08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Tuning charge order in (TMTTF)2X by partial anion substitution(2021) Pustogow, Andrej; Dizdarevic, Daniel; Erfort, Sebastian; Iakutkina, Olga; Merkl, Valentino; Untereiner, Gabriele; Dressel, MartinIn 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.Item Open Access Field- and frequency dependent transport in the two-dimensional organic conductor α-(BEDT-TTF)2I3(1994) Dressel, Martin; Grüner, George; Pouget, Jean-Paul; Breining, Angela; Schweitzer, DieterWe studied the electrodynamic response of α-(BEDT-TTF)2I3 in a wide range of frequency, covering microwave and millimeter wave frequencies as well as the optical spectral range, and found a frequency dependent conductivity up to 1000 cm-1 in the low temperature phase. This is accompanied be a non-linear transport with a smooth onset at about 10 V /cm. Our X-ray studies show no indication of superstructure reflections and clearly rule out the formation of a charge density wave ground state. The lack of a temperature dependence in the millimeter wave conductivity between 20 K and 100 K makes hopping transport unlikely.Item Open Access Charge-order phase transition in the quasi one-dimensional organic conductor (TMTTF)2NO3(2020) Majer, Lena N.; Miksch, Björn; Lesseux, Guilherme Gorgen; Untereiner, Gabriele; Dressel, MartinLow-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the (TMTTF)2X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor (TMTTF)2NO3. We found that the (TMTTF)2NO3 compound develops a peculiar anisotropy with a doubled periodicity (ab′-plane) of the ESR linewidth below about TCO=(250±10) K. This behavior is similar to observations in the related compounds (TMTTF)2X (X=PF6, SbF6 and AsF6), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous (TMTTF)2ClO4, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below TCO≈250 K.Item Open Access Microwave hall mobility and conductivity in crystals of various (BEDT-TTF) radical salts(1991) Dressel, Martin; Helberg, Hans Wilhelm; Schweitzer, DieterWe report investigations of the microwave transport properties carried out on the organic conductors and superconductors of (BEDT-TTF)-salts: α-, αt- and β-(BEDT-TTF)2I3, α-(BEDT-TTF)3(NO3)2 and x-(BEDT-TTF)2Cu(NCS)2. The anisotropy and temperature dependence (300 K to 4 K) of the high-frequency conductivity was measured in a microwave cavity at 10.3 GHz. There is low anisotropy in the high conducting plane of the quasi two-dimensional crystals of the (BEDT-TTF)-family, e.g. the (a,b)-plane in β-(BEDT-TTF)2I3; perpendicular to this the microwave conductivity is one order of magnitude lower. At room temperature the microwave Hall mobility at 9.5 GHz of the different phases of (BEDT-TTF)2I3 and (BEDT-TTF)3(NO3)2 is 100 to 200 cm2/Vs in the high conducting planes of the crystals.Item Open Access 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, SimoneLandau 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.Item Open Access 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.Item Open Access 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, MartinCoulomb 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.Item Open Access Dielectric anomaly and charge fluctuations in the non-magnetic dimer Mott insulator λ-(BEDT-STF)2GaCl4(2021) Iakutkina, Olga; Rösslhuber, Roland; Kawamoto, Atsushi; Dressel, MartinThe 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.Item Open Access Internal strain tunes electronic correlations on the nanoscale(2018) Pustogow, Andrej; McLeod, Alexander S.; Saito, Yohei; Basov, Dmitri N.; Dressel, MartinDa die Strukturen innerhalb von Festkörpern am Phasenübergang von Metallen zu Isolatoren meist kleiner sind als die Wellenlänge des Lichts, kann man sie nicht mit einem normalen Mikroskop beobachten. Daher nutzten die Stuttgarter Physiker ein Nahfeld-Mikroskop. Bei diesem macht man sich zunutze, dass eine atomar dünne Spitze ganz knapp über dem Material Licht streut und tiefe Blicke in die lokalen elektronischen Eigenschaften gibt. So konnten die Wissenschaftler auch an einem molekularen Kristall den Metall-Isolator-Phasenübergang untersuchen, der dort bei -138 Grad Celsius (136 K) auftritt. Es wurden scharfe Grenzen zwischen metallischen und isolierenden Gebieten beobachtet, was zweifelsfrei einen Phasenübergang erster Ordnung nachgeweist, der durch elektronische Wechselwirkungen getrieben wird. Zudem entsteht ein charakteristisches ("Zebra-") Streifenmuster als Folge mechanischer Verspannungen im Kristall. Dies liefert wichtige Erkenntnisse, welch wichtigen Einfluss die mechanische Integrität einer chemisch reinen Probe auf die makroskopisch gemessenen physikalischen Eigenschaften haben kann.Item Open Access Spectroscopic trace of the Lifshitz transition and multivalley activation in thermoelectric SnSe under high pressure(2021) Biesner, Tobias; Li, Weiwu; Tsirlin, Alexander A.; Roh, Seulki; Wei, Pai-Chun; Uykur, Ece; Dressel, MartinMultivalley systems offer not only exciting physical phenomena but also the possibility of broad utilization. Identifying an important platform and understanding its physics are paramount tasks to improve their capability for application. Here, we investigate a promising candidate, the semiconductor SnSe, by optical spectroscopy and density functional theory calculations. Upon applying pressure to lightly doped SnSe, we directly monitored the phase transition from semiconductor to semimetal. In addition, heavily doped SnSe exhibited a successive Lifshitz transition, activating multivalley physics. Our comprehensive study provides insight into the effects of pressure and doping on this system, leading to promising routes to tune the material properties for advanced device applications, including thermoelectrics and valleytronics.