Disentangling lattice and electronic instabilities in the excitonic insulator candidate Ta2_2NiSe5_5 by nonequilibrium spectroscopy

Ta$_2$NiSe$_5$ is an excitonic insulator candidate showing the semiconductor/semimetal-to-insulator (SI) transition below $T_{\text{c}}$ = 326 K. However, since a structural transition accompanies the SI transition, deciphering the role of electronic and lattice degrees of freedom in driving the SI transition has remained controversial. Here, we investigate the photoexcited nonequilibrium state in Ta$_2$NiSe$_5$ using pump-probe Raman and photoluminescence (PL) spectroscopies. The combined nonequilibrium spectroscopic measurements of the lattice and electronic states reveal the presence of a photoexcited metastable state where the insulating gap is suppressed, but the low-temperature structural distortion is preserved. We conclude that electron correlations play a vital role in the SI transition of Ta$_2$NiSe$_5$.Comment: 13 pages, 10 figure

Strong electron-phonon coupling and enhanced phonon Gr\"uneisen parameters in valence-fluctuating metal EuPd2_2Si2_2

We study the valence crossover and strong electron-phonon coupling of EuPd$_2$Si$_2$ by polarization-resolved Raman spectroscopy. The fully-symmetric phonon mode shows strongly asymmetric lineshape at low temperature, indicating Fano-type interaction between this mode and a continuum of electron-hole excitations. Moreover, the frequency and linewidth of the phonon modes exhibit anomalies across the valence-crossover temperature, suggesting the coupling between valence fluctuations and lattice vibration. In particular, two phonon modes show significantly enhanced Gr\"uneisen parameter, suggesting proximity to a critical elasticity regime. The relative contribution of the structural change and valence change to the phonon anomalies is evaluated by density functional theory calculations

An electronic nematic liquid in BaNi2_2As2_2

Understanding the organizing principles of interacting electrons and the emergence of novel electronic phases is a central endeavor of condensed matter physics. Electronic nematicity, in which the discrete rotational symmetry in the electron fluid is broken while the translational one remains unaffected, is a prominent example of such a phase. It has proven ubiquitous in correlated electron systems, and is of prime importance to understand Fe-based superconductors. Here, we find that fluctuations of such broken symmetry are exceptionally strong over an extended temperature range above phase transitions in BaNi$_2$(As$_{1−x}$P$_x$)$_2$, the nickel homologue to the Fe-based systems. This lends support to a type of electronic nematicity, dynamical in nature, which exhibits a particularly strong coupling to the underlying crystal lattice. Fluctuations between degenerate nematic configurations cause splitting of phonon lines, without lifting degeneracies nor breaking symmetries, akin to spin liquids in magnetic systems

Strain-Tuning of 2D and 3D Charge-Density Waves in High-Temperature Superconducting YBa2_{2}Cu3_{3}Oy_{\rm{y}}

Uniaxial pressure experiments in underdoped YBa$_{2}$Cu$_{3}$O$_{\rm{y}}$ provide an efficient approach to the control of the competition between charge-density waves (CDWs) and superconductivity. It can enhance the correlation volume of ubiquitous short-range CDW correlations and above a critical value, even induce a long-range CDW order otherwise only accessible through the suppression of superconductivity by large magnetic fields. Here we use x-ray diffraction with access to large areas of reciprocal space to study the evolution of long- and short-range CDWs with in-plane strains and as a function of doping. This further allows us to precisely monitor in-situ the structural changes induced by uniaxial pressurization of the crystals for a precise strain estimation in measurements up to $-0.85 \%$ compression. Interestingly, we uncover direct evidence for a competition between long- and short-range CDWs and show that the long-range CDW modulation remains incommensurate at all investigated strains and temperatures, showing neither signs of discommensurations nor a pair-density wave component at $\lambda_{\rm{PDW}} = 2\lambda_{\rm{CDW}}$ below $T_c$. We discuss the impact of structural disorder and the relationship of our findings to previous reports on nematicity in high-temperature superconducting cuprates. More generally, our results underscore the potential of strain tuning as a powerful tool for probing and manipulating competing orders in quantum materials.Comment: I. Vinograd and S. M. Souliou contributed equally to this wor

