arXiv subjects

A. Rydh

Publications and source records attributed to A. Rydh.

At least 19 recordsLinked to original sources

arXiv2024

Quantum Critical Scaling of Specific Heat in a Quasicrystal

In strongly correlated systems, interactions give rise to critical fluctuations surrounding the quantum critical point (QCP) of a quantum phase transition. Quasicrystals allow the study of quantum critical phenomena in aperiodic systems with frustrated magnetic interactions. Here, we study the magnetic field and temperature scaling of the low-temperature specific heat for the quantum critical Yb-Au-Al quasicrystal. We devise a scaling function that encapsulates the limiting behaviors as well as the area where the system goes from a temperature-limited to a field-limited quantum critical region, where magnetic field acts as a cutoff for critical fluctuations. The zero-field electronic specific heat is described by a power-law divergence, ${C_{el}/T \propto T^{-0.54}}$, aligning with previously observed ac-susceptibility and specific heat measurements. The field dependence of the electronic specific heat at high magnetic fields shows a similar power-law ${C_{el}/T \propto B^{-0.50}}$. In the zero-field and low-field region, we observe two small but distinct anomalies in the specific heat, located at 0.7 K and 2.1 K.

cond-mat.str-el↗

arXiv2024

Precursor to Quantum Criticality in Ce-Au-Al Quasicrystal Approximants

Rare-earth element containing aperiodic quasicrystals and their related periodic approximant crystals can exhibit non-trivial physical properties at low temperatures. Here, we investigate the 1/1 and 2/1 approximant crystal phases of the Ce-Au-Al system by studying the ac-susceptibility and specific heat at low temperatures and in magnetic fields up to 12 T. We find that these systems display signs of quantum criticality similar to the observations in other claimed quantum critical systems, including the related Yb-Au-Al quasicrystal. In particular, the ac-susceptibility at low temperatures shows a diverging behavior $\chi \propto 1/T$ as the temperature decreases as well as cutoff-behavior in magnetic field. Notably, the field dependence of $\chi$ closely resembles that of quantum critical systems. However, the ac-susceptibility both in zero and nonzero magnetic fields can be understood from the splitting of a ground state Kramers doublet of Ce$^{3+}$. The high-temperature Curie-Weiss fit yields an effective magnetic moment of approximately 2.54$\mu_{\mathrm{B}}$ per Ce for both approximant systems, which is reduced to $\sim$2.0$\mu_{\mathrm{B}}$ at temperatures below 10 K. The low-temperature specific heat is dominated by the Schottky anomaly originating from the splitting of the Ce$^{3+}$ Kramers doublet, resulting in an entropy of $R\ln 2$ at around 10 K.

cond-mat.str-el↗

arXiv2023

Magnetic field as a dynamic energy scale in quantum-critical CeCoIn$_5$

Whether magnetic field enters the quantum-critical dynamics as a competing energy scale in strange metals remains an open question. Using thermal impedance spectroscopy, we simultaneously measure the electronic specific heat and spin-relaxation dynamics of CeCoIn$_5$. Both properties show scale invariance with temperature and magnetic field, where a single energy scale governs their crossover. Magnetic field thus competes directly with temperature to set the infrared cutoff, establishing it as a dynamic energy scale in the critical dynamics of this strange metal.

cond-mat.str-el↗

arXiv2023

Wide Range Thin-FIlm Ceramic Metal-Alloy Thermometers with Low Magnetoresistance

Many thermal measurements in high magnetic fields require thermometers that are sensitive over a wide temperature range, are low mass, have a rapid thermal response, and have a minimal, easily correctable magnetoresistance. Here we report the development of a new granular-metal oxide ceramic composite (cermet) for this purpose formed by co-sputtering of the metallic alloy nichrome Ni$_{0.8}$Cr$_{0.2}$ and the insulator silcon dioxide SiO$_2$. The resulting thin films are sensitive enough to be used from room temperature down to below 100 mK in magnetic fields up to at least 35 tesla.

