arXiv subjects

R. K. Kremer

Publications and source records attributed to R. K. Kremer.

At least 19 recordsLinked to original sources

arXiv2026

Directional Andreev-Reflection Signatures of Inter-Orbital Pairing in Sr$_2$RuO$_4$

Unconventional superconductivity in quasi--two-dimensional systems is commonly identified through the emergence of Andreev bound states (ABS) at in-plane edges, while surfaces perpendicular to out-of-plane direction remain fully gapped due to weak interlayer coherence. This directional anisotropy has long served as a key paradigm for constraining pairing symmetries. Here, we show that Sr$_2$RuO$_4$ exhibits a striking reversal of this behavior. Using edge- and surface-sensitive spectroscopy, we observe pronounced in-gap ABS at surfaces perpendicular to the out-of-plane direction, whereas in-plane edges exhibit a reduced intensity of the in-gap spectral features. We show that this anomalous anisotropy can arise from the inter-orbital character of the superconducting pairing. Both even- and odd-parity inter-orbital pairing channels naturally generate robust surface ABS while suppressing planar edge modes and can also provide a mechanism for the appearance of a horizontal line node. Supported by \textit{ab initio} and model calculations, including Sr$_2$RuO$_4$/Ag interface reconstructions, our results highlight the possible role of inter-orbital correlations in shaping the spectroscopic response and provide constraints on the structure of the superconducting order parameter in Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2024

Thermal and Magnetoelastic Properties of the van der Waals Ferromagnet Fe$_{3-δ}$GeTe$_2$: Anisotropic Spontaneous Magnetostriction and Ferromagnetic Magnon Excitations

By determining the lattice parameters as a function of temperature of the hexagonal van der Waals ferromagnet Fe$_{2.92(1)}$Ge$_{1.02(3)}$Te$_2$ we obtain the temperature dependence of the spontaneous in-plane magnetostriction in the ferromagnetic and the linear thermal expansion coefficients in the paramagnetic state. The spontaneous magnetostriction is clearly seen in the temperature dependence of the in-plane lattice parameter $a(T)$, but less well pronounced perpendicular to the planes along $c$. Below $T_{\rm C}$ the spontaneous magnetostriction follows the square of the magnetization and leads to an expansion of the hexagonal layers. Extrapolating to $T\rightarrow$ 0~K we obtain a spontaneous in-plane saturation magnetostriction of $λ_{{\rm sp},a}(T \rightarrow 0) \approx-220 ~\times~10^{-6}$. In the paramagnetic state the linear thermal expansion coefficients amount to 13.9(1)$\times$10$^{-6}$~K$^{-1}$ and to 23.2(2)$\times$10$^{-6}$~K$^{-1}$ for the in-plane and out-of-plane direction, respectively, indicating a linear volume thermal expansion coefficient of 50.8(4)$\times$10$^{-6}$K$^{-1}$ which we use to estimate the volume thermal expansion contribution to the heat capacity determined at constant pressure. A Sommerfeld-type linear term in the low-temperature heat capacities can be quantitatively ascribed to 2dim ferromagnetic magnon excitations.

cond-mat.str-el↗

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

Magnetic correlations in infinite-layer nickelates: an experimental and theoretical multi-method study

We report a comprehensive study of magnetic correlations in LaNiO$_{2}$, a parent compound of the recently discovered family of infinite-layer (IL) nickelate superconductors, using multiple experimental and theoretical methods. Our specific heat, muon-spin rotation ($μ$SR), and magnetic susceptibility measurements on polycrystalline LaNiO$_{2}$ show that long-range magnetic order remains absent down to 2 K. Nevertheless, we detect residual entropy in the low-temperature specific heat, which is compatible with a model fit that includes paramagnon excitations. The $μ$SR and low-field static and dynamic magnetic susceptibility measurements indicate the presence of short-range magnetic correlations and glassy spin dynamics, which we attribute to local oxygen non-stoichiometry in the average infinite-layer crystal structure. This glassy behavior can be suppressed in strong external fields, allowing us to extract the intrinsic paramagnetic susceptibility. Remarkably, we find that the intrinsic susceptibility shows non-Curie-Weiss behavior at high temperatures, in analogy to doped cuprates that possess robust non-local spin fluctuations. The distinct temperature dependence of the intrinsic susceptibility of LaNiO$_{2}$ can be theoretically understood by a multi-method study of the single-band Hubbard model in which we apply complementary cutting-edge quantum many-body techniques (dynamical mean-field theory, cellular dynamical mean-field theory and the dynamical vertex approximation) to investigate the influence of both short- and long-ranged correlations. Our results suggest a profound analogy between the magnetic correlations in parent (undoped) IL nickelates and doped cuprates.