Untersuchung von Hochtemperatursupraleitern unter hohem Druck

The current thesis studies experimentally the effect of high external pressure on high-Tc superconductors. The structure and lattice dynamics of several members of the high-Tc cuprate and Fe-based superconductors families were investigated by means of Raman spectroscopy and x-ray diffraction under well-controlled, hydrostatic high pressure and low temperature conditions. The lattice dynamics of the high-Tc superconductor YBa(2)Cu(3)O(6+x) have been investigated systematically by Raman spectroscopy as a function of doping (x = 0.95, 0.75, 0.60, 0.55, and 0.45) and external pressure. Under ambient pressure conditions, in addition to the Raman modes expected from group theory, we observe new Raman active phonons upon cooling the underdoped samples, at temperatures well above the superconducting transition temperature. The doping dependence and the onset temperatures of the new Raman features suggest that they are associated with the incommensurate charge density wave (CDW) state recently discovered in underdoped cuprates using synchrotron x-ray scattering techniques. Under high pressure conditions (from 2 to 12 GPa), our Raman measurements on highly ordered underdoped YBa(2)Cu(3)O(6.55) samples do not show any of the new Raman phonons seen at ambient pressure. High pressure and low temperature Raman measurements have been performed on the underdoped superconductor YBa(2)Cu(4)O(8). A clear renormalization of some of the Raman phonons is seen below Tc as a result of the changes in the phonon self-energy upon the opening of the superconducting gap, with the most prominent one being that of the B1g-like buckling phonon mode. The amplitude of this renormalization strongly increases with pressure, resembling the effect of hole doping in YBa(2)Cu(3)O(6+x). At 10 GPa, the system undergoes a reversible pressure-induced structural phase transition to a non-centrosymmmetric structure (space group Imm2). The structural transition is clearly reflected in the high pressure Raman data through the appearance of several new modes, allowing us to map in detail the (P,T) phase diagram and determine the transition line between the two phases. In the new phase, the renormalization of the buckling mode is completely suppressed, while no anomalies are observed in any of the other Raman active phonons. According to ab initio calculations, the coupling of the buckling mode to the electronic system is not significantly affected by the structural phase transition. The absence of phonon renormalizations in the presence of sizable electron-phonon coupling, indicate that, in contrast to earlier transport studies, YBa(2)Cu(4)O(8) is not superconducting anymore under hydrostatic pressures higher than 10 GPa. Finally we proceeded with the investigation of the high pressure structural and vibrational properties of SmFeAsO, a member of the "1111" family (space group P4/nmm) of the Fe-based superconductors, in which superconductivity is commonly induced either by substituting F/H for O or by applying high pressures on the parent magnetic compound. The magnetic transition of the undoped compound is accompanied with a tetragonal-to-orthorhombic structural distortion, both of which are commonly suppressed upon the emergence of superconductivity. In the SmFeAsO(x)F(1-x) system while the magnetic transition is totally suppressed already at low doping levels, structural studies have reported either the gradual suppression of the orthorhombic distortion or its retention over a wide regime of the superconducting phase. We addressed this controversy using high pressure as an alternative tuning parameter to suppress the magneto-structural transition and induce superconductivity in the parent compound. Our high pressure, low temperature x-ray diffraction measurements on single crystals of SmFeAsO have revealed that the tetragonal-to-orthorhombic transition survives with the application of high pressures up to 85 kbars. In addition, our Raman data reveal a linewidth renormalization of the c-axis polarized A1g and B1g Raman phonons through the magnetic transition, similar to the one previously observed in the "122" family and attributed to the opening of the spin density wave gap. The renormalization is gradually suppressed under high pressure, in line with an earlier high pressure magnetization study which reported the pressure-induced decrease of the magnetic transition temperature. The linewidth anomaly disappears at 8 GPa suggesting the complete suppression of the magnetic transition at this pressure.Die vorliegende Arbeit beschäftigt sich experimentell mit dem Einfluss von hohem, externem Druck auf Hochtemperatursupraleiter. Die Dynamik