arXiv subjects

G. Güntherodt

Publications and source records attributed to G. Güntherodt.

At least 37 records · Page 2Linked to original sources

arXiv2006

Charge density waves in Sr_14-xCa_xCu_24O_41: electron correlations vs. structural effects

We report inelastic light scattering experiments in the spin-chain and ladder system Sr_14-xCa_xCu_24O_41. A depletion of an electronic background is observed for 0 < x < 5 at low temperatures and with light polarization both parallel to the leg and rung directions. This points to the formation of 2D charge density wave in the ladders. Furthermore, the Ca-substitution dependence of the 305- and 249-cm-1 modes, sensitive to the lattice modulations of the respective chain and ladder system, suggests that the charge dynamics in the ladders is strongly influenced by the structural incommensurability.

cond-mat.str-el↗

arXiv2005

Asymmetric magnetization reversal in the exchange bias system Fe/FeF_2 studied by MOKE

The asymmetry of the magnetization reversal process in exchange biased Fe/FeF$_2$ has been studied by magneto-optical Kerr effect. Qualitatively different transverse magnetization loops are observed for different directions of the cooling and the measuring field. These loops can be simulated by a simple calculation of the total energy density which includes the relevant magnetic anisotropies and coherent magnetization rotation only. Asymmetric magnetization reversal is shown to originate from the unidirectional anisotropy and may be observed if the external measuring field is not collinear with either the exchange bias or the easy axis of the antiferromagnetic epitaxial FeF$_2$(110) layer.

cond-mat.mtrl-sci↗

arXiv2005

Role of structural defects on exchange bias in the epitaxial CoO/Co system

We have studied the influence of non-magnetic defects throughout the antiferromagnet Co_{1-y}O on the exchange bias (EB) in epitaxially grown Co_{1-y}O/Co bilayers. These defects are either substitutional or structural (twin boundaries and surface morphology) which both lead to an increase of the EB-field. We find a dominance of twin boundaries over surface morphology (roughness) in enhancing EB which is consistent with the domain state model for exchange bias. In contrast, the crystal orientation of the Co_{1-y}O layer does not show a significant effect on the EB in this system.

cond-mat.mtrl-sci↗

arXiv2005

Magneto-optical study of magnetization reversal asymmetry in exchange bias

The asymmetric magnetization reversal in exchange biased Fe/MnF$_{2}$ involves coherent (Stoner-Wohlfarth) magnetization rotation into an intermediate, stable state perpendicular to the applied field. We provide here experimentally tested analytical conditions for the unambiguous observation of both longitudinal and transverse magnetization components using the magneto-optical Kerr effect. This provides a fast and powerful probe of coherent magnetization reversal as well as its chirality. Surprisingly, the sign and asymmetry of the transverse magnetization component of Fe/MnF$_{2}$ change with the angle between cooling and measurement fields.

cond-mat.mtrl-sci↗

arXiv2005

Hole stripe and orbital fluctuating state in LaMnO$_{3+δ}$ ($0.085\leq δ\leq0.125$) unveiled by Raman spectroscopy

Giant softening by 30 cm$^{-1}$ of the 490- and 620-cm$^{-1}$ modes is observed by Raman scattering measurements below the Curie temperature of single crystalline LaMnO$_{3+δ}$ ($0.085\leq δ\leq0.125$). A pseudogap-like electronic continuum and a Fano antiresonance at 144 cm$^{-1}$ appear below the charge ordering temperature. This gives evidence for the presence of an orbital fluctuating state and the formation of a hole stripe, respectively. This is further corroborated by a unstructured broadening and shifting of multiphonon features with increasing doping $δ$. Our study suggests the significance of double exchange mechanism in the charged ordered insulating state.

cond-mat.str-el↗

arXiv2005

Orbiton-mediated multi-phonon scattering in La$_{1-x}$Sr$_x$MnO$_3$

We report on Raman scattering measurements of single crystalline La$_{1-x}$Sr$_x$MnO$_3$ ($x$=0, 0.06, 0.09 and 0.125), focusing on the high frequency regime. We observe multi-phonon scattering processes up to fourth-order which show distinct features: (i) anomalies in peak energy and its relative intensity and (ii) a pronounced temperature-, polarization-, and doping-dependence. These features suggest a mixed orbiton-phonon nature of the observed multi-phonon Raman spectra.