cond-mat.str-el↗

arXiv2020

Phase separation and pairing fluctuations in oxide materials

We investigate the microscopic mechanism of charge instabilities and the formation of inhomogeneous states in systems with strong electron correlations. It is demonstrated that within a strong coupling expansion the single-band Hubbard model shows an instability towards phase separation and extend the approach also for an analysis of phase separation in the Hubbard-Kanamori hamiltonian as a prototypical multiband model. We study the pairing fluctuations on top of an inhomogeneous stripe state where superconducting correlations in the extended s-wave and d-wave channels correspond to (anti)bound states in the two-particle spectra. Whereas extended s-wave fluctuations are relevant on the scale of the local interaction parameter U, we find that d-wave fluctuations are pronounced in the energy range of the active subband which crosses the Fermi level. As a result low energy spin and charge fluctuations can transfer the d-wave correlations from the bound states to the low energy quasiparticle bands. Our investigations therefore help to understand the coexistence of stripe correlations and d-wave superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2020

Magnetoelastic study on the frustrated quasi-one-dimensional spin-1/2 magnet LiCuVO$_4$

We investigated the magnetoelastic properties of the quasi-one-dimensional spin-1/2 frustrated magnet LiCuVO$_4$. Longitudinal-magnetostriction experiments were performed at 1.5 K in high magnetic fields of up to 60 T applied along the $b$ axis, i.e., the spin-chain direction. The magnetostriction data qualitatively resemble the magnetization results, and saturate at $H_{\text{sat}} \approx 54$ T, with a relative change in sample length of $ΔL/L \approx 1.8\times10^{-4}$. Remarkably, both the magnetostriction and the magnetization evolve gradually between $H_{\text{c3}} \approx 48$ T and $H_{\text{sat}}$, indicating that the two quantities consistently detect the spin-nematic phase just below the saturation. Numerical analyses for a weakly coupled spin-chain model reveal that the observed magnetostriction can overall be understood within an exchange-striction mechanism. Small deviations found may indicate nontrivial changes in local correlations associated with the field-induced phase transitions.

cond-mat.str-el↗

arXiv2018

Impact of Rare-earth site Substitution on the Structural, Magnetic and Thermal Properties of Ce$_{1-x}$Eu$_x$CrO$_3$ Orthochromite Solid-solutions

The role of slight changes of the chemical composition on antiferromagnetic ordering of Cr in rare-earth orthochoromites was investigated on a series of ceramic solid-solutions Ce$_{1-x}$Eu${_x}$O$_3$ where x varied from 0 to 1. Gradual replacement of Ce with Eu reduces the cell volume and acts equivalently to applying external pressure. Full replacement of Ce by Eu, on the other hand, reduces the Néel temperature from 260 K for CeCrO$_3$ to 178 K for EuCrO$_3$ as established by magnetization, heat capacity and neutron powder diffraction measurements. High resolution x-ray powder diffraction measurements on Ce$_{1-x}$Eu${_x}$O$_3$ and neutron powder diffraction studies on CeCrO$_3$ enable to correlate the magnetic properties of the Cr magnetic subsystem with the size of the lattice and minute changes of the bonding and torsion angles within and between the CrO$_6$ octahedra. We find that the sizes and the shapes of the CrO$6$ octahedra remain essentially unchanged as the size of the rare-earth cations is reduced whereas decreasing Cr - O - Cr bonding angles and increasing inclination of neighboring octahedra enable to compensate for the decreasing lattice size.

cond-mat.str-el↗

arXiv2017

Frustration-driven C4 symmetric orders in a hetero-structured iron-based superconductor

A subtle balance between competing interactions in strongly correlated systems can be easily tipped by additional interfacial interactions in a heterostructure. This often induces exotic phases with unprecedented properties, as recently exemplified by high-Tc superconductivity in FeSe monolayer on the nonmagnetic SrTiO3. When the proximity-coupled layer is magnetically active, even richer phase diagrams are expected in iron-based superconductors (FeSCs), which however has not been explored due to the lack of a proper material system. One promising candidate is Sr2VO3FeAs, a naturally-assembled heterostructure of a FeSC and a Mott-insulating vanadium oxide. Here, using high-quality single crystals and high-accuracy 75As and 51V nuclear magnetic resonance (NMR) measurements, we show that a novel electronic phase is emerging in the FeAs layer below T0 ~ 155 K without either static magnetism or a crystal symmetry change, which has never been observed in other FeSCs. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order with C4-symmetry in the FeAs layers, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling is effective to stabilize a hidden order in FeSCs and, more generally, in strongly correlated heterostructures.

