arXiv subjects

Andreas Rydh

Publications and source records attributed to Andreas Rydh.

16 recordsLinked to original sources

arXiv2026

Magnetoresistive Memory in the Paramagnetic Phase of Eu$_5$In$_2$As$_6$

Magnetoresistive materials that respond sensitively to applied fields are central to modern data storage technologies. Here we unveil a novel Magnetoresistive Memory (MRM) in Eu$_5$In$_2$As$_6$, where the electrical resistivity depends not only on the magnitude but also on the history of the applied magnetic field. Such an effect has been reported in only two classes of strongly correlated electron systems: perovskite manganites and pyrochlore iridates. In both cases, the effect has been observed in the magnetically ordered phase. It has been attributed to metastable magnetic states in manganites and conducting domain walls in iridates. Remarkably, the MRM in Eu$_5$In$_2$As$_6$ onsets at twice the antiferromagnetic transition temperature, well within the paramagnetic phase. The temperature, field, and time dependence of resistivity suggest that either a hidden order or a fluctuating phase with short-range correlations underlies this effect. Our results offer MRM as a new platform for quantum sensing and memory technologies, and encourage searching for MRM in related materials.

cond-mat.str-el↗

arXiv2025

Split-gasket approach to the integration of electrical leads into diamond anvil cells

Transport and heat capacity measurements under pressure must reconcile the limited available space and complicated geometry of a high-pressure cell with the need for multiple electrical connections. One solution for diamond anvil cells is to use customized diamonds with deposited electrical leads. Here, we instead address the problem through a split-gasket approach, intended for diamond anvil cells at moderate pressures and low temperature. A key component is the use of a substrate with lithographically defined leads, which enables connections to components such as thermometer, heater, and/or sample within the confined sample volume of the cell. The design includes an elaborate BeCu gasket sandwich with a preparation method that ensures electrical contact integrity. Using this configuration, we bring 12 leads to within 100 $\mu$m of the center of the diamond anvil at a pressure of about 2 GPa, comparable to the pressure reached with a regular gasket, demonstrating the setup's capability for high-pressure experiments. The split-gasket approach may come at the cost of reduced maximum pressure, but brings versatility and reproducibility, and alleviates the experimental efforts of maintaining multiple electrical leads both intact and electrically isolated.

physics.ins-det↗

arXiv2025

Miniaturized chip calorimeter for high-pressure cells at low temperature

Heat capacity measurements under high pressure places high demands on the calorimeter. Here we describe the development of a miniaturized nanocalorimeter for high-pressure heat capacity measurements at low temperature. The device, fabricated on a silicon substrate, employs a high-frequency AC calorimetry technique and features a design with an outer diameter of 300 ${\mu}$m and thickness of 25-40 ${\mu}$m, small enough to fit into high pressure diamond anvil cells. Miniaturization is achieved by stacking all components, including thermometer and heaters, within a central area. The thin-film calorimeter thermometer measures 40 ${\mu}$m square and maintains the sensitivity and properties of larger thermometers. The fabrication process uses controlled anisotropic etch to produce calorimeter chips with a balance between robustness and thickness, suitable for experiments at high pressures and low temperatures. The calorimeter operates at a relatively high characteristic frequency between 10 Hz and 1 kHz, constraining the thermal oscillation to an effective volume dominated by the sample, thereby avoiding the use of a suspended membrane that is the basis for conventional nanocalorimeters.

physics.ins-det↗

arXiv2024

Two types of colossal magnetoresistance with distinct mechanisms in Eu5In2As6

Recent reports of colossal negative magnetoresistance (CMR) in a few magnetic semimetals and semiconductors have attracted attention, because these materials are devoid of the conventional mechanisms of CMR such as mixed valence, double exchange interaction, and Jahn-Teller distortion. New mechanisms have thus been proposed, including topological band structure, ferromagnetic clusters, orbital currents, and charge ordering. The CMR in these compounds has been reported in two forms: either a resistivity peak or a resistivity upturn suppressed by a magnetic field. Here we reveal both types of CMR in a single antiferromagnetic semiconductor Eu5In2As6. Using the transport and thermodynamic measurements, we demonstrate that the peak-type CMR is likely due to the percolation of magnetic polarons with increasing magnetic field, while the upturn-type CMR is proposed to result from the melting of a charge order under the magnetic field. We argue that similar mechanisms operate in other compounds, offering a unifying framework to understand CMR in seemingly different materials.

cond-mat.str-el↗

arXiv2023

Carrier density crossover and quasiparticle mass enhancement in a doped 5$d$ Mott insulator

