H. S. Jeevan

University of Augsburg

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P. Gegenwart

University of Augsburg

C. Geibel

Max Planck Institute for Chemical Physics of Solids

F. Steglich

Max Planck Institute for Chemical Physics of Solids

O. Stockert

Max Planck Institute for Chemical Physics of Solids

Y. Tokiwa

University of Augsburg

M. Loewenhaupt

Heinz Maier-Leibnitz Zentrum

E. Faulhaber

Technical University of Munich

J. Fink

Leibniz Institute for Solid State and Materials Research

Z. Hossain

Indian Institute of Technology Kanpur

S. Thirupathaiah

S.N. Bose National Centre for Basic Sciences

Cooperative Institutions

Max Planck Institute for Chemical Physics of Solids

Max Planck Society

TU Dresden

Leibniz Institute for Solid State and Materials Research

Zhejiang University

University of Stuttgart

Chinese Academy of Sciences

Kyoto University

University of Augsburg

The University of Tokyo

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Transport properties of under hydrostatic pressure

Physica B Condensed Matter (2005)

Takeshi Nakanishi G. Sparn H. S. Jeevan M. Deppe C. Geibel

Hydrostatic pressure

Hydrostatic equilibrium

10.1016/j.physb.2005.01.021

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Citations (2)

Quasiquartet crystal-electric-field ground state with possible quadrupolar ordering in the tetragonal compoundYbRu2Ge2

Physical Review B (2006)

H. S. Jeevan C. Geibel Z. Hossain

We have investigated the magnetic properties of $\mathrm{Yb}{\mathrm{Ru}}_{2}{\mathrm{Ge}}_{2}$ by means of magnetic susceptibility $\ensuremath{\chi}(T)$, specific heat $C(T)$ and electrical resistivity $\ensuremath{\rho}(T)$ measurements performed on flux-grown single crystals. The Curie-Weiss behavior of $\ensuremath{\chi}(T)$ along the easy plane, the large magnetic entropy at low temperatures and the weak Kondo-like increase in $\ensuremath{\rho}(T)$ proves a stable trivalent Yb state. Anomalies in $C(T)$, $\ensuremath{\rho}(T)$, and $\ensuremath{\chi}(T)$ at ${T}_{0}=10.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, ${T}_{1}=6.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and ${T}_{2}=5.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ evidence complex ordering phenomena. The magnetic entropy just above ${T}_{0}$ amounts to almost $R\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}4$, indicating that the crystal-electric-field (CEF) ground state is a quasiquartet instead of the expected doublet. The behavior at ${T}_{0}$ is rather unusual and suggests that this transition is related to quadrupolar ordering, being a consequence of the CEF quasiquartet ground state. The combination of a quasiquartet CEF ground state, a high ordering temperature, and the relevance of quadrupolar interactions makes $\mathrm{Yb}{\mathrm{Ru}}_{2}{\mathrm{Ge}}_{2}$ a rather unique system among Yb-based compounds.

Tetragonal crystal system

Specific heat

10.1103/physrevb.73.020407

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Transport and thermal properties of weakly ferromagnetic Sr₂IrO₄

Journal of Physics Condensed Matter (2006)

N.S. Kini A. M. Strydom H. S. Jeevan C. Geibel S. Ramakrishnan

We have synthesized polycrystalline Sr2IrO4 and measured its magnetic susceptibility, electrical resistivity, specific heat, Seebeck coefficient, and thermal conductivity. The magnetic susceptibility χ(T) shows a ferromagnetic transition at 250 K while the behaviour above the transition temperature is well described by a Curie–Weiss fit with a small effective moment μeff = 0.33 μB and a paramagnetic Curie–Weiss temperature, θCW = +251 K, consistent with previous studies on this compound. However, specific heat, Seebeck coefficient, and thermal conductivity are all dominated by the phonon contribution and show no anomalies at the ferromagnetic transition. Electrical resistivity, unlike the single crystal, shows a huge increase, three orders of magnitude, with decreasing temperature. The temperature dependence of resistivity is logarithmic at high temperatures (210 K

10.1088/0953-8984/18/35/008

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Coexistence of superconductivity and ferromagnetism in P-dopedEuFe2As2

Physical Review B (2014)

S. Nandi Wentao Jin Yinguo Xiao Y. Su Stephen J. Price

The magnetic structure of the Eu${}^{2+}$ moments in the superconducting EuFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ sample with $x=0.15$ has been determined using element-specific x-ray resonant magnetic scattering. Combining magnetic, thermodynamic, and scattering measurements, we conclude that the long-range ferromagnetic order of the Eu${}^{2+}$ moments aligned primarily along the c axis coexists with the bulk superconductivity at zero field. At an applied magnetic field $\ensuremath{\ge}$0.6 T, superconductivity still coexists with the ferromagnetic Eu${}^{2+}$ moments, which are polarized along the field direction. We propose a spontaneous vortex state for the coexistence of superconductivity and ferromagnetism in EuFe${}_{2}$(As${}_{0.85}$P${}_{0.15}$)${}_{2}$.

