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Orbital control of effective dimensionality: from spin-orbital fractionalization to confinement in the anisotropic ladder system CaCu2O3
Authors:
Valentina Bisogni,
Krzysztof Wohlfeld,
Satoshi Nishimoto,
Claude Monney,
Jan Trinckauf,
Kejin Zhou,
Roberto Kraus,
Klaus Koepernik,
Chinnathambi Sekar,
Vladimir Strocov,
Bernd Buechner,
Thorsten Schmitt,
Jeroen van den Brink,
Jochen Geck
Abstract:
Fractionalization of an electronic quasiparticle into spin, charge and orbital parts is a fundamental and characteristic property of interacting electrons in one dimension. However, real materials are never strictly one-dimensional and the fractionalization phenomena are hard to observe. Here we studied the spin and orbital excitations of the anisotropic ladder material CaCu2O3, whose electronic s…
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Fractionalization of an electronic quasiparticle into spin, charge and orbital parts is a fundamental and characteristic property of interacting electrons in one dimension. However, real materials are never strictly one-dimensional and the fractionalization phenomena are hard to observe. Here we studied the spin and orbital excitations of the anisotropic ladder material CaCu2O3, whose electronic structure is not one-dimensional. Combining high-resolution resonant inelastic x-ray scattering experiments with theoretical model calculations we show that: (i) spin-orbital fractionalization occurs in CaCu2O3 along the leg direction x through the xz orbital channel as in a 1D system; and (ii) no fractionalization is observed for the xy orbital, which extends in both leg and rung direction, contrary to a 1D system. We conclude that the directional character of the orbital hopping can select different degrees of dimensionality. Using additional model calculations, we show that spin-orbital separation is generally far more robust than the spin-charge separation. This is not only due to the already mentioned selection realized by the orbital hopping, but also due to the fact that spinons are faster than the orbitons.
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Submitted 5 March, 2015; v1 submitted 30 October, 2013;
originally announced October 2013.
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Femtosecond dynamics of magnetic excitations from resonant inelastic x-ray scattering in CaCu2O3
Authors:
Valentina Bisogni,
Stefanos Kourtis,
Claude Monney,
Kejin Zhou,
Roberto Kraus,
Chinnathambi Sekar,
Vladimir Strocov,
Bernd Buechner,
Jeroen van den Brink,
Lucio Braicovich,
Thorsten Schmitt,
Maria Daghofer,
Jochen Geck
Abstract:
Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them and determine their res…
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Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them and determine their respective timescales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are to a very good approximation independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence.
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Submitted 31 July, 2013;
originally announced July 2013.
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Effect of rotation of feed and seed rods on the quality of Na0.75CoO2 single crystal grown by traveling solvent floating zone method
Authors:
C. Sekar,
S. Paulraj,
P. Kanchana,
B. Schupp-Niewa,
R. Klingeler,
G. Krabbes
Abstract:
High purity Na0.75CoO2 single crystals have been grown by floating zone method. We found the rotation of feed and seed rods play a crucial role in growing high quality single crystal. Systematic investigations suggest the occurrence of a phase separation at microscopic level, such as the separation into Na-rich and Na-poor domains during the growth, and formation of impurity phase(s) depending on…
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High purity Na0.75CoO2 single crystals have been grown by floating zone method. We found the rotation of feed and seed rods play a crucial role in growing high quality single crystal. Systematic investigations suggest the occurrence of a phase separation at microscopic level, such as the separation into Na-rich and Na-poor domains during the growth, and formation of impurity phase(s) depending on growth conditions. NaxCoO2 (x = 0.30, 0.60) crystals have been prepared by sodium deintercalation from Na0.75CoO2. Powder X-ray and energy dispersive X-ray analyses have confirmed the phase purity and homogeneity of the samples. Magnetic susceptibility measurements of x = 0.60 and 0.75 crystals indicate a bulk phase transition at 22 K and an anomaly around 339 K and 334 K respectively which are attributed to structural transition. In addition some unique features related to Na0.60CoO2 were observed.
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Submitted 19 March, 2011;
originally announced March 2011.
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Confinement of fractional quantum number particles in a condensed matter system
Authors:
B. Lake,
A. M. Tsvelik,
S. Notbohm,
D. A. Tennant,
T. G. Perring,
M. Reehuis,
C. Sekar,
G. Krabbes,
B. Büchner
Abstract:
The idea of confinement states that in certain systems constituent particles can be discerned only indirectly being bound by an interaction whose strength increases with increasing particle separation. Though the most famous example is the confinement of quarks to form baryons and mesons in (3+1)-dimensional Quantum Chromodynamics, confinement can also be realized in the systems of condensed mat…
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The idea of confinement states that in certain systems constituent particles can be discerned only indirectly being bound by an interaction whose strength increases with increasing particle separation. Though the most famous example is the confinement of quarks to form baryons and mesons in (3+1)-dimensional Quantum Chromodynamics, confinement can also be realized in the systems of condensed matter physics such as spin-ladders which consist of two spin-1/2 antiferromagnetic chains coupled together by spin exchange interactions. Excitations of individual chains (spinons) carrying spin S=1/2, are confined even by an infinitesimal interchain coupling. The realizations studied so far cannot illustrate this process due to the large strength of their interchain coupling which leaves no energy window for the spinon excitations of individual chains. Here we present neutron scattering experiments for a weakly-coupled ladder material. At high energies the spectral function approaches that of individual chains; at low energies it is dominated by spin 0,1 excitations of strongly-coupled chains.