High-temperature superconductivity in cuprates emerges upon doping the parent Mott insulator. Robust signatures of the low-doped electronic state include a Hall carrier density that initially tracks the number of doped holes and the emergence of an anisotropic pseudogap; the latter characterised by disconnected Fermi arcs, closure at a critical doping level $p^* \approx 0.19$, and, in some cases, a strongly enhanced carrier effective mass. In Sr$_2$IrO$_4$, a spin-orbit-coupled Mott insulator often regarded as a 5$d$ analogue of the cuprates, surface probes have revealed the emergence of an anisotropic pseudogap and Fermi arcs under electron doping, though neither the corresponding $p^*$ nor bulk signatures of pseudogap closing have as yet been observed. Here, we report electrical transport and specific heat measurements on Sr$_{2-x}$La$_x$IrO$_4$ over an extended doping range 0 $\leq x \leq$ 0.20. The effective carrier density $n_{\rm H}$ at low temperatures exhibits a crossover from $n_{\rm H} \approx x$ to $n_{\rm H} \approx 1+x$ near $x$ = 0.16, accompanied by \textcolor{blue}{a five-orders-of-magnitude increase in conductivity} and a six-fold enhancement in the electronic specific heat. These striking parallels in the bulk pseudogap phenomenology, coupled with the absence of superconductivity in electron-doped Sr$_2$IrO$_4$, disfavour the pseudogap as a state of precursor pairing and thereby narrow the search for the key ingredient underpinning the formation of the superconducting condensate in doped Mott insulators.

cond-mat.str-el↗

arXiv2023

Bogoliubov Quasiparticle on the Gossamer Fermi Surface in Electron-Doped Cuprates

In contrast to hole-doped cuprates, electron-doped cuprates consistently exhibit strong antiferromagnetic correlations with a commensurate (π, π) ordering wave vector, leading to the prevalent belief that antiferromagnetic spin fluctuations mediate Cooper pairing in these unconventional superconductors. However, early investigations produced two paradoxical findings: while antiferromagnetic spin fluctuations create the largest pseudogap at "hot spots" in momentum space, Raman scattering and angle-resolved photoemission spectroscopy measurements using the leading-edge method seem to suggest the superconducting gap is also maximized at these locations. This presented a dilemma for spin-fluctuation-mediated pairing: Cooper pairing is strongest at momenta where normal state low energy spectral weight is most suppressed. Here we investigate this dilemma in Nd2-xCexCuO4 using angle-resolved photoemission spectroscopy under significantly improved experimental conditions. The unprecedented signal-to-noise ratio and resolution allow us to directly observe the Bogoliubov quasiparticles, demonstrating the existence and importance of two sectors of states: 1. The reconstructed main band and the states gapped by the antiferromagnetic pseudogap around the hot spots. 2. The gossamer Fermi surface states with distinct dispersion inside the pseudogap, from which Bogoliubov quasiparticle coherence peaks emerge below Tc. Supported by numerical results, we propose that the non-zero modulus of the antiferromagnetic order parameter causes the former, while fluctuations in the antiferromagnetic order parameter orientation are responsible for the latter. Our revelations of the gossamer Fermi surface reconcile the paradoxical observations, deepening our understanding of superconductivity in electron-doped cuprates in particular, and unconventional superconductivity in general.

cond-mat.supr-con↗

arXiv2022

Magnetic monopole relaxation effects in spin ice Dy$_2$Ti$_2$O$_7$

Spin ice compounds enable the exploration of the dynamics of magnetic monopoles in condensed matter systems. In this study, we use ac calorimetry to probe the dynamical response of the heat capacity of the classical spin-ice compounds Dy$_2$Ti$_2$O$_7$ at low temperatures (0.5-5 K). Using frequencies of 0.01-500 Hz, we find a strong frequency dependence in the measured heat capacity and are able to study thermal relaxation effects on the corresponding timescales. The relaxation time $\tau$ is determined from the frequency dependence of the heat capacity as the characteristic frequency below which the heat capacity saturates. The extracted $\tau$ shows a divergent behavior below 1 K reaching $\sim$6 s at 0.65 K, similar to the relaxation time seen in previous studies. Corresponding specific heat shows a maximum around this temperature. Performing dynamic Monte Carlo simulations, we verify that the specific heat frequency response has its origin in the slow magnetic monopole dynamics indigenous to spin ice. We find a timescale of 20 ms per Monte Carlo step at 4 K in contrast to 2.5 ms mentioned in previous studies by other techniques.