cond-mat.supr-con↗

arXiv2016

Fermi-surface topological phase transition and horizontal order-parameter nodes in CaFe$_2$As$_2$ under pressure

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe$_2$As$_2$, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe$_2$As$_2$ crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the hole-like Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line.

cond-mat.supr-con↗

arXiv2016

Magnetic structure, magnetoelastic coupling, and thermal properties of EuCrO$_3$ nano-powders

We carried out detailed studies of the magnetic structure, magnetoelastic coupling, and thermal properties of EuCrO$_3$ nano-powders from room temperature to liquid helium temperature. Our neutron powder diffraction and X-ray powder diffraction measurements provide precise atomic positions of all atoms in the cell, especially for the light oxygen atoms. The low-temperature neutron powder diffraction data revealed extra Bragg peaks of magnetic origin which can be attributed to a $G_x$ antiferromagnetic structure with an ordered moment of $\sim$ 2.4 $μ_{\rm B}$ consistent with the $3d^3$ electronic configuration of the Cr$^{3+}$ cations. Apart from previously reported antiferromagnetic and ferromagnetic transitions in EuCrO$_3$ at low temperatures, we also observed an anomaly at about 100 K. This anomaly was observed in temperature dependence of sample's, lattice parameters, thermal expansion, Raman spectroscopy, permittivity and conductance measurements. This anomaly is attributed to the magnetoelastic distortion in the EuCrO$_3$ crystal.

cond-mat.str-el↗

arXiv2014

Spin excitations in the two-dimensional strongly coupled dimer system malachite

The mineral malachite, Cu2(OD)2CO3, has a quantum spin-liquid ground state and no long-range magnetic order down to at least T=0.4 K. Inelastic neutron scattering measurements show that the excitation spectrum consists of dispersive gapped singlet-triplet excitations, characteristic of spin-1/2 dimer-forming Heisenberg antiferromagnets. We identify a new two-dimensional dimerized coupling scheme with strong interdimer coupling J'/J1~0.3 that places malachite between strongly coupled alternating chains, square lattice antiferromagnets, and infinite-legged ladders. The geometry of the interaction scheme resembles the staggered dimer lattice, which may allow unconventional quantum criticality.

cond-mat.str-el↗

arXiv2014

Magnetic properties of the RbMnPO4 zeolite-ABW type material: a frustrated zigzag spin chain

The crystal structure and magnetic properties of the RbMnPO4 zeolite-ABW type material have been studied by temperature-dependent neutron powder diffraction, low temperature magnetometry and heat capacity measurements. RbMnPO4 represents a rare example of a weak ferromagnetic polar material, containing Mn2+ ions with TN = 4.7 K. The neutron powder diffraction pattern recorded at T = 10 K shows that the compound crystallizes in the chiral and polar monoclinic space group P21 (No. 4) with the unit-cell parameters: a = 8.94635(9) Å, b = 5.43415(5) Å, c = 9.10250(8) Å and beta = 90.4209(6)°. A close inspection of the crystal structure of RbMnPO4 shows that this material presents two different types of zigzag chains running along the b axis. This is a unique feature among the zeolite-ABW type materials exhibiting the P21 symmetry. At low temperature, RbMnPO4 exhibits a canted antiferromagnetic structure characterized by the propagation vector k1 = 0 resulting in the magnetic symmetry P21. The magnetic moments lie mostly along the b axis with the ferromagnetic component being in the ac plane. Due to the geometrical frustration present in this system, an intermediate phase appears within the temperature range 4.7 - 5.1 K characterized by the propagation vector k2 = (kx, 0, kz) with kx/kz = 2. This ratio is reminiscent of the multiferroic phase of the orthorhombic RMnO3 phases (R = rare earth). This suggests that RbMnPO4 could present some multiferroic properties at low temperature. Our density functional calculations confirm the presence of magnetic frustration, which explains this intermediate incommensurate phase. Taking into account the strongest magnetic interactions, we are able to reproduce the magnetic structure observed experimentally at low temperature.

