, indicating deviation from power law form outside the strange-metal regime. Our study suggests the strange metal exhibits a new type of charge dynamics in which excitations are local to such a degree that space and time axes are decoupled.

Quasiparticle interference and strong electron-mode coupling in the quasi-one-dimensional bands of Sr2RuO4

The normal state of the ruthenate Sr2RuO4 is not that of a conventional metal but one with enhanced correlation effects, which may help to elucidate the origin of the unconventional superconductivity observed in this material.

Structural contributions to the pressure-tuned charge-density-wave to superconductor transition in ZrTe 3 : Raman scattering studies

Author(s): Gleason, SL; Gim, Y; Byrum, T; Kogar, A; Abbamonte, P; Fradkin, E; MacDougall, GJ; Van Harlingen, DJ; Zhu, Xiangde; Petrovic, C; Cooper, SL | Abstract: Superconductivity evolves as functions of pressure or doping from charge-ordered phases in a variety of strongly correlated systems, suggesting that there may be universal characteristics associated with the competition between superconductivity and charge order in these materials. We present an inelastic light (Raman) scattering study of the structural changes that precede the pressure-tuned charge-density-wave (CDW) to superconductor transition in one such system, ZrTe3. In certain phonon bands, we observe dramatic linewidth reductions that accompany CDW formation, indicating that these phonons couple strongly to the electronic degrees of freedom associated with the CDW. The same phonon bands, which represent internal vibrations of ZrTe3 prismatic chains, are suppressed at pressures above ~10 kbar, indicating a loss of long-range order within the chains, specifically amongst intrachain Zr-Te bonds. These results suggest a distinct structural mechanism for the observed pressure-induced suppression of CDW formation and provide insights into the origin of pressure-induced superconductivity in ZrTe3.

Reexamination of the effective fine structure constant of graphene as measured in graphite

Author(s): Gan, Yu; de la Pena, Gilberto A; Kogar, Anshul; Uchoa, Bruno; Casa, Diego; Gog, Thomas; Fradkin, Eduardo; Abbamonte, Peter | Abstract: We present a refined and improved study of the influence of screening on the effective fine structure constant of graphene,

Crystallographic refinement of collective excitations using standing wave inelastic X-ray scattering

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Surface Collective Modes in the Topological Insulators Bi2Se3 and Bi0.5Sb1.5Te3-xSex

, as measured in graphite using inelastic x-ray scattering. This follow-up to our previous study [J. P. Reed, et al., Science 330, 805 (2010)] was carried out with two times better energy resolution, five times better momentum resolution, and improved experimental setup with lower background. We compare our results to RPA calculations and evaluate the relative importance of interlayer hopping, excitonic corrections, and screening from high energy excitations involving the

Measurement of the dynamic charge response of materials using low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS)

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