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Head of the Institute
Prof. Dr. Matthieu Le Tacon
Karlsruher Institut für Technologie
Institut für Festkörperphysik
Hermann-von-Helmholtz-Platz 1
D-76344 Eggenstein-Leopoldshafen
Postal Address
Karlsruhe Institute of Technology
Institut für Festkörperphysik
P.O. Box 3640
D-76021 Karlsruhe
Phone ++49-721608-26751
Fax       ++49-721608-24624
Carmen DoerflingerCyd8∂kit edu

KIT Campus North
Building 425


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New Materials, Transport, Thermodynamics and Mesoscopics

Head: Dr. Christoph Meingast

We investigate thermodynamic and transport properties of solid state systems with strong electronic correlations. We also study electronic transport at mesoscopic length scales. The experiments strongly rely upon high-quality single crystals which are grown in-house. Our current interests focuses on unconventional superconductivity, quantum critical phenomena, magnetism, single-electron devices and nanoscaled Josephson junctions arrays.

We use a variety of experimental techniques (high-resolution capacitance dilatometry, calorimetry, magnetometry and state-of-the-art transport measurements) in a broad temperature range (10 mK - 500 K) and in magnetic fields up to 14 Tesla.

Fe-based Superconductors

Superconductivity in the Fe-based materials occurs in the vicinity of a magnetic/structural critical point, suggesting that magnetic/structural fluctuations may be important for the pairing mechanism. The nematic susceptibility, i.e., the susceptibility of the electronic order driving the structural transition, has turned out to be a good measure of such fluctuations. In order to gain access to the nematic susceptibility of hole-doped Ba1-xKxFe2As2 and electron-doped Ba(Fe1-xCox)2As2 superconductors, a new technique was developed by which the shear modulus is measured using a three-point bending setup in a capacitance dilatometer. Nematic fluctuations, although weakened by doping, were found to extend over the whole superconducting dome in both systems, suggesting their close link to superconductivity. Evidence for quantum critical behavior of the nematic susceptibility was, surprisingly, only found for Ba(Fe1-xCox)2As2, the system with the lower maximal Tc value. Further, a scaling relation between nuclear magnetic resonance (NMR) and shear modulus data in the tetragonal phase of electron-doped Ba(Fe1-xCox)2As2 compounds was found, which provides strong evidence for a magnetically-driven structural transition.

Nematic susceptibility (color-coded) of hole- and electron-doped BaFe2As2 superconductors as a function of dopant concentration and temperature.

Selected publications

  • Nematic susceptibility of hole- and electron-doped BaFe2As2 iron-based superconductors
    A.E. Böhmer, P. Burger, F. Hardy, T. Wolf, P. Schweiss, R. Fromknecht, M. Reinecker, W. Schranz, C. Meingast
    Phys. Rev. Lett. 112 (2014) 47001.
  • Scaling between magnetic and lattice fluctuations in iron-pnictide superconductors
    R. M. Fernandes, A. E. Böhmer, C. Meingast, J. Schmalian
    Phys. Rev. Lett. 111 (2013) 137001.
  • Doping Evolution of Superconducting Gaps and Electronic Densities of States
    in Ba(Fe1-xCox)2As2 Iron Pnictides

    F. Hardy, P. Burger, T. Wolf, R.A. Fisher, P. Schweiss, P. Adelmann, R. Heid, R. Fromknecht, R. Eder, D. Ernst, H. v. Löhneysen, C. Meingast
    Europhys. Lett. 91 (2010) 47008.
  • Pressure versus Concentration Tuning of the Superconductivity in Ba(Fe1-xCox)2As2
    S. Drotziger, P. Schweiss, K. Grube, T. Wolf, P. Adelmann, C. Meingast, H.v.Löhneysen
    J. Phys. Soc. Jpn. 79 (2010) 124705.

Heavy-Fermion Compounds

Continuous magnetic phase transitions driven to zero temperature by a non-thermal control parameter yielding a quantum critical point (QCP). The excitations close to the QCP as measured, e.g., by the specific heat, do not follow the predictions of Fermi-liquid theory. Furthermore, often novel phases emerge in the proximity of a QCP. The heavy-fermion system CeCoIn5 is a well-established member of the growing family of compounds exhibiting unconventional superconductivity close to antiferromagnetic order. Locating the the exact position of a QCP is however difficult, because it is masked by superconductivity. Our thermal-expansion and magnetostriction measurements on CeCoIn5 single crystals indicate that the pronounced deviations from Fermi-liquid behavior arise from a quantum critical line, i.e., an antiferromagnetic phase boundary in the field and pressure plane at zero temperature that is shielded by superconductivity. Together with pressure and doping data, a three-dimensional phase diagram showing this phase boundary could be constructed.

