arXiv subjects

Maarit Karppinen

Publications and source records attributed to Maarit Karppinen.

9 recordsLinked to original sources

arXiv2026

Superconductivity in strongly overdoped cuprates: beyond the single-band model

In order to explain the observation of an extended superconducting region in several overdoped cuprates, which contrasts the dome scenario, by means of neutron and synchrotron x-ray powder diffraction we study the crystal structure of YBa$_2$Cu$_3$O$_{y}$, where strong oxygen overdoping up to $y = 7.4$ is achieved under high-pressure. A bond valence sum analysis indicates that 1/5 of the extra holes created by the excess oxygen are transferred to the CuO$_2$ planes, thus increasing the hole density up to $p=0.27$ hole/Cu, where superconductivity is expected to vanish according to the dome scenario. Instead, our data confirm a previous observation [Okai, Ono and Mitsuhashi, Physica C: Superconductivity {\bf 366}, 164 (2002)] that the superconducting critical temperature, $T_c$, remains constant with $y$. Our data analysis accounts for this discrepancy in terms of the much shorter bond between the apical oxygen and the planar Cu ion, which suggests that the extra holes occupy the $a_1$-symmetry states formed by $d_{3z^2-r^2}$ orbitals, instead of the usual $b_1$-symmetry Zhang-Rice singlet states formed by $d_{x^2-y^2}$ orbitals. Suitable spectroscopic measurements on single crystals may support such a two-band scenario, which would require a totally different theoretical approach to explain superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2023

Structure, spin correlations and magnetism of the $S = 1/2$ square-lattice antiferromagnet Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ ($0 \leq x \leq 1$)

Quantum spin liquids are highly entangled magnetic states with exotic properties. The $S = 1/2$ square-lattice Heisenberg model is one of the foundational models in frustrated magnetism with a predicted, but never observed, quantum spin liquid state. Isostructural double perovskites Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are physical realizations of this model, but have distinctly different types magnetic order and interactions due to a $d^{10}/d^0$ effect. Long-range magnetic order is suppressed in the solid solution Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ in a wide region of $x = 0.05-0.6$, where the ground state has been proposed to be a disorder-induced spin liquid. Here we show that the spin-liquid-like $x = 0.2$ and $x = 0.5$ samples have distinctly different local spin correlations, which suggests they have different ground states. Furthermore, the previously ignored interlayer coupling between the square-planes is likely to play a role in the suppression of magnetic order on the W-rich side at $x \approx 0.6$. These results highlight the complex magnetism of Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ and hint at a new quantum critical point at $x \approx 0.3$.

cond-mat.str-el↗

arXiv2022

Correlation of strontium anharmonicity with charge-lattice dynamics of the apical oxygens and their coupling to cuprate superconductivity

Cu K edge X-ray absorption spectra of overdoped superconducting $YSr_2Cu_{2.75}Mo_{0.25}O_{7.54}$ and $Sr_2CuO_{3.3}$ show a remarkably strong correlation of their superconductivity with the local dynamics of their Cu-Sr and Cu-apical-O pairs. This finding that the entire alkaline earth cation-apical O "dielectric" layer has an active role in the unusual electronic properties of cuprates has not been previously considered and has far reaching implications. We develop this idea of a possible role for the apical oxygen charge dynamics via a soft mode of the Sr by applying Kuramoto's synchronization technique to exact diagonalization calculations of two neighboring Cu-apical O pairs bridged by Sr and a planar O atom. These calculations show a first order phase transition to a synchronized state of the Internal Quantum Tunneling Polarons (IQTPs) in which a fraction of the hole originally confined to the apical O atoms of the cluster is transferred onto the planar O. This combination of experimental results and theory demonstrates that the Sr-O dielectric layer of cuprates most likely plays an important role in high temperature superconductivity via its collective charge dynamics that extends into the $CuO_2$ conducting planes.

cond-mat.supr-con↗

arXiv2022

Randomness and Frustration in a S = 1/2 Square-Lattice Heisenberg Antiferromagnet

We explore the interplay between randomness and magnetic frustration in the series of $S = 1/2$ Heisenberg square-lattice compounds Sr$_2$CuTe$_{1-x}$W$_x$O$_6$. Substituting W for Te alters the magnetic interactions dramatically, from strongly nearest-neighbor to next-nearest-neighbor antiferromagnetic coupling. We perform neutron scattering measurements to probe the magnetic ground state and excitations over a range of $x$. We propose a bond-disorder model that reproduces ground states with only short-ranged spin correlations in the mixed compounds. The calculated neutron diffraction patterns and powder spectra agree well with the measured data and allow detailed predictions for future measurements. We conclude that quenched randomness plays the major role in defining the physics of Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ with frustration being less significant.

cond-mat.str-el↗

arXiv2020

Fe3Se4: A Possible Ferrimagnetic Half-Metal?