cond-mat.mtrl-sci↗

arXiv2023

Calorimetric measurement of nuclear spin-lattice relaxation rate in metals

The quasiparticle density of states in correlated and quantum-critical metals directly probes the effect of electronic correlations on the Fermi surface. Measurements of the nuclear spin-lattice relaxation rate provide one such experimental probe of quasiparticle mass through the electronic density of states. By far the most common way of accessing the spin-lattice relaxation rate is via nuclear magnetic resonance and nuclear quadrupole resonance experiments, which require resonant excitation of nuclear spin transitions. Here we report non-resonant access to spin-lattice relaxation dynamics in AC-calorimetric measurements. The nuclear spin-lattice relaxation rate is inferred in our measurements from its effect on the frequency dispersion of the thermal response of the calorimeter-sample assembly. We use fast, lithographically-defined nanocalorimeters to access the nuclear spin-lattice relaxation times in metallic indium from 0.3~K to 7~K and in magnetic fields up to 35~T.

cond-mat.str-el↗

arXiv2022

Element-specific probe of quantum criticality in $\mathrm{CeCoIn_{5}}$

Employing the elemental sensitivity of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), we study the valence and magnetic order in the heavy fermion superconductor CeCoIn$_5$. We probe spin population of the f-electrons in Ce and d-electrons in Co as a function of temperature (down to 0.1 K) and magnetic field (up to 6 T). From the XAS we find a pronounced contribution of Ce$^{4+}$ component at low temperature and a clear temperature dependence of the Ce valence below 5 K, suggesting enhanced valence fluctuations, an indication for the presence of a nearby quantum critical point (QCP). We observe no significant corresponding change with magnetic field. The XMCD displays a weak signal for Ce becoming clear only at 6 T. This splitting of the Kramers doublet ground state of Ce$^{3+}$ is significantly smaller than expected for independent but screened ions, indicating strong antiferromagnetic pair interactions. The unconventional character of superconductivity in CeCoIn$_5$ is evident in the extremely large specific heat step at the superconducting transition.

cond-mat.str-el↗

arXiv2022

Superconducting properties of the spin Hall candidate Ta3Sb with eightfold degeneracy

We report the synthesis and characterization of phase pure Ta3Sb, a material predicted to be topological with eightfold degenerate fermionic states [Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Sci. Adv. 5, eaav8575 (2019]. We observe superconductivity in Ta3Sb with Tc~ 0.67 K in both electrical resistivity \r{ho}(T) and specific heat C(T) measurements. Field dependent measurements yield the superconducting phase diagram with an upper critical field of Hc2(0) ~ 0.95 T, corresponding to a superconducting coherence length of ξ ~18.6 nm. The gap ratio deduced from specific heat anomaly, 2Δ0/kBTc is 3.46, a value close to the Bardeen-Cooper-Schrieffer (BCS) value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of \k{appa} ~90 is obtained identifying Ta3Sb as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions Ta3Sb as an appealing platform to further explore exotic quantum states in multifold degenerate systems.

cond-mat.supr-con↗

arXiv2022

Magnetoquantum Oscillations in the Specific Heat of a Topological Kondo Insulator

Surprisingly, magnetoquantum oscillations (MQO) characteristic of a metal with a Fermi surface have been observed in measurements of the topological Kondo insulator SmB6. As these MQO have only been observed in measurements of magnetic torque (dHvA) and not in measurements of magnetoresistance (SdH), a debate has arisen as to whether the MQO are an extrinsic effect arising from rare-earth impurities, defects, and/or aluminum inclusions or an intrinsic effect revealing the existence of charge-neutral excitations. We report here the first observation of magnetoquantum oscillations in the low-temperature specific heat of SmB6. The observed frequencies and their angular dependence for these flux-grown samples are consistent with previous results based on magnetic torque for SmB6 but the inferred effective masses are significantly larger than previously reported. Such oscillations can only be observed if the MQO are of bulk thermodynamic origin; the measured magnetic-field dependent oscillation amplitude and effective mass allow us to rule out suggestions of an extrinsic, aluminium inclusion-based origin for the MQO.

cond-mat.str-el↗

arXiv2022

Observation of Quantum Oscillations in The Low Temperature Specific Heat of SmB$_6$

