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The $c$-axis dimer and its electronic break-up: the insulator-to-metal transition in Ti$_2$O$_3$
Authors:
C. F. Chang,
T. C. Koethe,
Z. Hu,
J. Weinen,
S. Agrestini,
J. Gegner,
H. Ott,
G. Panaccione,
Hua Wu,
M. W. Haverkort,
H. Roth,
A. C. Komarek,
F. Offi,
G. Monaco,
Y. -F. Liao,
K. -D. Tsuei,
H. -J. Lin,
C. T. Chen,
A. Tanaka,
L. H. Tjeng
Abstract:
We report on our investigation of the electronic structure of Ti$_2$O$_3$ using (hard) x-ray photoelectron and soft x-ray absorption spectroscopy. From the distinct satellite structures in the spectra we have been able to establish unambiguously that the Ti-Ti $c$-axis dimer in the corundum crystal structure is electronically present and forms an $a_{1g}a_{1g}$ molecular singlet in the low tempera…
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We report on our investigation of the electronic structure of Ti$_2$O$_3$ using (hard) x-ray photoelectron and soft x-ray absorption spectroscopy. From the distinct satellite structures in the spectra we have been able to establish unambiguously that the Ti-Ti $c$-axis dimer in the corundum crystal structure is electronically present and forms an $a_{1g}a_{1g}$ molecular singlet in the low temperature insulating phase. Upon heating we observed a considerable spectral weight transfer to lower energies with orbital reconstruction. The insulator-metal transition may be viewed as a transition from a solid of isolated Ti-Ti molecules into a solid of electronically partially broken dimers where the Ti ions acquire additional hopping in the $a$-$b$ plane via the $e_g^π$ channel, the opening of which requires the consideration of the multiplet structure of the on-site Coulomb interaction.
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Submitted 24 October, 2017;
originally announced October 2017.
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Spin-orbital excitation continuum and anomalous electron-phonon interaction in the Mott insulator LaTiO$_3$
Authors:
C. Ulrich,
G. Khaliullin,
M. Guennou,
H. Roth,
T. Lorenz,
B. Keimer
Abstract:
Raman scattering experiments on stoichiometric, Mott-insulating LaTiO$_3$ over a wide range of excitation energies reveal a broad electronic continuum which is featureless in the paramagnetic state, but develops a gap of $\sim 800$ cm$^{-1}$ upon cooling below the Néel temperature $T_N = 146$ K. In the antiferromagnetic state, the spectral weight below the gap is transferred to well-defined spectr…
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Raman scattering experiments on stoichiometric, Mott-insulating LaTiO$_3$ over a wide range of excitation energies reveal a broad electronic continuum which is featureless in the paramagnetic state, but develops a gap of $\sim 800$ cm$^{-1}$ upon cooling below the Néel temperature $T_N = 146$ K. In the antiferromagnetic state, the spectral weight below the gap is transferred to well-defined spectral features due to spin and orbital excitations. Low-energy phonons exhibit pronounced Fano anomalies indicative of strong interaction with the electron system for $T > T_N$, but become sharp and symmetric for $T < T_N$. The electronic continuum and the marked renormalization of the phonon lifetime by the onset of magnetic order are highly unusual for Mott insulators and indicate liquid-like correlations between spins and orbitals.
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Submitted 17 September, 2015;
originally announced September 2015.
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Elemental Topological Insulator with a Tunable Fermi Level: Strained α-Sn on InSb(001)
Authors:
A. Barfuss,
L. Dudy,
M. R. Scholz,
H. Roth,
P. Höpfner,
C. Blumenstein,
G. Landolt,
J. H. Dil,
N. C. Plumb,
M. Radovic,
A. Bostwick,
E. Rotenberg,
A. Fleszar,
G. Bihlmayer,
D. Wortmann,
G. Li,
W. Hanke,
R. Claessen,
J. Schäfer
Abstract:
We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-pola…
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We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants.
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Submitted 9 September, 2013; v1 submitted 4 August, 2013;
originally announced August 2013.
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Momentum dependence of orbital excitations in Mott-insulating titanates
Authors:
C. Ulrich,
L. J. P. Ament,
G. Ghiringhelli,
L. Braicovich,
M. Moretti Sala,
N. Pezzotta,
T. Schmitt,
G. Khaliullin,
J. van den Brink,
H. Roth,
T. Lorenz,
B. Keimer
Abstract:
High-resolution resonant inelastic x-ray scattering has been used to determine the momentum dependence of orbital excitations in Mott-insulating LaTiO$_3$ and YTiO$_3$ over a wide range of the Brillouin zone. The data are compared to calculations in the framework of lattice-driven and superexchange-driven orbital ordering models. A superexchange model in which the experimentally observed modes a…
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High-resolution resonant inelastic x-ray scattering has been used to determine the momentum dependence of orbital excitations in Mott-insulating LaTiO$_3$ and YTiO$_3$ over a wide range of the Brillouin zone. The data are compared to calculations in the framework of lattice-driven and superexchange-driven orbital ordering models. A superexchange model in which the experimentally observed modes are attributed to two-orbiton excitations yields the best description of the data.