10.1103/physrevb.89.014512

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Unique phase diagram with narrow superconducting dome in EuFe2(As1−xPx…

Physical Review B (2012)

Y. Tokiwa Sebastian Hübner O. Beck H. S. Jeevan P. Gegenwart

The interplay between superconductivity and Eu$ ^{2+}$ magnetic moments in EuFe$_2$(As$_{1-x}$P$_x$)$_2$ is studied by electrical resistivity measurements under hydrostatic pressure on $x=0.13$ and $x=0.18$ single crystals. We can map hydrostatic pressure to chemical pressure $x$ and show, that superconductivity is confined to a very narrow range $0.18\leq x \leq 0.23$ in the phase diagram, beyond which ferromagnetic (FM) Eu ordering suppresses superconductivity. The change from antiferro- to FM Eu ordering at the latter concentration coincides with a Lifshitz transition and the complete depression of iron magnetic order.

Hydrostatic pressure

Hydrostatic equilibrium

10.1103/physrevb.86.220505

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Citations (31)

Far-infrared optical conductivity of CeCu₂Si₂

Journal of Physics Condensed Matter (2013)

J. Sichelschmidt Alexander Herzog H. S. Jeevan C. Geibel F. Steglich

We investigated the optical reflectivity of the heavy-fermion metal CeCu 2 Si 2 in the energy range 3 meV -30 eV for temperatures between 4K -300K.The results for the charge dynamics indicate a behavior that is expected for the formation of a coherent heavy quasiparticle state: Upon cooling the spectra of the optical conductivity indicate a narrowing of the coherent response.Below temperatures of 30 K a considerable suppression of conductivity evolves below a peak structure at 13 meV.We assign this gap-like feature to strong electron correlations due to the 4f -conduction electron hybridization.

Optical conductivity

Charge carrier

10.1088/0953-8984/25/6/065602

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Scanning tunneling spectroscopy studies on heavy fermion superconductors

Verhandlungen der Deutschen Physikalischen Gesellschaft (2008)

Stefan Ernst S. Wirth H. S. Jeevan C. Geibel F. Steglich

Scanning tunneling spectroscopy (STS) is a powerful tool for mapping the local electronic density of states of conducting sample surfaces. Of particular interest are experiments with superconducting (SC) materials, as information about the SC energy gap can directly be obtained. The application of STS to heavy fermion (HF) superconductors is expected to make a valuable contribution to the understanding of this class of materials. Fundamental properties such as the symmetry of the SC order parameter or, possibly, excitations due to the SC pairing interaction might be revealed. This work reports on STM measurements carried out at low temperatures (320 mK) and under UHV conditions. A magnetic field of up to 12 T could be applied to confirm superconductivity of clean samples. For the materials investigated here, spectroscopic features in the order of a few hundred {mu}eV are expected, calling for an excellent energy resolution during STS. The sufficiently high resolution of our STM has been verified by resolving the sub-meV SC energy gap of aluminum. Clean sample surfaces were prepared by in situ cleaving of the single crystals. Preliminary STS measurements were conducted on single crystalline samples of the HF superconductors CeCoIn{sub 5} and CeCu{sub 2}Si{sub 2}.

Heavy fermion superconductor

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YbCo4Ge2 and YbNi5Ge3: two compounds with a stable trivalent, antiferromagnetically ordered Yb state

Physica B Condensed Matter (2005)

H. S. Jeevan Z. Hossain C. Geibel

10.1016/j.physb.2005.01.047

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Citations (2)

Varying Eu2+magnetic order by chemical pressure in EuFe2(As1

Physical Review B (2011)

S. Zapf Dan Wu Lapo Bogani H. S. Jeevan P. Gegenwart

Based on low-field magnetization measurements on a series of single crystals, we present a scheme of the Eu${}^{2+}$ spin alignment in EuFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$. We explain observations of the Eu${}^{2+}$ ordering previously reported, reconciling different existing phase diagrams. The magnetic moments of the Eu${}^{2+}$ ions are slightly canted, yielding a ferromagnetic contribution along the $c$ direction that becomes stronger with pressure, until superconductivity sets in. The spin-density wave as well as the superconducting phase coexist with an antiferromagnetic interlayer coupling of the canted spins. Reducing the interlayer distance finally leads to a ferromagnetic Eu${}^{2+}$ interlayer coupling and to the suppression of superconductivity.

10.1103/physrevb.84.140503

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Citations (55)

Physical Review B (2011)

S. Thirupathaiah E. D. L. Rienks H. S. Jeevan Ruslan Ovsyannikov E. Slooten

We have studied the electronic structure of EuFe2As2-xPx using high resolution angle-resolved photoemission spectroscopy. Upon substituting As with the isovalent P, which leads to a chemical pressure and to superconductivity, we observe a non-rigid-band like change of the electronic structure along the center of the Brillouin zone (BZ): an orbital and kz dependent increase or decrease in the size of the hole pockets near the Gamma - Z line. On the other hand, the diameter of the Fermi surface cylinders at the BZ corner forming electron pockets, hardly changes. This is in stark contrast to p and n-type doped iron pnictides where, on the basis of ARPES experiments, a more rigid-band like behavior has been proposed. These findings indicate that there are different ways in which the nesting conditions can be reduced causing the destabilization of the antiferromagnetic order and the appearance of the superconducting dome.

Inverse photoemission spectroscopy

10.1103/physrevb.84.014531

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Citations (38)