cond-mat.str-el↗

arXiv2021

Quartic metal: Spontaneous breaking of time-reversal symmetry due to four-fermion correlations in Ba$_{1-x}$K$_x$Fe$_2$As$_2$

Discoveries of ordered quantum states of matter are of great fundamental interest, and often lead to unique applications. The most well known example -- superconductivity -- is caused by the formation and condensation of pairs of electrons. A key property of superconductors is diamagnetism: magnetic fields are screened by dissipationless currents. Fundamentally, what distinguishes superconducting states from normal states is a spontaneously broken symmetry corresponding to long-range coherence of fermion pairs. Here we report a set of experimental observations in hole doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ which are not consistent with conventional superconducting behavior. Our specific-heat measurements indicate the formation of fermionic bound states when the temperature is lowered from the normal state. However, for $x \sim 0.8$, instead of the standard for superconductors, zero resistance and diamagnetic screening, for a range of temperatures, we observe the opposite effect: the generation of self-induced magnetic fields measured by spontaneous Nernst effect and muon spin rotation experiments. The finite resistance and the lack of any detectable diamagnetic screening in this state exclude the spontaneously broken symmetry associated with superconducting two-fermion correlations. Instead, combined evidence from transport and thermodynamic measurements indicates that the formation of fermionic bound states leads to spontaneous breaking of time-reversal symmetry above the superconducting transition temperature. These results demonstrate the existence of a broken-time-reversal-symmetry bosonic metal state. In the framework of a multiband theory, such a state is characterized by quartic correlations: the long-range order exists only for {\it pairs} of fermion pairs.

cond-mat.supr-con↗

arXiv2019

Phase transition preceding magnetic long-range order in the double perovskite Ba2NaOsO6

Recent theoretical studies [Chen et al., Phys. Rev. B 82, 174440 (2010), Ishizuka et al., Phys. Rev. B 90, 184422 (2014)] for the magnetic Mott insulator Ba2NaOsO6 have proposed a low-temperature order parameter that breaks lattice rotational symmetry without breaking time reversal symmetry leading to a nematic phase just above magnetic ordering temperature. We present high-resolution calorimetric and magnetization data of the same Ba2NaOsO6 single crystal and show evidence for a weakly field-dependent phase transition occurring at a temperature of Ts ~ 9.5K, above the magnetic ordering temperature of Tc ~ 7.5K. This transition appears as a broadened step in the low-field temperature dependence of the specific heat. The evolution of the phase boundary with applied magnetic field suggests that this phase coincides with the phase of broken local point symmetry seen in high field NMR experiments [Lu et al., Nat.\ Comm. 8 14407 (2017)]. Furthermore, the magnetic field dependence of the specific heat provides clear indications for magnetic correlations persisting at temperatures between Tc and Ts where long-range magnetic order is absent giving support for the existence of the proposed nematic phase.

cond-mat.str-el↗

arXiv2012

Photoconductivity effects in mixed-phase BSCCO whiskers

We report on combined photoconductivity and annealing experiments in whisker-like crystals of the Bi-Sr-Ca-Cu-O (BSCCO) high-Tc superconductor. Both single-phase Bi2Sr2CaCu2O8+δ(Bi-2212) samples and crystals of the mixed phases Bi2Sr2Ca2Cu3O10+x (Bi-2223)/Bi-2212 have been subjected to annealing treatments at 90°C in air in a few hours steps, up to a maximum total annealing time of 47 h. At every step, samples have been characterized by means of electrical resistance vs temperature (R vs T) and resistance vs time at fixed temperature (R vs t) measurements, both in the dark and under illumination with a UV-VIS halogen arc lamp. A careful comparison of the results from the two techniques has shown that, while for single-phase samples no effect is recorded, for mixed-phase samples an enhancement in the conductivity that increases with increasing the annealing time is induced by the light at the nominal temperature T = 100 K, i.e. at an intermediate temperature between the critical temperatures of the two phases. A simple pseudo-1D model based on the Kudinov's scheme [Kudinov et al., Phys. Rev. B 47, 9017-28, (1993)] has been developed to account for the observed effects, which is based on the existence of Bi-2223 filaments embedded in the Bi-2212 matrix and on the presence of electronically active defects at their interfaces. This model reproduces fairly well the photoconductive experimental results and shows that the length of the Bi-2223 filaments decreases and the number of defects increases with increasing the annealing time.

cond-mat.supr-con↗

arXiv2012

Strong polaritonic interaction between flux-flow and phonon resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions: Angular dependence and the alignment procedure