(p,B,T) phase diagram of CeCoIn5 (AF: antiferromagnetic phase, SC: superconducting phase, QC: quantum critical line in the (B,p) plane at T=0, not observable for small magnetic fields due to superconductivity)

Selected publications

  • Towards the Identification of a Quantum Critical Line in the (p,B) Phase Diagram of CeCoIn5 with Thermal-Expansion Measurements
    S. Zaum, K. Grube, R. Schäfer, E.D. Bauer, J.D. Thompson, H. v. Löhneysen
    Phys. Rev. Lett. 106 (2011) 87003.
  • Evolution of the magnetic structure in CeCu5.5Au0.5 under pressure towards quantum criticality
    A. Hamann, O. Stockert, V. Fritsch, K. Grube, A. Schneidewind, H.v. Löhneysen
    Phys. Rev. Lett. 110 (2013) 96404.
  • Hidden Low-Temperature Instability in PrOs4Sb12
    K. Grube, S. Zaum, E.D. Bauer, M.B. Maple, H. v. Löhneysen
    Phys. Status Solidi B 247 (2010) 571.

Electronic Transport in Normalconducting and Superconducting Nanostructures
(R. Schäfer)

We fabricate and investigate single-charge devices of normal metals and superconductors. Superconducting tunnel contacts ('Josephson contacts') may carry a supercurrent if the phase of the superconducting order parameter on both sides of the contact is suitably fixed. However, fixing the phase phase of the superconducting condensate excludes determination of the involved charge. This leads to a peculiar transport behavior of mesoscopic Josephson-junction devices which is the focus of our present research. In particular, we investigate chains where supercurrents can be switched on and off by magnetic fields.

Top: Scanning electron micrograph of a Josephson junction chain formed by bone shaped aluminum islands. The actual Josephson junctions result from overlaps of the islands seen at the top and bottom of the right inset.
Bottom: Current through such a chain of 256 mesoscopic Josephson junction at 0.01 K as function of voltage and magnetic field.

Selected publications

  • Josephson vortex coupled to a flux qubit
    K. G. Fedorov, A. V. Shcherbakova, R. Schäfer, A. V. Ustinov
    Appl. Phys. Lett. 102 (2013) 132602.
  • Aluminum Hard Mask Technique for the Fabrication of High Quality Submicron Nb/Al-AlOx/Nb Josephson Junctions
    Ch. Kaiser, J.M. Meckbach, K.S. Ilin, J. Lisenfeld, R. Schäfer, A.V. Ustinov, M. Siegel
    Supercond. Sci. Technol. 24 (2011) 35005.
  • Two-dimensional simulations of temperature and current-density distribution in electromigrated structures
    B. Kießig, H. v. Löhneysen, R. Schäfer
    New J. Phys. 16 (2014) 13017.
  • Thermally activated conductance in arrays of small Josephson junctions
    J. Zimmer, N. Vogt, A. Fiebig, S. V. Syzranov, A. Lukashenko, R. Schäfer, H. Rotzinger, A. Shnirman, M. Marthaler, A. V. Ustinov
    Phys. Rev. B 88 (2013) 144506.

Crystal Growth (A.-A. Haghighirad)

Our laboratory is equipped for the preparation of samples in single-crystalline and polycrystalline form. Single crystals are grown using mainly growth from the melt or flux, as well as vapor-growth techniques. The growth parameters are optimized according to a detailed compositional and structural characterization of the crystals in order to continuously improve the degree of perfection of the crystals. If required crystals with a particular impurity doping concentration can be grown.

For sample preparation, chamber furnaces, top-loading furnaces and tubular furnaces up to working temperatures of 1800 °C, Czochralski furnace, 3-zone furnace, induction furnace, arc-melt furnace, mirror furnace, autoclave furnace are available.

