arXiv subjects

A. Bussmann-Holder

Publications and source records attributed to A. Bussmann-Holder.

At least 19 recordsLinked to original sources

arXiv2023

K. Alex Müller: Science between ferroelectricity and superconductivity

K. Alex Müller started his scientific career in 1958 when he was about thirty-one years old. After his wife passed away and being in his nineties, his interest in physics gradually faded. In those years shortly before he passed away, on January 9, 2023, he was no longer interested in superconductivity or ferroelectricity, he had become essentially devoted to philosophy and psychology. It is of note that altogether his research in physics comprised sixty years of activity. Almost in the middle of this period, he was awarded the Nobel Prize in Physics for the discovery of high-temperature superconductivity in ceramic copper oxides in 1987. This discovery made him truly famous. While most researchers are familiar with his Nobel Prize work, many are not aware of the fact that K. Alex Müller was already an acclaimed scientist before that and highly respected in the field of perovskite oxides, ferroelectricity, structural phase transitions, Jahn-Teller physics, electron paramagnetic resonance (EPR) and, especially so in the properties of SrTiO3.

cond-mat.supr-con↗

arXiv2021

Filamentary superconductivity of resistively-switched strontium titanate

SrTiO$_3$, although a wide gap insulator, has long been known to become metallic and superconducting at extremely low doping levels. This has given rise to questions concerning the coexistence or interdependence of metallicity, superconductivity, and the material's polar properties. This issue becomes especially intriguing in conjunction with the observation that filamentary metallicity can be induced by means of resistive switching at conditions well below relevant doping levels for homogeneous metallicity. In this study, we demonstrate that resistive switching can also be employed to generate superconductivity at the superconducting transition temperature of $T_c$ $\approx$ 0.2 K in SrTiO$_3$. By combining local characterization of the conductivity with theoretical analysis, we propose that the superconducting properties are associated with the electro-formation of columnar-like bundles with a diameter of 40-50 nm, consisting of metallic filaments surrounded by polar regions. We provide a theoretical model identifying the coexistence of metallic and polar regions as a prerequisite for the filamentary-like superconductivity observed.

cond-mat.supr-con↗

arXiv2021

Magnetic field driven novel phase transitions in EuTiO$_3$

The influence of an external static magnetic field (up to 480 mT)on the structural properties of EuTiO$_3$ (ETO) polycrystalline samples was examined by powder XRD at the Elettra synchrotron facilities in the temperature range 100-300K. While the cubic to tetragonal structural phase transition temperature in this magnetic field range remains almost unaffected, significant lattice effects appear at two characteristic temperatures (~200K and 250K), which becomes more pronounced at a critical threshold magnetic field. At ~200K a change in the sign of the magnetostriction is detected attributed to a modification of the local magnetic properties from intrinsic ferromagnetism to intrinsic antiferromagnetism. These data are a clear indication that strong spin-lattice interactions govern also the high temperature phase of ETO and trigger the appearance of magnetic domain formation and novel phase transitions

cond-mat.mtrl-sci↗

arXiv2020

Magnetic interactions and the puzzling absence of any Raman mode in EuTiO3

Polycrystalline ceramic samples and a single crystal of EuTiO3 have been investigated by Raman spectroscopy in the temperature range 80-300 K. Although synchrotron XRD data clearly indicated the cubic to tetragonal phase transition around 282 K, no mode from the symmetry allowed Raman active phonons was found in the tetragonal phase, contrary to the case of the homologous SrTiO3. In order to study the evolution of this unique characteristic, ceramics of EuxSr1-xTiO3 (x=0.03-1.0) characterized by synchrotron XRD for the structural phase transition have been also investigated by Raman spectroscopy, verifying the very strong influence on the Raman yield by Eu substitution. By applying an external magnetic field or alternatively hydrostatic pressure modes are activated in the Raman spectra. The temperature dependence of the main mode that is activated shows remarkable agreement with theoretical predictions. We attribute the puzzling absence of the Raman modes to a mechanism related to strong spin-lattice interaction that drives the cubic to tetragonal structural phase transition and makes the Raman tensor antisymmetric. On the contrary, the external perturbations induce a symmetric Raman tensor allowing even symmetry modes to be present in the spectra. Previous EPR, muon scattering and magnetic measurements indicated the presence of small magnetic interactions deep inside the paramagnetic phase. In order to probe those magnetic interactions in our EuTiO3 polycrystalline sample and test our hypothesis, we have performed temperature dependant XAS/XMCD, which support the existence of magnetic nanodomains even close to room temperature.

cond-mat.mtrl-sci↗

arXiv2016

Evidence for strong lattice effects as revealed from huge unconventional oxygen isotope effects on the pseudogap temperature in La$_{2-x}$Sr$_{x}$CuO$_{4}$

The oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) on the pseudogap (charge-stripe ordering) temperature $T^{\ast}$ is investigated for the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ as a function of doping $x$ by means of x-ray absorption near edge structure (XANES) studies. A strong $x$ dependent and sign reversed OIE on $T^{\ast}$ is observed. The OIE exponent $α_{T^{\ast}}$ systematically decreases from $α_{T^{\ast}} = - 0.6(1.3)$ for $x = 0.15$ to $α_{T^{\ast}} = - 4.4(1.1)$ for $x = 0.06$, corresponding to increasing $T^{\ast}$ and decreasing superconducting transition temperature $T_{c}$. Both $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ exhibit a linear doping dependence with different slopes and critical end points (where $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ fall to zero) at $x_{c}(^{16}{\rm O}) = 0.201(4)$ and $x_{c}(^{18}{\rm O}) = 0.182(3)$, indicating a large positive OIE of $x_{c}$ with an exponent of $α_{x_{c}} = 0.84(22)$. The remarkably large and strongly doping dependent OIE on $T^{\ast}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.

cond-mat.supr-con↗

arXiv2016

Multigap superconductivity at extremely high temperature: a model for the case of pressurized H2S

It is known that in pressurized H2S the complex electronic structure in the energy range of 200 meV near the chemical potential can be separated into two electronic components, the first characterized by steep bands with a high Fermi velocity and the second by flat bands with a vanishing Fermi velocity. Also the phonon modes interacting with electrons at the Fermi energy can be separated into two components: hard modes with high energy around 150 meV and soft modes with energies around 60 meV. Therefore we discuss here a multiband scenario in the standard BCS approximation where the effective BCS coupling coefficient is in the range 0.1- 0.32. We consider a first (second) BCS condensate in the strong (weak) coupling regime 0.32 (0.15). We discuss different scenario segregated in different portions of the material. The results show the phenomenology of unconventional superconducting phases in this two-gap superconductivity scenario where there are two electronic components in two Fermi surface spots, the pairing is mediated by either by a soft or a hard phonon branch where the inter-band exchange term, also if small, plays a key role for the emergence of high temperature superconductivity in pressurized sulfur hydride.

cond-mat.str-el↗

arXiv2015

Pressure-Temperature phase diagram of multiferroic EuTiO$_3$

The structural transformation of multiferroic EuTiO$_3$ has been intensively investigated by synchrotron x-ray diffraction at pressures up to 50.3 GPa and temperatures from 50 to 500 K. An antiferrodistortive phase transition from cubic Pm-3m to tetragonal I4/mcm space group has been observed, identical to the one that has been previously explored at ambient pressure and low temperatures. Several compression/decompression cycles at different temperatures have been carried out to accurately map the transition, and as a result a P-T phase diagram for EuTiO$_3$ has been constructed. The observed phase transition exhibits many similarities with isostructural SrTiO$_3$, although the absence of magnetoelectric interactions in the latter accounts for the different phase boundaries between the two materials.

cond-mat.mtrl-sci↗

arXiv2015

Oxygen Isotope Effects on Lattice Properties of La_{2-x}Ba_xCuO_4 (x = 1/8)

A novel negative oxygen-isotope (16O/18O) effect (OIE) on the low-temperature tetragonal phase transition temperature T_{LTT} was observed in La_{2-x}Ba_xCuO_4 (x = 1/8) using high-resolution neutron powder diffraction. The corresponding OIE exponent alpha(T_{LTT}) = - 0.36(5) has the same sign as alpha(T_{so}) = -0.57(6) found for the spin-stripe order temperature T_so. The fact that the LTT transition is accompanied by charge ordering (CO) implies the presence of an OIE also for the CO temperature T_co. Furthermore, a temperature dependent shortening of the c-axis with the heavier isotope is observed. These results combined with model calculations demonstrate that anharmonic electron-lattice interactions are essential for all transitions observed in the stripe phase of cuprates.

cond-mat.supr-con↗

arXiv2014

Spin-lattice coupling induced weak dynamical magnetism in EuTiO_3 at high temperatures

EuTiO_3, which is a G-type antiferromagnet below T_N = 5.5 K, has some fascinating properties at high temperatures, suggesting that macroscopically hidden dynamically fluctuating weak magnetism exists at high temperatures. This conjecture is substantiated by magnetic field dependent magnetization measurements, which exhibit pronounced anomalies below 200 K becoming more distinctive with increasing magnetic field strength. Additional results from muon spin rotation ($μ$SR) experiments provide evidence for weak fluctuating bulk magnetism induced by spin-lattice coupling which is strongly supported in increasing magnetic field.

cond-mat.mtrl-sci↗

arXiv2013

Lattice and polarizability mediated spin activity in EuTiO_3

EuTiO_3 is shown to exhibit novel strong spin-charge-lattice coupling deep in the paramagnetic phase. Its existence is evidenced by an, until now, unknown response of the paramagnetic susceptibility at temperatures exceeding the structural phase transition temperature T_S = 282K. The "extra" features in the susceptibility follow the rotational soft zone boundary mode temperature dependence above and below T_S. The theoretical modeling consistently reproduces this behavior and provides reasoning for the stabilization of the soft optic mode other than quantum fluctuations.

