08 Fakultät Mathematik und Physik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9

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2021 - 202423

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Institute: search.filters.UBSInstitut.1. Physikalisches InstitutStart date: 2021

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Now showing 1 - 10 of 23
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    Tuning charge order in (TMTTF)2X by partial anion substitution
    (2021) Pustogow, Andrej; Dizdarevic, Daniel; Erfort, Sebastian; Iakutkina, Olga; Merkl, Valentino; Untereiner, Gabriele; Dressel, Martin
    In the quasi-one-dimensional (TMTTF)2X compounds with effectively quarter-filled bands, electronic charge order is stabilized from the delicate interplay of Coulomb repulsion and electronic bandwidth. The correlation strength is commonly tuned by physical pressure or chemical substitution with stoichiometric ratios of anions and cations. Here, we investigate the charge-ordered state through partial substitution of the anions in (TMTTF)2[AsF6]1-x[SbF6]x with x≈0.3, determined from the intensity of infrared vibrations, which is sufficient to suppress the spin-Peierls state. Our dc transport experiments reveal a transition temperature TCO = 120 K and charge gap ΔCO=430 K between the values of the two parent compounds (TMTTF)2AsF6 and (TMTTF)2SbF6. Upon plotting the two parameters for different (TMTTF)2X, we find a universal relationship between TCO and ΔCO yielding that the energy gap vanishes for transition temperatures TCO≤60 K. While these quantities indicate that the macroscopic correlation strength is continuously tuned, our vibrational spectroscopy results probing the local charge disproportionation suggest that 2δ is modulated on a microscopic level.
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    Microwave properties of superconducting SrTiO3 at mK-temperatures
    (2022) Beydeda, Cenk
    In this thesis the properties of superconducting Nb-doped SrTiO3 are investigated, more concrete the optical conductivity was obtained as function of temperature, magnetic flux density and frequency. Superconducting Stripline resonators were used to probe the optical properties of Nb:SrTiO3. The optical conductivity of Nb:SrTiO3 reveals features that are typically associated with a dirty single-gap superconductor. At low frequencies the coherence peak predicted by the BCS theory is observed. In the type II superconductor Nb:SrTiO3 two critical magnetic flux densities are observed that correspond to two superconducting bands. The real part of the optical conductivity displays a strong initial increase in dependence of magnetic flux density even at lowest achieved temperature to values multiple times of the DC conductivity. The critical magnetic flux densities and the critical temperatures show a dome-shaped dependence on the Nb-doping.
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    Charge degrees of freedom in quasi-two-dimentional organic conductors
    (2022) Iakutkina, Olga; Dressel, Martin (Prof. Dr.)
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    Low-energy optical conductivity of TaP : comparison of theory and experiment
    (2021) Yaresko, Alexander; Pronin, Artem V.
    The ab-plane optical conductivity of the Weyl semimetal TaP is calculated from the band structure and compared to the experimental data. The overall agreement between theory and experiment is found to be best when the Fermi level is slightly (20 to 60 meV) shifted upwards in the calculations. This confirms a small unintentional doping of TaP, reported earlier, and allows a natural explanation of the strong low-energy (50 meV) peak seen in the experimental ab-plane optical conductivity: this peak originates from transitions between the almost parallel non-degenerate electronic bands split by spin-orbit coupling. The temperature evolution of the peak can be reasonably well reproduce by calculations using an analog of the Mott formula.
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    Spectroscopic investigations of kagome - honeycomb quantum magnets
    (2022) Biesner, Tobias; Dressel, Martin (Prof. Dr.)
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    Rise and fall of Landau’s quasiparticles while approaching the Mott transition
    (2021) Pustogow, Andrej; Saito, Yohei; Löhle, Anja; Sanz Alonso, Miriam; Kawamoto, Atsushi; Dobrosavljević, Vladimir; Dressel, Martin; Fratini, Simone
    Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m⋆. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe-Regel-Mott limit is accompanied by a ‘displaced Drude peak’ in the optical conductivity. Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition.