Superconducting and related materials:
TiN, ZrN, HfN, TiC, YBa2Cu3Ox-family (La-Lu), BaCuO2, Ba2CuO3, CuO, (Nd1-xCex)2CuO4, (Gd1-xNdx)2CuO4, BaFe2As2, SrFe2As2, CaFe2As2, KFe2As2, RbFe2As2, CsFe2As2, KxFe2Se2, FeAs, FeSe, FeTe, FeTe2, FeI2, MgB2, AlB2, (Ca/Sr)CuO2, Ca(Al0.5Si0.5)2, NbSe2.
Quantum critical materials:
YbNiSi3, YbNiGe3, YbPdSn, LaCu6, CeCu6-xAux,YbB2, YbB4, YbAlB4, CrB2, Cr2B3, CrAlB4.
Magnetic and other materials:
BaNi2V2O8, BaNi2-xMgxV2O8, Ni3V2O8, Co3V2O8, MnSi (Al, Co, Fe), Ba2V3O9, Ba2V2O7, Ba3V4O13, Cs2CuCl4, Ti4O7, Zn4Sb3, YBaCo4O7, La2Mo2O9, Co3TeO6.

Fe2GeO4 (left) and FeSe single crystals (right)

Selected publications

The Helmholtz Young Investigator Group (YIG) "Strain Tuning of Correlated Electronic Phases " (A. E. Böhmer)

In many interesting materials the electronic properties which are based on the interaction of the electrons are closely related to the structural properties of the crystal lattice. We make use of this relation to manipulate the electronic properties. The simplest way to realize this is to use the different thermal expansion of a sample and a substrate. As an example, non-superconducting samples of Co-doped CaFe2As2 glued to glass subtrates thus become superconducting [s. A. E. Böhmer et al., Phys. Rev. Lett. 118, 107002 (2017)]. Moreover, we use piezo actuators to generate uniaxial deformation. The change of the electrical resistance induced by such a deformation, the so-called elastoresistance, is particularly interesting. E.g. it is used to investigate the nematic susceptibility of iron-based superconductors.

Shift of the phase diagram of Ca(Fe1-xCox)2As2 induced by biaxial strain. For high Co-content strain induces superconductivity in an actually non-superconducting material
[A. E. Böhmer et al., Phys. Rev. Lett. 118, 107002 (2017)].

Selected publications

  • Distinct pressure evolution of coupled nematic and magnetic orders in FeSe
    A. E. Böhmer, K. Kothapalli, W. T. Jayasekara, J. M. Wilde, B. Li, A. Sapkota, B. G. Ueland, P. Das, Y. Xiao, W. Bi, J. Zhao, E. E. Alp, S. L. Bud'ko, P. C. Canfield, A. I. Goldman, A. Kreyssig
    Phys. Rev. B 100 (2019) 64515.
  • Persistent correlation between superconductivity and antiferromagnetic fluctuations near a nematic quantum critical point in FeSe1−xSx
    P. Wiecki, K. Rana, A. E. Böhmer, Y. Lee, S. L. Bud'ko, P. C. Canfield
    Phys. Rev. B 98 (2018) 20507.
  • Ingredients for the electronic nematic phase in FeSe revealed by its anisotropic optical response
    M. Chinotti, A. Pal, L. Degiorgi, A. E. Böhmer, and P. C. Canfield
    Phys. Rev. B 98 (2018) 94506.

Current group members (in alphabetical order)

Former group members

Dr. Peter Adelmann, Dr. Samira Barakat, Thomas Brugger, Bernd Burbulla, Dr. Philipp Burger, Dr. Georg Burkhart, Dr. May Chiao, Dr. Wanyin Cui, Prof. Dr. Gordon Czjzek, Dr. Stefanie Drobnik, Dr. Sandra Drotziger, Dipl.-Phys. Felix Eilers, Doris Ernst, Dr. Erxi Feng, Andreas Fiebig, Dr. Rainer Fromknecht, Mingquan He, Dr. Chien-Lung Huang, Dr. Devang Joshi, Dr. Heinz Küpfer, Steffen Kaus, Dr. Birgit Kiessig, Dr. William Knafo, Sebastian Kuntz, Sebastian Kuntz, Dr. Bernhard Limbach, Prof. Dr. Rolf Lortz, Dr. Nata Matskevich, Dr. Peter Nagel, Dr. Volker Pasler, Dr. Paul Popovich, Prof. Dr. Mark E. Reeves, Dr. Burkhard Renker, Dr. Peter Schweiss, Anastasia Shcherbakova, Dr. Tatjana Skipa, Prof. Dr. Alexander Soldatov, Dr. Anke Sypli, Dr. Peter vom Stein, Dr. V. Voronkova, Dr. Christoph Wallisser, Dr. Liran Wang, Thomas Woisczyk, Dr. Thomas Wolf, Prof. Dr. I. K. Yanson, Dr. Sebastian Zaum, Dr. Qin Zhang, et al.