cond-mat.mtrl-sci↗

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↗

arXiv2014

Characterization of the Spin-1/2 Linear-Chain Ferromagnet CuAs$_2$O$_4$

The magnetic and lattice properties of the $S$=1/2 quantum-spin-chain ferromagnet, CuAs$_2$O$_4$, mineral name trippkeite, were investigated. The crystal structure of CuAs$_2$O$_4$ is characterized by the presence of corrugated CuO$_2$ ribbon chains. Measurements of the magnetic susceptibility, heat capacity, electron paramagnetic resonance and Raman spectroscopy were performed. Our experiments conclusively show that a ferromagnetic transition occurs at $\sim$7.4 K. $\textit{Ab initio}$ DFT calculations reveal dominant ferromagnetic nearest-neighbor and weaker antiferromagnetic next- nearest-neighbor spin exchange interactions along the ribbon chains. The ratio of $J_{\rm nn}$/$J_{\rm nnn}$ is near -4, placing CuAs$_2$O$_4$ in close proximity to a quantum critical point in the $J_{\rm nn}$ - $J_{\rm nnn}$ phase diagram. TMRG simulations used to analyze the magnetic susceptibility confirm this ratio. Single-crystal magnetization measurements indicate that a magnetic anisotropy forces the Cu$^{2+}$ spins to lie in an easy plane perpendicular to the $c$-axis. An analysis of the field and temperature dependent magnetization by modified Arrott plots reveals a 3d-XY critical behavior. Lattice perturbations induced by quasi-hydrostatic pressure and temperature were mapped via magnetization and Raman spectroscopy.

cond-mat.mtrl-sci↗

arXiv2014

Investigation of the strongly correlated one-dimensional magnetic behavior of NiTa2O6

The magnetic properties of NiTa$_2$O$_6$ were investigated by magnetic susceptibility, specific heat, electron paramagnetic resonance, neutron powder diffraction and pulse field magnetization measurements. Accompanying \textit{ab initio} DFT calculations of the spin-exchange constants complemented and supported our experimental findings that NiTa$_2$O$_6$ must be described as a quasi-1D Heisenberg $S$ = 1 spin chain system with a nearest-neighbor only anti-ferromagnetic spin-exchange interaction of 18.92(2) K. Inter-chain coupling is by about two orders of magnitude smaller. Electron paramagnetic resonance measurements on Mg$_{1-x}$Ni$_x$Ta$_2$O$_6$ ($x \approx$ $1\%$) polycrystalline samples enabled us to estimate the single-ion zero-field splitting of the $S$ = 1 states which amounts to less than $4\%$ of the nearest-neighbor spin-exchange interaction. At 0 T NiTa$_2$O$_6$ undergoes long-range anti-ferromagnetic ordering at 10.3(1) K evidenced by a $λ$-type anomaly in the specific heat capacity. On application of a magnetic field the specific heat anomaly is smeared out. We confirmed the magnetic structure by neutron powder diffraction measurements and at 2.00(1) K refined a magnetic moment of 1.93(5) $μ_{\rm{B}}$ per Ni$^{2+}$ ion. Additionally, we followed the magnetic order parameter as a function of temperature. Lastly we found saturation of the magnetic moment at 55.5(5) T with a $g$-factor of 2.14(1), with an additional high field phase above 12.8(1) T. The onset of the new high field phase is not greatly effected by temperature, but rather smears out as one approaches the long-range ordering temperature.

cond-mat.str-el↗

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

EuFe2(As1-xPx)2: reentrant spin glass and superconductivity

By systematic investigations of the magnetic, transport and thermodynamic properties of single crystals of EuFe2(As1-xPx)2 we explore the complex interplay of superconductivity and Eu2+ magnetism. Below 30K, two magnetic transitions are observed for all P substituted crystals suggesting a revision of the phase diagram. In addition to the canted A-type antiferromagnetic order of Eu2+ at approximately 20K, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of EuFe2(As1-xPx)2 coexists with superconductivity around x = 0.2.

cond-mat.supr-con↗

arXiv2013

Point-contact spectroscopy in Co-doped CaFe2As2: nodal superconductivity and topological Fermi surface transition

We performed point-contact Andreev reflection spectroscopy measurements in Ca(Fe_{1-x}Co_x)2As2 single crystals with effective x=0.060 +- 0.005. The spectra of ab-plane contacts show a zero-bias maximum and broad shoulders at about 5-6 meV. Their fit with the three-dimensional Blonder-Tinkham-Klapwijk model (making use of a analytical expression for the Fermi surface that mimics the one calculated from first principles) shows that this compound presents a large isotropic gap on the quasi-2D electronlike Fermi surface sheets and a smaller anisotropic (possibly nodal) gap on the 3D holelike Fermi surface pockets centered at the Z point in the Brillouin zone. These results nicely fit into the theoretical picture for the appearance of nodal superconductivity in 122 compounds.

cond-mat.supr-con↗