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Group of Prof. Lorenz

Preparation and investigations of highly correlated transition metal oxides

Crystal Growth and Characterization

Our group focuses on complex transition metal compounds with partially filled d- or f-shells. Such materials show a plethora of unusual and fascinating phenomena, such as high-temperature superconductivity, colossal magnetoresistance, multiferroic ordering phenomena, low-dimensional quantum magnetism or quantum phase transitions. All these effects arise from many-body physics of interacting electrons, which cause fluctuations and/or complex ordering phenomena of the spin, charge, lattice or orbital degrees of freedom. The understanding of the interplay of these degrees of freedom is of fundamental importance and goes far beyond the material-specific questions. The continuous search of new phenomena and systematic studies of proper model systems in order to identify the underlying microscopic mechanisms are of fundmental interest for modern solid-state physics.

Thermodynamics (Magnetism, Specific Heat, Thermal Expansion)

Our in-house experimental techniques include crystal growth and characterization, measurements of different thermodynamic and magnetic properties as well as the study of electric and thermal transport properties. Depending on the experimental demands, these measurements can be performed in a temperature range from about 20 mK up to 1000K and in external magnetic fields up to 17 Tesla.

Transport (e.g. resistivity, Hall effect, thermal conductivity)

One focus of our research is the study of low-dimensional spin systems. Here, magnetic-field induced quantum phase transitions as well as the influence of magnetic frustration are of particular interest [1-4]. Other resaerch topics are the study of the thermal transport properties of so-called spin-ice, the magnetic analogon to water-ice [5,6], and the inluence of spin-state transitions on magnetic, structural and electric properties [7-9]. More literature about our work can be found here.

All topics above are open for bachelor and master students to conduct part of our research within a thesis project. Depending on your personal interests, either more technical aspects (e.g. setting up new measurement environments) or more theoretical and analytical interpretation of experimental data may become the focus of the work. In case of interest on our research, please do not hesistate to contact us for a meeting.  Thomas Lorenz

 

 

[1] see e.g.: S. Sachdev and B. Keimer. Quantum criticality Phys. Today 64, 29 (2011) http://dx.doi.org/10.1063/1.3554314

[2] M. Valldor, O. Heyer, A. C. Komarek, A. Senyshyn, M. Braden and T. Lorenz. Magnetostrictive Neel ordering of the spin-5/2 ladder compound BaMn2O3: Distortion-induced lifting of geometrical frustration Phys. Rev. B 83, 024418 (2011)

[3] J. Rohrkamp, M.D. Phillips, M.M. Turnbull and T. Lorenz. Thermal expansion of the spin-1/2 Heisenberg-chain compound Cu(C4H4N2)(NO3)2 JPCS 200, 012169, (2010)

[4] T. Lorenz, O. Heyer, M. Garst, F. Anfuso, A. Rosch, Ch. Rüegg and K. Krämer. Diverging thermal expansion of the spin-ladder system (C5H12N)2CuBr4 Phys. Rev. Lett. 100, 067208, (2008)

[5] see e.g.: C. Castelnovo, R. Moessner and S. L. Sondhi Magnetic monopoles in spin ice Nature 451, 42 (2008)

[6] G. Kolland, O. Breunig, M. Valldor, M. Hiertz, J. Frielingsdorf and T. Lorenz. Thermal conductivity and specific heat of the spin-ice compound Dy2Ti2O7: Experimental evidence for monopole heat transport Phys. Rev. B 86, 060402(R) (2012)

[7] N. Hollmann, M. W. Haverkort, M. Benomar, M. Cwik, M. Braden and T. Lorenz. Evidence for a temperature-induced spin-state transition of Co3+ in La2-xSrxCoO4 Phys. Rev. B, 83, 174435 (2011)

[8] A. V. Kalinov, O. Yu. Gorbenko, A. N. Taldenkov, J. Rohrkamp, O. Heyer, S. Jodlauk, N. A. Babushkina, L. M. Fisher, A. R. Kaul, A. A. Kamenev, T. G. Kuzmova, D. I. Khomskii, K. I. Kugel and T. Lorenz. Phase diagram and isotope effect in (Pr1−yEuy)0.7Ca0.3CoO3 cobaltites exhibiting spin-state transitions Phys. Rev. B 81, 134427 (2010)

[9] M. Kriener, M. Braden, H. Kierspel, D. Senff, O. Zabara, C. Zobel and T. Lorenz. Magnetic and structural transitions in La1−xAxCoO3 (A=Ca, Sr, and Ba) Phys. Rev. B 79, 224104 (2009)