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Submitted 18 August, 2009;
originally announced August 2009.
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Magnetoelastic coupling in RETiO3 (RE = La, Nd, Sm, Gd, Y)
Authors:
A. C. Komarek,
H. Roth,
M. Cwik,
W. -D. Stein,
J. Baier,
M. Kriener,
F. Bouree,
T. Lorenz,
M. Braden
Abstract:
A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm, Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and the lattice which cannot be fully attributed to the structural deformation arising from bond-length mismatch. The TiO6 octahedra in this series are all irregular with the shape of the distortion depending on the RE ionic radius. These octahed…
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A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm, Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and the lattice which cannot be fully attributed to the structural deformation arising from bond-length mismatch. The TiO6 octahedra in this series are all irregular with the shape of the distortion depending on the RE ionic radius. These octahedron distortions vary more strongly with temperature than the tilt and rotation angles. Around the Ti magnetic ordering all compounds exhibit strong anomalies in the thermal-expansion coefficients, these anomalies exhibit opposite signs for the antiferromagnetic and ferromagnetic compounds. Furthermore the strongest effects are observed in the materials close to the magnetic cross-over from antiferromagnetic to ferromagnetic order.
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Submitted 18 January, 2007;
originally announced January 2007.
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Mott-Hubbard exciton in the optical conductivity of YTiO3 and SmTiO3
Authors:
A. Gössling,
R. Schmitz,
H. Roth,
M. W. Haverkort,
T. Lorenz,
J. A. Mydosh,
E. Müller-Hartmann,
M. Grüninger
Abstract:
In the Mott-Hubbard insulators YTiO3 and SmTiO3 we study optical excitations from the lower to the upper Hubbard band, d^1d^1 -> d^0d^2. The multi-peak structure observed in the optical conductivity reflects the multiplet structure of the upper Hubbard band in a multi-orbital system. Absorption bands at 2.55 and 4.15 eV in the ferromagnet YTiO3 correspond to final states with a triplet d^2 confi…
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In the Mott-Hubbard insulators YTiO3 and SmTiO3 we study optical excitations from the lower to the upper Hubbard band, d^1d^1 -> d^0d^2. The multi-peak structure observed in the optical conductivity reflects the multiplet structure of the upper Hubbard band in a multi-orbital system. Absorption bands at 2.55 and 4.15 eV in the ferromagnet YTiO3 correspond to final states with a triplet d^2 configuration, whereas a peak at 3.7 eV in the antiferromagnet SmTiO3 is attributed to a singlet d^2 final state. A strongly temperature-dependent peak at 1.95 eV in YTiO3 and 1.8 eV in SmTiO3 is interpreted in terms of a Hubbard exciton, i.e., a charge-neutral (quasi-)bound state of a hole in the lower Hubbard band and a double occupancy in the upper one. The binding to such a Hubbard exciton may arise both due to Coulomb attraction between nearest-neighbor sites and due to a lowering of the kinetic energy in a system with magnetic and/or orbital correlations. Furthermore, we observe anomalies of the spectral weight in the vicinity of the magnetic ordering transitions, both in YTiO3 and SmTiO3. In the G-type antiferromagnet SmTiO3, the sign of the change of the spectral weight at T_N depends on the polarization. This demonstrates that the temperature dependence of the spectral weight is not dominated by the spin-spin correlations, but rather reflects small changes of the orbital occupation.
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Submitted 20 May, 2008; v1 submitted 24 August, 2006;
originally announced August 2006.