Bi2Sr2CaCu2O8+x single crystals represent natural stacks of atomic scale intrinsic Josephson junctions, formed between metallic CuO2-Ca-CuO2 and ionic insulating SrO-2BiO-SrO layers. Electrostriction effect in the insulating layers leads to excitation of c-axis phonons by the ac-Josephson effect. Here we study experimentally the interplay between and velocity matching (Eck) electromagnetic resonances in the flux-flow state of small mesa structures with c-axis optical phonons. A very strong interaction is reported, which leads to formation of phonon-polaritons with infrared and Raman-active transverse optical phonons. A special focus in this work is made on analysis of the angular dependence of the resonances. We describe an accurate sample alignment procedure that prevents intrusion of Abrikosov vortices in fields up to 17 Tesla, which is essential for achieving high-quality resonances at record high frequencies up to 13 THz.

cond-mat.supr-con↗

arXiv2012

The anti-ordinary Hall effect in NiPt thin films

We study the anomalous Hall effect in binary alloys between the group-10 elements Ni and Pt. It is observed that the ordinary Hall effect is negative (electron-like) at any composition of the alloy. The extraordinary Hall effect is also negative except in the vicinity of the ferromagnetic quantum critical point. Close to the critical point the sign of the extraordinary Hall effect can be changed to positive (hole-like) by tuning either the temperature or the composition of the alloy. We attribute such an "anti-ordinary" Hall effect with opposite signs of the ordinary and the extraordinary contributions to a Berry phase singularity, moving away from the Fermi energy with increasing the ferromagnetic exchange energy.

cond-mat.mtrl-sci↗

arXiv2012

Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures

Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x mesa structures leads to persistent doping of the mesas. Here we employ this effect for analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are able to controllably and reversibly change the doping state of the same single crystal from underdoped to overdoped state, without changing its chemical composition. It is observed that such physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping the critical temperature and the superconducting gap decrease, with underdoping the c-axis critical current rapidly decreases due to progressively more incoherent interlayer tunneling and the pseudogap rapidly increases, indicative for the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even in voltage contribution, attributed to a charge transfer effect, and (ii) odd in voltage contribution, attributed to reordering of oxygen impurities.

cond-mat.supr-con↗

arXiv2011

Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures

We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi2Sr2CaCu2O8+x and Bi1.75Pb0.25Sr2CaCu2O8+x single crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on lambdaJ(Jc) and occur at about 0.4 and 1.3 fluxons per lambdaJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

cond-mat.supr-con↗

arXiv2010

Observation of polaritons in Bi2Sr2CaCu2O8+x single crystals

The Bi2Sr2CaCu2O8+x high-temperature superconductor represents a natural metamaterial, composed of metallic CuO bilayers sandwiched between ionic BiO planes. Each pair of CuO bilayers forms an atomic-scale Josephson junction. Here we employ the intrinsic Josephson effect for in situ generation and detection of electromagnetic waves in Bi2Sr2CaCu2O8+x single crystals. We observe that electromagnetic waves form polaritons with several transverse optical phonons. This indicates the presence of unscreened polar response in cuprates, which may lead to strong electronphonon interaction. Our technique can provide intense local sources of coherent, monochromatic phonon-polaritons with kW/cm2 power densities.

cond-mat.supr-con↗

arXiv2010

Detection of the phase shift from a single quantized superconducting vortex

An Abrikosov vortex in a superconductor carries a flux quantum, Phi_0 = hc/2e, localized at its center, but induces a global 2pi phase rotation in the superconducting condensate. This long-range gauge field outside the area pierced by a magnetic field is due to the Aharonov-Bohm effect, which is a non-classical phenomenon that illustrates the significance of potentials rather than forces in quantum mechanics. In the London gauge, the phase of the condensate is given by the polar angle around the vortex. Here we raise the question whether this phase shift could be detected by means of Cooper pair interferometry using Josephson junctions as phase-sensitive detectors. We introduce a single Abrikosov vortex into a superconducting lead with a detector junction made at the edge of the lead. We observe that the vortex induces a Josephson phase shift equal to the polar angle of the vortex within the junction length. When the vortex is close to the junction it induces a pi-step in the Josephson phase difference, leading to a controllable and reversible switching of the junction into the 0 - pi state. This in turn results in an unusual Phi_0/2 quantization of the flux in the junction. The vortex may hence act as a tunable "phase battery" for quantum electronics.

cond-mat.supr-con↗