von Struktur- und Gitterparametern verschiedener Vertreter der cuprat- und eisenbasierten Supraleitern wurde unter präzise kontrolliertem, hohem hydrostatischem Druck und tiefen Temperaturen mittels Ramanspektroskopie und Röntgendiffraktion untersucht. Die Gitter-Schwingungen des Hochtemperatur-Supraleiters YBa(2)Cu(3)O(6+x) wurden systematisch mittels Raman-Spektroskopie als Funktion von Dotierung (x = 0.95, 0.75, 0.60, 0.55, und 0.45) und externem Druck untersucht. Bei Normaldruck und tiefen Temperaturen, jedoch noch deutlich oberhalb des Übergangs zum supraleitenden Zustand, beobachten wir für unterdotierte Proben neue Raman-aktive Phononen zusätzlich zu den gemäß der Gruppen-Theorie erwarteten Raman-Moden. Das Einsetzen der neuen Raman-Moden unterhalb bestimmter Temperaturen und ihre Dotierungsabhängigkeit legen einen Zusammenhang mit der inkommensurablen Ladungsdichtewelle (CDW) nahe, die kürzlich durch Methoden der Synchrotron- Röntgenstreuung in unterdotierten Cupraten entdeckt wurde. Unter hohem Druck (von 2 bis 12 GPa) zeigen unsere Raman-Messungen an hochgeordnetem, unterdotiertem YBa(2)Cu(3)O(6.55) keine der neuen Raman-Phononen, die bei Normaldruck beobachtet wurden. Ramanmessungen unter hohem Druck und tiefen Temperaturen wurden an dem unterdotierten Supraleiter YBa(2)Cu(4)O(8) durchgeführt. Dabei wird unterhalb der kritischen Temperatur eine deutliche Renormalisierung einiger Ramanmoden beobachtet, die eine Folge von Veränderungen in der Phononeigenenergie durch die Öffnung der supraleitenden Bandlücke ist. Die markanteste hiervon ist die B1g-ähnliche Buckling-Mode. Das Ausmaß dieser Renormalisierung nimmt mit dem Druck stark zu, ähnlich dem Einfluss von Lochdotierung in YBa(2)Cu(3)O(6+x). Bei einem Druck von etwa 10 GPa geht das System durch einen reversiblen, druckinduzierten, strukturellen Phasenübergang zu einer nicht-punktsymmetrischen Struktur über (Raumgruppe Imm2). Der Strukturübergang spiegelt sich durch das Auftreten neuer Moden eindeutig in den Ramandaten wider. Dadurch ist es uns möglich das (P,T)-Phasendiagramm im Detail abzubilden und die Grenze zwischen den beiden Phasen zu bestimmen. In der neuen Phase wird die Renormalisierung der Buckling-Mode vollständig unterdrückt. Zugleich werden in keinem anderen Raman-aktiven Phonon Anomalien beobachtet. Ab-initio-Berechnungen zeigen, dass die Kopplung zwischen der Buckling-Mode und dem elektronischen System nicht signifikant durch den Phasenübergang beeinflusst wird. Die Abwesenheit von Phononrenormalisierungen bei gleichzeitiger Anwesenheit beträchtlicher Elektronen-Phononen-Kopplung weist darauf hin, dass, im Gegensatz zu älteren Transportmessungen, YBa(2)Cu(4)O(8) unter einem hydrostatischen Druck von über 10 GPa nicht mehr supraleitend ist. Abschließend untersuchten wir die strukturellen Eigenschaften und Gitterschwingungen von SmFeAsO unter hohem Druck. Dieses ist ein Vertreter der "1111"-Familie (Raumgruppe P4/nmm) der eisen-basierten Supraleiter, in der die Supraleitung für gewöhnlich entweder durch die Substitution von O durch F/H oder durch den Einsatz hoher Drücke im undotierten Material hervorgerufen wird, dessen magnetischer Übergang durch eine Änderung der Struktur von tetragonal zu orthorhombisch begleitet wird. Beides wird normalerweise durch das Auftreten der Supraleitung unterdrückt. Während der magnetische Übergang in dem System von SmFeAsO(x)F(1-x) bereits bei niedriger Dotierung komplett unterdrückt wird, zeigten strukturelle Untersuchungen entweder die allmähliche Verdrängung der orthorhombischen Verzerrung oder ihre Beibehaltung über einen weiten Bereich der supraleitenden Phase. Diese Kontroverse brachte uns dazu, hohe Drücke als weiteren, fein justierbaren Parameter anzuwenden, um den magneto-strukturellen Übergang zu unterdrücken und im undotierten Material Supraleitung hervorzurufen. Unsere Röntgenbeugungsmessungen unter hohem Druck und bei niedrigen Temperaturen an SmFeAsO-Einkristallen zeigten, dass der strukturelle Übergang bei Drücken bis zu 85 kbar fortbesteht. Außerdem zeigten unsere Ramandaten eine Renormalisierung der Linienbreite der in Richtung der c-Achse polarisierten A1g- und B1g-Raman- Phononen beim Durchschreiten des magnetischen Übergangs. Ähnliches wurde zuvor im "122"-System beobachtet und der Öffnung einer Bandlücke durch das Auftreten einer Spindichtewelle zugesprochenen. Die Renormalisierung wird durch hohe Drücke graduell unterdrückt. Dies ist in Übereinstimmung mit früheren Magnetisierungsmessungen, die gezeigt hatten, dass die magnetische Übergangstemperatur mit höheren Drücken sinkt. Die Anomalie in der Linienbreite verschwindet bei 8 GPa, was die vollständige Unterdrückung des magnetischen Übergangs bei diesem Druck andeutet