cond-mat.mtrl-sci↗

arXiv2013

Nonlinear pressure dependence of T_N in almost multiferroic EuTiO_3

The antiferromagnetic (AFM) phase transition temperature T_N of EuTiO_3 has been studied as a function of pressure p. The data reveal a nonlinear dependence of T_N on p with T_N increasing with increasing pressure. The exchange interactions exhibit an analogous dependence on p as T_N (if the absolute value of the nearest neighbor interaction is considered) and there is evidence that the AFM transition is robust with increasing pressure. The corresponding Weiss temperature Θ_W remains anomalous since it always exhibits positive values. The data are analyzed within the Bloch power law model and provide excellent agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2013

Tuning the structural instability of SrTiO_3 by Eu doping: The phase diagram of Sr_1-xEu_xTiO_3

The phase diagram of Sr_1-xEu_xTiO_3 is determined experimentally by electron paramagnetic resonance and resistivity measurements and analyzed theoretically within the self-consistent phonon approximation as a function of x ([0.03-1.0]). The transition temperature of the structural instability of the system increases nonlinearly to higher temperatures with increasing x. This is interpreted theoretically by a substantial alteration in the dynamics caused by a change in the double-well potential from broad and shallow to narrow and deep.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic field enhanced structural instability in EuTiO_{3}

EuTiO_{3} undergoes a structural phase transition from cubic to tetragonal at T_S = 282 K which is not accompanied by any long range magnetic order. However, it is related to the oxygen ocathedra rotation driven by a zone boundary acoustic mode softening. Here we show that this displacive second order structural phase transition can be shifted to higher temperatures by the application of an external magnetic field (increased by 4 K for mu_{0}H = 9 T). This observed field dependence is in agreement with theoretical predictions based on a coupled spin-anharmonic-phonon interaction model.

cond-mat.mtrl-sci↗

arXiv2012

Hybrid paramagnon phonon modes at elevated temperatures in EuTiO3

EuTiO3 (ETO) has recently experienced an enormous revival of interest because of its possible multiferroic properties which are currently in the focus of research. Unfortunately ETO is an unlikely candidate for enlarged multifunctionality since the mode softening - typical for ferroelectrics - remains incomplete, and the antiferromagnetic properties appear at 5.5K only. However, a strong coupling between lattice and Eu spins exists and leads to the appearance of a magnon-phonon-hybrid mode at elevated temperatures as evidenced by electron paramagnetic resonance (EPR), muon spin rotation (μSR) experiments and model predictions based on a coupled spin-polarizability Hamiltonian. This novel finding supports the notion of strong magneto-dielectric (MD) effects being realized in ETO and opens new strategies in material design and technological applications.

cond-mat.mtrl-sci↗

arXiv2012

Structural instability of EuTiO3 from X-ray powder diffraction

We have recently predicted and subsequently verified experimentally by specific heat measurements that EuTiO3 undergoes a structural phase transition at elevated temperature TS = 282 K. The origin of the phase transition has been attributed to the softening of a transverse acoustic mode stemming from an oxygen octahedral rotation analogous to SrTiO3. Here we demonstrate that the theoretical interpretation is correct by using high resolution laboratory X-ray powder diffraction which evidences a cubic to tetragonal phase transition in EuTiO3. The room temperature structure could be refined in with a = 3.9082(2) Å and at 100 K the refinement in the tetragonal space group resulted in a = 5.5192(2) and c = 7.8164(8) Å.

cond-mat.mtrl-sci↗

arXiv2011

Analogies of structural instabilities in EuTiO3 and SrTiO3

Specific heat measurements and theoretical calculations reveal an intimate analogy between EuTiO3 and SrTiO3. For EuTiO3 a hitherto unknown specific heat anomaly is discovered at TA=282(1)K which is analogous to the well known specific heat anomaly of SrTiO3 at TA=105K caused by an antiferrodistortive transition. Since the zone center soft phonon mode observed in both systems can be modeled with the same parameters we ascribe the new 282(1)K instability of EuTiO3 to an antiferrodistortive phase transition. The higher transition temperature of EuTiO3 as compared to SrTiO3 results from spin phonon coupling.

cond-mat.str-el↗

arXiv2010

Field Dependent Superfluid Density in the Optimally Doped SmFeAsO_(1-x)F_y Superconductor

The magnetic field dependence of the in-plane magnetic penetration depth for optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density as the field is increased up to 1.4 T. This behavior is analysed within a two-band model with self-consistently derived coupled gaps, where the superfluid density related to the larger gap is field independent and the superfluid density related to the smaller gap is strongly suppressed with increasing field.

cond-mat.supr-con↗

arXiv2010

Intrinsic and structural isotope effects in Fe-based superconductors

The currently available results of the isotope effect on the superconducting transition temperature T_c in Fe-based high-temperature superconductors (HTS) are highly controversial. The values of the Fe isotope effect (Fe-IE) exponent α_Fe for various families of Fe-based HTS were found to be as well positive, as negative, or even be exceedingly larger than the BCS value α_BCS=0.5. Here we demonstrate that the Fe isotope substitution causes small structural modifications which, in turn, affect T_c. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of α~0.35-0.4 is observed for at least three different families of Fe-based HTS.

cond-mat.supr-con↗