Unveiling electronic correlations in layered molecular conductors by optical spectroscopy

Diese Arbeit handelt von den elektronischen Eigenschaften stark korrelierter Elektronensysteme, insbesondere optischen Untersuchungen nahe des Mott Metall-Isolator-Übergangs. Die experimentelle Bestimmung der Korrelationsstärke erlaubt eine einheitliche Darstellung aller Mott-Isolatoren in einem universellen Phasendiagramm. Unser Konzept ermöglicht erstmals einen quantitativen Vergleich unter verschiedenen Verbindungen, sowie zwischen Experiment und Theorie. Damit begründen wir ein neues Fundament für das Verständnis von Elektron-Elektron Wechselwirkungen in Mott-Systemen und den darauf basierendenden Phänomenen, allen voran unkonventioneller Supraleitung

High thermoelectric performance in metallic NiAu alloys

Thermoelectric (TE) materials seamlessly convert thermal into electrical energy and vice versa, making them promising for applications such as power generation or cooling. Although historically the TE effect was first discovered in metals, state-of-the-art research mainly focuses on doped semiconductors with large figure of merit, $zT$, that determines the conversion efficiency of TE devices. While metallic alloys have superior functional properties, such as high ductility and mechanical strength, they have mostly been discarded from investigation in the past due to their small Seebeck effect. Here, we realize unprecedented TE performance in metals by tuning the energy-dependent electronic scattering. Based on our theoretical predictions, we identify binary NiAu alloys as promising candidate materials and experimentally discover colossal power factors up to 34 mWm$^{-1}$K$^{-2}$ (on average 30 mWm$^{-1}$K$^{-2}$ from 300 to 1100 K), which is more than twice larger than in any known bulk material above room temperature. This system reaches a $zT$ up to 0.5, setting a new world record value for metals. NiAu alloys are not only orders of magnitude more conductive than heavily doped semiconductors, but also have large Seebeck coefficients originating from an inherently different physical mechanism: within the Au s band conduction electrons are highly mobile while holes are scattered into more localized Ni d states, yielding a strongly energy-dependent carrier mobility. Our work challenges the common belief that good metals are bad thermoelectrics and presents an auspicious paradigm for achieving high TE performance in metallic alloys through engineering electron-hole selective s-d scattering

Internal strain tunes electronic correlations on the nanoscale

Da 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

Evidence for even parity unconventional superconductivity in Sr2RuO4

Funding: A.C. is grateful for support from the Julian Schwinger Foundation for Physics Research. A.P. acknowledges support by the Alexander von Humboldt Foundation through the Feodor Lynen Fellowship. Work at Los Alamos was funded by Laboratory Directed Research and Development (LDRD) program, and A.P. acknowledges partial support through the LDRD. N.K. acknowledges the support by the Grants-in-Aid for Scientific Research (KAKENHI, Grant JP18K04715 and JP21H01033) from Japan Society for the Promotion of Science (JSPS). The work at Dresden was funded by the Deutsche Forschungsgemeinschaft - TRR 288 - 422213477 (projects A10 and B01). The work at University of California, Los Angeles, was supported by NSF Grants 1709304 and 2004553.Unambiguous identification of the superconducting order parameter symmetry in Sr2RuO4 has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid 3He-A was ruled out only very recently, other proposed triplet-pairing scenarios are still viable. Establishing the condensate magnetic susceptibility reveals a sharp distinction between even-parity (singlet) and odd-parity (triplet) pairing since the superconducting condensate is magnetically polarizable only in the latter case. Here field-dependent 17O Knight shift measurements, being sensitive to the spin polarization, are compared to previously reported specific heat measurements for the purpose of distinguishing the condensate contribution from that due to quasiparticles. We conclude that the shift results can be accounted for entirely by the expected field-induced quasiparticle response. An upper bound for the condensate magnetic response of < 10% of the normal state susceptibility is sufficient to exclude all purely odd-parity candidates. PostprintPeer reviewe

Upper Critical Field of Sr2_2RuO4_4 under In-Plane Uniaxial Pressure

In-plane uniaxial pressure has been shown to strongly tune the superconducting state of Sr$_2$RuO$_4$ by approaching a Lifshitz transition and associated Van Hove singularity (VHS) in the density of states. At the VHS, $T_c$ and the in- and out-of-plane upper critical fields are all strongly enhanced, and the latter has changed its curvature as a function of temperature from convex to concave. However, due to strain inhomogeneity it has not been possible so far to determine how the upper critical fields change with strain. Here, we show the strain dependence of both upper critical fields, which was achieved due to an improved sample preparation. We find that the in-plane upper critical field is mostly linear in $T_c$. On the other hand, the out-of-plane upper critical field varies with a higher power in $T_c$, and peaks strongly at the VHS. The strong increase in magnitude and the change in form of $H_\mathrm{c2||c}$ occur very close to the Van Hove strain, and points to a strong enhancement of both the density of states and the gap magnitude at the Lifshitz transition