Evidence for nesting-driven charge density wave instabilities in the quasi-two-dimensional material LaAgSb2_{2}

Since their theoretical prediction by Peierls in the 1930s, charge density waves (CDWs have been one of the most commonly encountered electronic phases in low-dimensional metallic systems. The instability mechanism originally proposed combines Fermi surface nesting and electron-phonon coupling but is, strictly speaking, only valid in one dimension. In higher dimensions, its relevance is questionable as sharp maxima in the static electronic susceptibility χ(q) are smeared out, and is, in many cases, unable to account for the periodicity of the observed charge modulations. Here, we investigate the quasi-two-dimensional LaAgSb2, which exhibits two CDW transitions, by a combination of diffuse x-ray scattering, inelastic x-ray scattering, and ab initio calculations. We demonstrate that the CDW formation is driven by phonon softening. The corresponding Kohn anomalies are visualized in three dimensions through the momentum distribution of the x-ray diffuse scattering intensity. We show that they can be quantitatively accounted for by considering the electronic susceptibility calculated from a Dirac-like band, weighted by anisotropic electron-phonon coupling. This remarkable agreement sheds new light on the importance of Fermi surface nesting in CDW formation

Raman Scattering from Higgs Mode Oscillations in the Two-Dimensional Antiferromagnet Ca2RuO4

We present and analyze Raman spectra of the Mott insulator Ca2RuO4, whose quasi-two-dimensional antiferromagnetic order has been described as a condensate of low-lying spin-orbit excitons with angular momentum J(eff) = 1. In the A(g) polarization geometry, the amplitude (Higgs) mode of the spin-orbit condensate is directly probed in the scalar channel, thus avoiding infrared-singular magnon contributions. In the B-1g geometry, we observe a single-magnon peak as well as two-magnon and two-Higgs excitations. Model calculations using exact diagonalization quantitatively agree with the observations. Together with recent neutron scattering data, our study provides strong evidence for excitonic magnetism in Ca2RuO4 and points out new perspectives for research on the Higgs mode in two dimensions.1111sciescopu