We report measurements of the low-temperature specific heat of Al-flux-grown samples of SmB$_6$ in magnetic fields up to 32 T. Quantum oscillations periodic in $1/H$ are observed between 8 and 32 T at selected angles between [001] and [111]. The observed frequencies and their angular dependence are consistent with previous magnetic torque measurements of SmB$_6$ but the effective masses inferred from Lifshitz-Kosevich theory are significantly larger and closer to those inferred from zero-field specific heat. Our results are thus consistent with a bulk density of states origin for the oscillations.

cond-mat.str-el↗

arXiv2020

Transforming physics laboratory work from 'cookbook' type to genuine inquiry

'Cookbook' style laboratory tasks have long been criticised for the lack of critical and independent thought that students need in order to complete them. We present an account of how we transformed a 'cookbook' lab to a genuine inquiry experiment in first year physics. Crucial features of the work were visits to see other teaching laboratories, understanding student preparedness and the selection of an appropriate experiment to develop. The new two session laboratory work is structured so students make decisions related to the method of a basic experiment in the first session and then have freedom to investigate any aspect they wish to in the second. Formative feedback on laboratory notebook keeping is provided by short online activities.

physics.ed-ph↗

arXiv2018

Unusual interplay between superconductivity and field-induced charge order in YBa2Cu3Oy

We present a detailed study of the temperature (T) and magnetic field (H) dependence of the electronic density of states (DOS) at the Fermi level, as deduced from specific heat and Knight shift measurements in underdoped YBa2Cu3Oy. We find that the DOS becomes field-independent above a characteristic field H_{DOS} and that the H_{DOS}(T) line displays an unusual inflection near the onset of the long range 3D charge-density wave order. The unusual S-shape of H_{DOS}(T) is suggestive of two mutually-exclusive orders that eventually establish a form of cooperation in order to coexist at low T. On theoretical grounds, such a collaboration could result from the stabilisation of a pair-density wave state, which calls for further investigations in this region of the phase diagram

cond-mat.supr-con↗

arXiv2018

Anisotropic superconductivity and magnetism in single-crystal RbEuFe$_4$As$_4$

We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe$_4$As$_4$ using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu-moments of $T_m$ = 15 K and a superconducting transition temperature of $T_c$ = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 kG/K and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of $Γ$ = 1.7. Ginzburg-Landau parameters of $κ_c \sim 67$ and $κ_{ab} \sim 108$ indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu-magnetism is quasi-two dimensional as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and $<$ 0.04 K. A consequence of the quasi-2D nature of the Eu-magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a cusp-like anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 kG and 4 kG.

cond-mat.supr-con↗

arXiv2018

Raising the superconducting $T_\mathrm{c}$ of gallium: in-situ characterization of the transformation of $α$-Ga into $β$-Ga

Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable $\mathrmβ$-Ga with a critical temperature $T_\mathrm{c}= 6.04(5)\,\mathrm{K}$, while stable $\mathrmα$-Ga has a much lower $T_\mathrm{c}<1.2\,\mathrm{K}$. Here we use a membrane-based nanocalorimeter to initiate the transition from $\mathrmα$-Ga to $\mathrmβ$-Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in-situ transformation is initiated by bringing the temperature to about $10\,\mathrm{K}$ above the melting temperature of $\mathrmα$-Ga. After such treatment, the liquid supercools down to $232\,\mathrm{K}$, where $\mathrmβ$-Ga solidifies. We find that $\mathrmβ$-Ga is a strong-coupling type-I superconductor with $Δ(0)/k_\mathrm{B}T_\mathrm{c} =2.00(5)$ and a Sommerfeld coefficient $γ_\mathrm{n} = 1.53(4)\,\mathrm{mJ/molK^2}$, 2.55 times higher than that in the $α$ phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.

cond-mat.supr-con↗

arXiv2015

Superconducting gap evolution in overdoped BaFe$_{2}$(As$_{1-x}$P$_x$)$_2$ single crystals through nanocalorimetry