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Magnetic heat transport in R$_2$CuO$_4$ (R = La, Pr, Nd, Sm, Eu, and Gd)
Authors:
K. Berggold,
T. Lorenz,
J. Baier,
M. Kriener,
D. Senff,
H. Roth,
A. Severing,
H. Hartmann,
A. Freimuth,
S. Barilo,
F. Nakamura
Abstract:
We have studied the thermal conductivity $κ$ on single crystalline samples of the antiferromagnetic monolayer cuprates R$_2$CuO$_4$ with R = La, Pr, Nd, Sm, Eu, and Gd. For a heat current within the CuO$_2$ planes, i.e. for $κ_{ab}$ we find high-temperature anomalies around 250 K in all samples. In contrast, the thermal conductivity $κ_c$ perpendicular to the CuO$_2$ planes, which we measured fo…
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We have studied the thermal conductivity $κ$ on single crystalline samples of the antiferromagnetic monolayer cuprates R$_2$CuO$_4$ with R = La, Pr, Nd, Sm, Eu, and Gd. For a heat current within the CuO$_2$ planes, i.e. for $κ_{ab}$ we find high-temperature anomalies around 250 K in all samples. In contrast, the thermal conductivity $κ_c$ perpendicular to the CuO$_2$ planes, which we measured for R = La, Pr, and Gd, shows a conventional temperature dependence as expected for a purely phononic thermal conductivity. This qualitative anisotropy of $κ_i$ and the anomalous temperature dependence of $κ_{ab}$ give evidence for a significant magnetic contribution $κ_{mag}$ to the heat transport within the CuO$_2$ planes. Our results suggest, that a large magnetic contribution to the heat current is a common feature of single-layer cuprates. We find that $κ_{mag}$ is hardly affected by structural instabilities, whereas already weak charge carrier doping causes a strong suppression of $κ_{mag}$.
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Submitted 9 March, 2006; v1 submitted 15 September, 2005;
originally announced September 2005.
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Pressure-driven orbital reorientation and change in Mott-Hubbard gap in YTiO3
Authors:
I. Loa,
X. Wang,
K. Syassen,
H. Roth,
T. Lorenz,
M. Hanfland,
Y. -L. Mathis
Abstract:
We investigate the crystal structure of YTiO3 at high pressures up to 30 GPa by synchrotron x-ray powder diffraction (T = 295 K). The variation of the Ti--O bond lengths with pressure evidences a distinct change in the distortion of the TiO_6 octahedra at around 10 GPa, indicating a pressure-driven spatial reorientation of the occupied Ti 3d(t_2g) orbitals. The pressure-induced reduction of the…
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We investigate the crystal structure of YTiO3 at high pressures up to 30 GPa by synchrotron x-ray powder diffraction (T = 295 K). The variation of the Ti--O bond lengths with pressure evidences a distinct change in the distortion of the TiO_6 octahedra at around 10 GPa, indicating a pressure-driven spatial reorientation of the occupied Ti 3d(t_2g) orbitals. The pressure-induced reduction of the optical band gap of YTiO3 is determined quantitatively by mid-infrared synchrotron micro-spectroscopy and discussed in terms of bond length and orbital orientation changes.
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Submitted 15 April, 2005;
originally announced April 2005.
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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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Orbital excitations in titanates
Authors:
C. Ulrich,
A. Gössling,
M. Grüninger,
M. Guennou,
H. Roth,
M. Cwik,
T. Lorenz,
G. Khaliullin,
B. Keimer
Abstract:
Raman scattering is used to observe pronounced electronic excitations around 230 meV - well above the two-phonon range - in the Mott insulators LaTiO$_3$ and YTiO$_3$. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of…
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Raman scattering is used to observe pronounced electronic excitations around 230 meV - well above the two-phonon range - in the Mott insulators LaTiO$_3$ and YTiO$_3$. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.
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Submitted 4 March, 2005;
originally announced March 2005.
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Determination of the orbital moment and crystal field splitting in LaTiO$_{3}$
Authors:
M. W. Haverkort,
Z. Hu,
A. Tanaka,
G. Ghiringhelli,
H. Roth,
M. Cwik,
T. Lorenz,
C. Schuessler-Langeheine,
S. V. Streltsov,
A. S. Mylnikova,
V. I. Anisimov,
C. de Nadai,
N. B. Brookes,
H. H. Hsieh,
H. -J. Lin,
C. T. Chen,
T. Mizokawa,
Y. Taguchi,
Y. Tokura,
D. I. Khomskii,
L. H. Tjeng
Abstract:
Utilizing a sum-rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO$_3$ is strongly reduced both below and above the Néel temperature. Using Ti $L_{2,3}$ x-ray absorption spectroscopy as a local probe, we found that the crystal field splitting in the $t_{2g}$ subshell is about 0.12-0.30 eV. This large splitting…
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Utilizing a sum-rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO$_3$ is strongly reduced both below and above the Néel temperature. Using Ti $L_{2,3}$ x-ray absorption spectroscopy as a local probe, we found that the crystal field splitting in the $t_{2g}$ subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.
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Submitted 23 September, 2004; v1 submitted 21 May, 2004;
originally announced May 2004.