cond-mat.str-el↗

arXiv2004

Growth and spin-resolved photoelectron spectroscopy of the MgO/Fe(110) system

Electronic and structural properties of ultrathin MgO layers grown on epitaxial Fe(110) films were investigated at room temperature by means of spin-resolved photoelectron spectroscopy, Auger-electron spectroscopy, and low energy electron diffraction. The spin polarization at the Fermi level of the Fe(110) film decreases abruptly with increasing thickness of MgO layer up to 7 Å. This behavior is attributed to the formation of a thin FeO layer at the MgO(111)/Fe(110) interface, attenuating the intrinsic spin polarization.

cond-mat.mtrl-sci↗

arXiv2004

Existence of orbital polarons in ferromagnetic insulating La$_{1-x}$Sr$_x$MnO$_{3}$ (0.11$<x<$0.14) evidenced by giant phonon softening

We present an inelastic light scattering study of single crystalline (La$_{1-y}$Pr$_y$)$_{1-x}$Sr$_{x}$MnO$_3$ ($0\leq x\leq0.14$,$y=0$ and $x=1/8$,$0\leq y\leq0.5$). A giant softening up to 20 - 30 cm$^{-1}$ of the Mn-O breathing mode has been observed only for the ferromagnetic insulating (FMI) samples ($0.11\leq x \leq 0.14$) upon cooling below the Curie temperature. With increasing Pr-doping the giant softening is gradually suppressed. This is attributed to a coupling of the breathing mode to orbital polarons which are present in the FMI phase.

cond-mat.str-el↗

arXiv1999

Magnetoelastic coupling and spin excitations in the spin-gap system (VO)$_2$P$_2$O$_7$: A Raman scattering study

Single crystals of the spin-gap system $\rm (VO)_2P_2O_7$ have been investigated by means of Raman scattering. At temperatures below T=75 K~$\approx 2Δ_{01}({\bf k}=0)$, with $Δ_{01}$ the singlet-triplet gap, a shoulder appears at 47 cm$^{-1}$. An analysis of the polarization selection rules and the temperature dependence of the scattering intensity leads to its identification as a singlet bound state with very small binding energy. Therefore in contrast to recent theoretical models the importance of frustration for this compound must be rejected. The observation of a strong anharmonicity of three phonons with energies close to $Δ_{01}$ at the zone boundary and additional quasielastic light scattering, however, both point to a strong spin-phonon coupling in this system. We propose this coupling to be the origin of the second bound triplet state observed in recent neutron scattering experiments.

cond-mat.supr-con↗

arXiv1999

Electron-phonon and spin-phonon coupling in NaV$_{2}$O$_{5}$: charge fluctuations effect

We show that the asymmetric crystal environment of the V site in the ladder compound NaV$_{2}$O$_{5}$ leads to a strong coupling of vanadium 3d electrons to phonons. This coupling causes fluctuations of the charge on the V ions, and favors a transition to a charge-ordered state at low temperatures. In the low temperature phase the charge fluctuations modulate the spin-spin superexchange interaction, resulting in a strong spin-phonon coupling.

cond-mat.str-el↗

arXiv1999

Quantum Spin Systems: From Spin Gaps to Pseudo Gaps

Many low dimensional spin systems with a dimerized or ladder-like antiferromagnetic exchange coupling have a gapped excitation spectrum with magnetic bound states within the spin gap. For spin ladders with an even number of legs the existence of spin gaps and within the t-J model a tendency toward superconductivity with d-wave symmetry is predicted. In the following we will characterize the spin excitation spectra of different low dimensional spin systems taking into account strong spin phonon interaction ($\rm CuGeO_3$), charge ordering ($\rm NaV_2O_5$) and doping on chains and ladders (\ladder). The spectroscopic characterization of the model systems mentioned above has been performed using magnetic inelastic light scattering originating from a spin conserving exchange scattering mechanism. This is also bound to yield more insight into the interrelation between these spin gap excitations and the origin of the pseudo gap in high temperature superconductors.

cond-mat.str-el↗

arXiv1999

Spin gap behavior and charge ordering in α^{\prime}-NaV_2O_5 probed by light scattering