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Submitted 25 September, 2009; v1 submitted 7 August, 2009;
originally announced August 2009.
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Magnetic heat conductivity in $\rm\bf CaCu_2O_3$: linear temperature dependence
Authors:
C. Hess,
H. ElHaes,
A. Waske,
B. Büchner,
C. Sekar,
G. Krabbes,
F. Heidrich-Meisner,
W. Brenig
Abstract:
We present experimental results for the thermal conductivity $κ$ of the pseudo 2-leg ladder material $\rm CaCu_2O_3$. The strong buckling of the ladder rungs renders this material a good approximation to a $S=1/2$ Heisenberg-chain. Despite a strong suppression of the thermal conductivity of this material in all crystal directions due to inherent disorder, we find a dominant magnetic contribution…
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We present experimental results for the thermal conductivity $κ$ of the pseudo 2-leg ladder material $\rm CaCu_2O_3$. The strong buckling of the ladder rungs renders this material a good approximation to a $S=1/2$ Heisenberg-chain. Despite a strong suppression of the thermal conductivity of this material in all crystal directions due to inherent disorder, we find a dominant magnetic contribution $κ_\mathrm{mag}$ along the chain direction.
$κ_\mathrm{mag}$ is \textit{linear} in temperature, resembling the low-temperature limit of the thermal Drude weight $D_\mathrm{th}$ of the $S=1/2$ Heisenberg chain. The comparison of $κ_\mathrm{mag}$ and $D_\mathrm{th}$ yields a magnetic mean free path of $l_\mathrm{mag}\approx 22 \pm 5$ Å, in good agreement with magnetic measurements.
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Submitted 12 December, 2006;
originally announced December 2006.
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X-ray absorption spectroscopy on layered cobaltates Na_xCoO_2
Authors:
T. Kroll,
M. Knupfer,
J. Geck,
C. Hess,
T. Schwieger,
G. Krabbes,
C. Sekar,
D. R. Batchelor,
H. Berger,
B. Buechner
Abstract:
Measurements of polarization and temperature dependent soft x-ray absorption have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co^{3+} and Co^{4+} sites. With the help of a cl…
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Measurements of polarization and temperature dependent soft x-ray absorption have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co^{3+} and Co^{4+} sites. With the help of a cluster calculation we are able to interpret the Co L_{23}-edge absorption spectrum and find a doping dependent energy splitting between the t_{2g} and the e_g levels (10Dq) in Na_xCoO_2.
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Submitted 20 June, 2006;
originally announced June 2006.
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Optical study of orbital excitations in transition-metal oxides
Authors:
R. Rückamp,
E. Benckiser,
M. W. Haverkort,
H. Roth,
T. Lorenz,
A. Freimuth,
L. Jongen,
A. Möller,
G. Meyer,
P. Reutler,
B. Büchner,
A. Revcolevschi,
S. -W. Cheong,
C. Sekar,
G. Krabbes,
M. Grüninger
Abstract:
The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging fr…
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The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.
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Submitted 24 June, 2005; v1 submitted 16 March, 2005;
originally announced March 2005.
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The unusual electronic structure of the "pseudo-ladder" compound CaCu2O3
Authors:
T. K. Kim,
H. Rosner,
S. -L. Drechsler,
Z. Hu,
C. Sekar,
G. Krabbes,
J. Malek,
M. Knupfer,
J. Fink,
H. Eschrig
Abstract:
Experimental and theoretical studies of the unoccupied electronic structure of CaCu2O3 single crystals have been performed using polarization-dependent x-ray absorption spectroscopy and band structure calculations. The measured hole distribution shows an unusual large number of holes in orbitals parallel to the interlayer direction which is in agreement with the theoretical analysis. CaCu2O3 dev…
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Experimental and theoretical studies of the unoccupied electronic structure of CaCu2O3 single crystals have been performed using polarization-dependent x-ray absorption spectroscopy and band structure calculations. The measured hole distribution shows an unusual large number of holes in orbitals parallel to the interlayer direction which is in agreement with the theoretical analysis. CaCu2O3 deviates significantly from the standard pd-sigma cuprate picture. The corresponding strong interlayer exchange is responsible for the missing spin gap generic for other two-leg ladder cuprates.
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Submitted 8 August, 2002;
originally announced August 2002.