We report on specific heat measurements on clean overdoped $\mathrm{BaFe_{2}(As_{1-x}P_x)_2}$ single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature $γ_\mathrm{r}={C/T}|_{T \to 0}$ is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave $α$ model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of $Δ_0 \sim 5.3\,\mathrm{meV}$, corresponding to $Δ_0 / k_\mathrm{B} T_\mathrm{c} \sim 2.2$. Increasing the phosphorus concentration $x$, the main gap reduces till a value of $Δ_0 \sim 1.9\,\mathrm{meV}$ for $x = 0.55$ and a second weaker gap becomes evident. From the magnetic field effect on $γ_\mathrm{r}$, all samples however show similar behavior [$γ_\mathrm{r}(H) - γ_\mathrm{r}(H=0) \propto H^n$, with $n$ between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

cond-mat.supr-con↗

arXiv2015

Microscopic Parameters from High-Resolution Specific Heat Measurements on Overdoped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ Single Crystals

We investigate the electronic specific heat of overdoped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the doping dependence of the condensation energy, superconducting gap $Δ$, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump $ΔC \propto T_{\mathrm{c}}^3$, found in many iron-based superconductors, in this system originates from a $T_\mathrm{c}$-dependent ratio $Δ/k_\mathrm{B}T_\mathrm{c}$ in combination with a doping-dependent density of states $N(\varepsilon_\mathrm{F})$. A clear enhancement is seen in the effective mass $m^{*}$ as the composition approaches the value that has been associated with a quantum critical point at optimum doping. However, a simultaneous increase in the superconducting carrier concentration $n_\mathrm{s}$ maintains the superfluid density, yielding an apparent penetration depth $λ$ that decreases with increasing $T_\mathrm{c}$ without sharp divergence at the quantum critical point. Uemura scaling indicates that $T_\mathrm{c}$ is governed by the Fermi temperature $T_\mathrm{F}$ for this multi-band system.

cond-mat.supr-con↗

arXiv2014

Thermodynamics around the First-Order Ferromagnetic Phase Transition of ${\rm Fe}_2{\rm P}$ Single Crystals

The specific heat and thermodynamics of ${\rm Fe}_2{\rm P}$ single-crystals around the first order paramagnetic (PM) to ferromagnetic (FM) phase transition at $T_{\rm C} = 217 \,{\rm K}$ are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived H-T phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the $c$-axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields, the phase transition is split into two, with an intermediate FM phase where there is no spontaneous magnetization along the $c$-axis. The zero-field transition displays a text-book example of a first order transition with different phase stability limits on heating and cooling. The results have special significance since ${\rm Fe}_2{\rm P}$ is the parent material to a family of compounds with outstanding magnetocaloric properties.

cond-mat.mtrl-sci↗

arXiv2013

Evidence for nonlocal electrodynamics in planar Josephson junctions

We study temperature dependence of the critical current modulation Ic(H) for two types of planar Josephson junctions: a low-Tc Nb/CuNi/Nb and a high-Tc YBa2Cu3O7 bicrystal grain-boundary junction. At low T both junctions exhibit a conventional behavior, described by the local sine-Gordon equation. However, at elevated T the behavior becomes qualitatively different: the Ic(H) modulation field deltaH becomes almost T-independent and neither deltaH nor the critical field for penetration of Josephson vortices vanish at Tc. Such an unusual behavior is in good agreement with theoretical predictions for junctions with nonlocal electrodynamics. We extract absolute values of the London penetration depth from our data and show that a crossover from local to nonlocal electrodynamics occurs with increasing T when London penetration depth becomes larger than the electrode thickness.

cond-mat.supr-con↗

arXiv2013

High bias anomaly in YBa2Cu3O7/LaMnO_{3+delta}/YBa2Cu3O7 Superconductor/Ferromagnetic Insulator/Superconductor junctions: Evidence for a long-range superconducting proximity effect through the conduction band of a ferromagnetic insulator

We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+δ}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a step-like onset of excess current that occurs below the superconducting transition temperature $T<T_c$ and is easily suppressed by a magnetic field. The phenomenon is attributed to a novel type of the superconducting proximity effect of non-equilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly non-equilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long-range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.

cond-mat.supr-con↗