We present a detailed analysis of light scattering experiments performed on the quarter-filled spin ladder compound $α^\prime$-NaV$_{2}$O$_{5}$ for the temperature range 5 K$\le$T$\le$300 K. This system undergoes a phase transition into a singlet ground state at T=34 K accompanied by the formation of a super structure. For T$\leq$34 K several new modes were detected. Three of these modes are identified as magnetic bound states. Experimental evidence for charge ordering on the V sites is detected as an anomalous shift and splitting of a V-O vibration at 422 cm$^{-1}$ for temperatures above 34 K. The smooth and crossover-like onset of this ordering at T$_{\rm CO}$= 80 K is accompanied by pretransitional fluctuations both in magnetic and phononic Raman scattering. It resembles the effect of stripe order on the super structure intensities in La$_2$NiO$_{4+δ}$.

cond-mat.str-el↗

arXiv1998

Magnetic bound states in the quarter-filled ladder system $α'-NaV_{2}O_{5}$}

Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5 shows in the dimerized singlet ground state ($T \leq T_{SP}=35K$) an unexpected sequence of three magnetic bound states. Our results suggest that the recently proposed mapping onto an effective spin chain for $T > T_{SP}$ has to be given up in favor of the full topology and exchange paths of a ladder in the dimerized phase for $T < T_{SP}$. As the new ground state we propose a dynamic superposition of energetically nearly degenerate dimer configurations on the ladder.

cond-mat.str-el↗

arXiv1998

Magnetic Bound States in Dimerized Quantum Spin Systems

Magnetic bound states are a general phenomenon in low dimensional antiferromagnets with gapped singlet states. Using Raman scattering on three compounds as dedicated examples we show how exchange topology, dimensionality, defects and thermal fluctuations influence the properties and the spectral weight of these states.

cond-mat.str-el↗

arXiv1997

Revival of the spin-Peierls transition in Cu_xZn_(1-x)GeO_3 under pressure

Pressure and temperature dependent susceptibility and Raman scattering experiments on single crystalline Cu_xZn_(1-x)GeO_3 have shown an unusually strong increase of the spin-Peierls phase transition temperature upon applying hydrostatic pressure. The large positive pressure coefficient (7.5 K/GPa) - almost twice as large as for the pure compound (4.5 K/GPa) - is interpreted as arising due to an increasing magnetic frustration which decreases the spin-spin correlation length, and thereby weakens the influence of the non-magnetic Zn-substitution.

cond-mat.str-el↗

arXiv1997

Observation of three-magnon light scattering in CuGeO_3

Temperature and magnetic field dependent polarized Brillouin spectra of CuGeO_3 are reported. In addition to a bound singlet state at 30 cm^{-1}, a new feature has been observed at 18 cm^{-1}. This feature is interpreted in terms of a novel three-magnon light scattering process between excited triplet states.

cond-mat.str-el↗

arXiv1997

Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3

The effect of in-chain and off-chain substitutions on 1D spin fluctuations in the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in order to understand the interplay between defect induced states, enhanced spin-spin correlations and the ground state of low dimensional systems. In-chain and off-chain substitutions quench the spin-Peierls state and induce 3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D gap-induced mode as well as a constant intensity of a spinon continuum are observed at low temperatures. A 3D two-magnon density of states now gradually extends to higher temperatures T\leq 60K compared with pure CuGeO_3. This effect is more pronounced in the case of off-chain substitutions (Si) for which a Néel state occurs over a larger substitution range, starting at very low concentrations. Besides, additional low energy excitations are induced. These effects, i.e. the shift of a dimensional crossover to higher temperatures are due to an enhancement of the spin-spin correlations induced by a small amount of substitutions. The results are compared with recent Monte Carlo studies on substituted spin ladders, pointing to a similar instability of coupled, dimerized spin chains and spin ladders upon substitution.

cond-mat.str-el↗

arXiv1997

Experimental evidence for a spin-Peierls-transition in $α'-NaV_2O_5$

We present the first measurements of magnetisation and Raman light scattering on $α'-NaV_2O_5$ single crystals. Below 34 K, we observe a pronounced isotropic decrease of the susceptibility indicating the opening of a spin gap. The transition temperature is slightly field dependent. Raman experiments reveal a crystallographic distortion at the transition. Our results clearly establish $α'-NaV_2O_5$ to be the second inorganic spin-Peierls system.

cond-mat.str-el↗