Half-metallic ferromagnets show 100% spin-polarization at the Fermi level and are ideal candidates for spintronic applications. Despite the extensive research in the field, very few materials have been discovered so far. Here we present results of electronic band structure calculations based on density functional theory and extensive physical-property measurements for Fe3Se4 revealing signatures of half-metallicity. The spin-polarized electronic band structure calculations predict half-metallic ferrimagnetism for Fe3Se4. The electrical resistivity follows exponentially suppressed electron-magnon scattering mechanism in the low-temperature regime and show a magnetoresistance effect that changes the sign from negative to positive with decreasing temperature around 100 K. Other intriguing observations include the anomalous behavior of Hall resistance below 100 K and an anomalous Hall coefficient that roughly follows the \r{ho}2 behavior.

cond-mat.str-el↗

arXiv2013

Spin state of Co$^{3+}$ in LaCo$_{1-x}$Rh$_{x}$O$_{3}$ investigated by structural phenomena

Neutron diffraction for a polycrystalline sample of LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ and synchrotron x-ray diffraction for polycrystalline samples of LaCo$_{0.9}$Rh$_{0.1}$O$_{3}$ and LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ have been carried out in order to investigate the structural properties related with the spin state of Co$^{3+}$ ions. We have found that the values of the Co(Rh)-O bond lengths in the Co(Rh)O$_{6}$ octahedron of LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ are nearly identical at 10 K. The lattice volume for the Rh$^{3+}$ substituted samples decreases with the thermal expansion coefficient similar to that of LaCoO$_{3}$ from room temperature, and ceases to decrease around 70 K. These experimental results favor a mixed state consisting of the high-spin-state and low-spin-state Co$^{3+}$ ions, and suggest that the high-spin-state Co$^{3+}$ ions are thermally excited in addition to those pinned by the substituted Rh$^{3+}$ ions.

cond-mat.str-el↗

arXiv2008

Cationic Ordering and Microstructural Effects in the Ferromagnetic Perovskite La0.5Ba0.5CoO3: Impact upon Magnetotransport Properties

The synthesis and structural study of the stoichiometric perovskite La0.5Ba0.5CoO3 have allowed three forms to be isolated. Besides the disordered La0.5Ba0.5CoO3 and the perfectly ordered layered LaBaCo2O6, a third form called nanoscale-ordered LaBaCo2O6, is obtained. As evidenced by transmission electron microscopy investigations, the latter consists of 112-type 90 degree oriented domains fitted into each other at a nanometer scale which induce large strains and consequently local atomic scale lattice distortions. These three ferromagnetic perovskites exhibit practically the same Tc (174-179 K), but differently from the other phases, the nanoscale-ordered LaBaCo2O6 is a hard ferromagnet, with Hc = 4.2 kOe, due to the strains which may pin domain walls, preventing the reversal of the spins in a magnetic field. The magnetotransport properties of these phases show that all of them exhibit a maximum intrinsic magnetoresistance, close to 6-7 % around Tc under 70 kOe but that the ordered phase exhibits a much higher tunnelling magnetoresistance effect at low temperature of about 15 % against 4 % due to the grain boundary effects.

cond-mat.mtrl-sci↗

arXiv2004

Oxidation State of Cobalt in the NaxCoO2-d yH2O Superconductor

Here we present results of accurate wet-chemical redox analyses, revealing that the oxidation number of cobalt in the newly discovered CoO2-layer superconductor, NaxCoO2 yH2O (x ~ 0.36, y ~ 1.3) is ~ 3.46. This value is significantly lower than the one (~ 3.64) expected on the basis of the value determined for the Na to Co stoichiometry ratio, x. The "lower-than-expected" value for the Co oxidation state may be alternatively explained by presence of oxygen vacancies or excess protons. Moreover, the result implies that rather than an electron-doped Co(IV) lattice the phase should be considered as a hole-doped Co(III) lattice.

cond-mat.supr-con↗

arXiv2003

Static magnetic order in Na$_{0.75}$CoO$_2$ detected by muon spin rotation and relaxation

The nature of the magnetic transition of the Na-rich thermoelectric Na$_{0.75}$CoO$_2$ at 22K was studied by positive muon-spin-rotation and relaxation ($μ^+$SR) spectroscopy, using a polycrystalline sample in the temperature range between 300 and 2.5 K. Zero field $μ$SR measurements indicated the existence of a static internal magnetic field at temperatures below 22 K (= $T_{\rm m}$). The observed muon spin precession signal below $T_{\rm m}$ consisted of three components with different precession frequencies, corresponding to three inequivalent muon$^+$ sites in the Na$_{0.75}$CoO$_2$ lattice. The total volume fraction of the three components was estimated as $\sim$21% at 2.5 K; thus, this magnetic transition was not induced by impurities but is an intrinsic change in the magnetism of the sample, although the sample was magnetically inhomogeneous otherwise. On the other hand, a similar experiment on a Na$_{0.65}$CoO$_2$ sample exhibited no magnetic transition down to 2.5 K; which indicates that the average valence of the Co ions is responsible for inducing the magnetic transition at 22 K.

cond-mat.str-el↗