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    Dielectric properties on the insulating side of the superconductor-insulator transition
    (2021) Ebensperger, Nikolaj Gabriel; Dressel, Martin (Prof. Dr.)
    Amorphous indium oxide is the prime example of strongly disordered superconductors showing a quantum phase transition at strong levels of disorder, where a disorder-driven superconductor-insulator transition (SIT) is found. While there are studies focusing on the superconducting side, the insulating side is much less explored because experimental means are lacking. In this work an experimental approach is presented, employing coplanar microwave resonators in GHz frequency range and down to mK-temperatures, giving access to dielectric properties and the finite-frequency conductivity of amorphous indium oxide as a function of temperature, frequency and disorder. It is found that depending on disorder, indium oxide has a large dielectric constant with a critical value of ~100, separating samples with and without localized Cooper-pairs. From scaling, the fractal nature of electronic eigenstates is inferred. Strong effects of a gap are found, giving direct evidence for a finite pairing pseudogap even in the insulating phase, perfectly matching fractal Anderson localization theory.
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    Fractional power-law intraband optical conductivity in the low-dimensional Dirac material CaMnBi2
    (2021) Schilling, Micha Benjamin; Wang, C. X.; Shi, You-Guo; Kremer, Reinhard Karl; Dressel, Martin; Pronin, Artem V.
    We studied the broadband optical conductivity of CaMnBi2, a material with two-dimensional Dirac electronic bands, and found that both components of the intraband conductivity follow a universal power law as a function of frequency at low temperatures. This conductivity scaling differs from the Drude(-like) behavior, generally expected for free carriers, but matches the predictions for the intraband response of an electronic system in a quantum critical region. Since no other indications of quantum criticality are reported for CaMnBi2 so far, the cause of the observed unusual scaling remains an open question.
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    Low-temperature dielectric anomaly arising from electronic phase separation at the Mott insulator-metal transition
    (2021) Pustogow, Andrej; Rösslhuber, Roland; Tan, Yuting; Uykur, Ece; Böhme, Anette; Wenzel, Maxim; Saito, Yohei; Löhle, Anja; Hübner, Ralph; Kawamoto, Atsushi; Schlueter, John A.; Dobrosavljević, Vladimir; Dressel, Martin
    Coulomb repulsion among conduction electrons in solids hinders their motion and leads to a rise in resistivity. A regime of electronic phase separation is expected at the first-order phase transition between a correlated metal and a paramagnetic Mott insulator, but remains unexplored experimentally as well as theoretically nearby T = 0. We approach this issue by assessing the complex permittivity via dielectric spectroscopy, which provides vivid mapping of the Mott transition and deep insight into its microscopic nature. Our experiments utilizing both physical pressure and chemical substitution consistently reveal a strong enhancement of the quasi-static dielectric constant ε1 when correlations are tuned through the critical value. All experimental trends are captured by dynamical mean-field theory of the single-band Hubbard model supplemented by percolation theory. Our findings suggest a similar ’dielectric catastrophe’ in many other correlated materials and explain previous observations that were assigned to multiferroicity or ferroelectricity.
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    Dielectric anomaly and charge fluctuations in the non-magnetic dimer Mott insulator λ-(BEDT-STF)2GaCl4
    (2021) Iakutkina, Olga; Rösslhuber, Roland; Kawamoto, Atsushi; Dressel, Martin
    The dimer Mott insulator l-(BEDT-STF)2GaCl4 undergoes no magnetic order down to the lowest temperatures, suggesting the formation of a novel quantum disordered state. Our frequency and temperature-dependent investigations of the dielectric response reveal a relaxor-like behavior below T ≈ 100 K for all three axes, similar to other spin liquid candidates. Optical measurement of the charge-sensitive vibrational mode n27(b1u) identifies a charge disproportionation Dr ≈ 0.04e on the dimer that exists up to room temperature and originates from inequivalent molecules in the weakly coupled dimers. The linewidth of the charge sensitive mode is broader than that of typical organic conductors, supporting the existence of a disordered electronic state.