arXiv subjects

J. B. Betts

Publications and source records attributed to J. B. Betts.

At least 19 recordsLinked to original sources

arXiv2022

Energy-scale competition in the Hall resistivity of a strange metal

Anomalous transport behavior -- both longitudinal and Hall -- is the defining characteristic of the strange-metal state of High-Tc cuprates. The temperature, frequency, and magnetic field dependence of the resistivity is understood within strange metal phenomenology as resulting from energy-scale competition to set the inelastic relaxation rate. The anomalously strong temperature dependence of the Hall coefficient, however, is at odds with this phenomenology. Here we report measurements of the Hall resistivity in the strange metal state of cuprates over a broad range of magnetic fields and temperatures. The observed field and temperature dependent Hall resistivity at very high magnetic fields reveals a distinct high-field regime which is controlled by energy-scale competition. This extends the strange metal phenomenology in the cuprates to include the Hall resistivity and suggests, in particular, that the direct effect of magnetic field on the relaxation dynamics of quantum fluctuations may be at least partially responsible for the anomalous Hall resistivity of the strange metal state.

cond-mat.str-el↗

arXiv2020

Skyrmion lattice creep at ultra-low current densities

Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities $j_c$ to depin from the underlying atomic lattice. Above $j_c$ skyrmions exhibit well-controlled motion, making them prime candidates for race-track memories. In thin films thermally-activated creep motion of isolated skyrmions was observed below $j_c$ as predicted by theory. Uncontrolled skyrmion motion is detrimental for race-track memories and is not fully understood. Notably, the creep of skyrmion lattices in bulk materials remains to be explored. Here we show using resonant ultrasound spectroscopy--a probe highly sensitive to the coupling between skyrmion and atomic lattices--that in the prototypical skyrmion lattice material MnSi depinning occurs at $j_c^*$ that is only 4 percent of $j_c$. Our experiments are in excellent agreement with Anderson-Kim theory for creep and allow us to reveal a new dynamic regime at ultra-low current densities characterized by thermally-activated skyrmion-lattice-creep with important consequences for applications.

cond-mat.str-el↗

arXiv2020

Scale-Invariance of a Spin Liquid in High Magnetic Fields

In RuCl$_3$, inelastic neutron scattering and Raman spectroscopy reveal a continuum of non-spin-wave excitations that persists to high temperature, suggesting the presence of a spin liquid state on a honeycomb lattice. In the context of the Kitaev model, magnetic fields introduce finite interactions between the elementary excitations, and thus the effects of high magnetic fields - comparable to the spin exchange energy scale - must be explored. Here we report measurements of the magnetotropic coefficient - the second derivative of the free energy with respect to magnetic field orientation - over a wide range of magnetic fields and temperatures. We find that magnetic field and temperature compete to determine the magnetic response in a way that is independent of the large intrinsic exchange interaction energy. This emergent scale-invariant magnetic anisotropy provides evidence for a high degree of exchange frustration that favors the formation of a spin liquid state in RuCl$_3$.

cond-mat.str-el↗

arXiv2020

Scale-invariant magnetic anisotropy in RuCl$_3$ at high magnetic fields

cond-mat.str-el↗

arXiv2019

Phase stabilization by electronic entropy in plutonium

(Pu) has an unusually rich phase diagram that includes seven distinct solid state phases and an unusually large 25% collapse in volume from its delta phase to its low temperature alpha phase via a series of structural transitions. Despite considerable advances in our understanding of strong electronic correlations within various structural phases of Pu and other actinides, the thermodynamic mechanism responsible for driving the volume collapse has continued to remain a mystery. Here we utilize the unique sensitivity of magnetostriction measurements to unstable f electron shells to uncover the crucial role played by electronic entropy in stabilizing delta-Pu against volume collapse. We find that in contrast to valence fluctuating rare earths, which typically have a single f electron shell instability whose excitations drive the volume in a single direction in temperature and magnetic field, delta-Pu exhibits two such instabilities whose excitations drive the volume in opposite directions while producing an abundance of entropy at elevated temperatures. The two instabilities imply a near degeneracy between several different configurations of the 5f atomic shell, giving rise to a considerably richer behavior than found in rare earth metals. We use heat capacity measurements to establish a robust thermodynamic connection between the two excitation energies, the atomic volume, and the previously reported excess entropy of delta-Pu at elevated temperatures.

cond-mat.str-el↗

arXiv2018

Cyclotron and combined phonon-assisted resonances in double-well heterostructure In$_{0.65}$Ga$_{0.35}$As/In$_{0.52}$Al$_{0.48}$As at megagauss magnetic fields

Experiments on resonances of conduction electrons in InGaAs/InAlAs double quantum wells at megagauss magnetic fields in the Faraday geometry are reported. We observe new cyclotron resonances assisted by emission of InAs-like and GaAs-like optic phonons and a combined (cyclotron-spin) resonance assisted by emission of InAs-like phonon. The observations are very well described for three laser frequencies with the use of an eight-band (three level) $\textbf{k}\cdot \textbf{p}$ model, taking into account position- and energy-dependent effective masses and spin g-factors. It is indicated that the new observations are possible due to the application very high magnetic fields.

physics.app-ph↗

arXiv2018

Scale-invariant magnetoresistance in a cuprate superconductor

The anomalous metallic state in high-temperature superconducting cuprates is masked by the onset of superconductivity near a quantum critical point. Use of high magnetic fields to suppress superconductivity has enabled a detailed study of the ground state in these systems. Yet, the direct effect of strong magnetic fields on the metallic behavior at low temperatures is poorly understood, especially near critical doping, $x=0.19$. Here we report a high-field magnetoresistance study of thin films of \LSCO cuprates in close vicinity to critical doping, $0.161\leq x\leq0.190$. We find that the metallic state exposed by suppressing superconductivity is characterized by a magnetoresistance that is linear in magnetic field up to the highest measured fields of $80$T. The slope of the linear-in-field resistivity is temperature-independent at very high fields. It mirrors the magnitude and doping evolution of the linear-in-temperature resistivity that has been ascribed to Planckian dissipation near a quantum critical point. This establishes true scale-invariant conductivity as the signature of the strange metal state in the high-temperature superconducting cuprates.

cond-mat.str-el↗

arXiv2018

Quantum limit transport and destruction of the Weyl nodes in TaAs

Weyl fermions are a new ingredient for correlated states of electronic matter. A key difficulty has been that real materials also contain non-Weyl quasiparticles, and disentangling the experimental signatures has proven challenging. We use magnetic fields up to 95 tesla to drive the Weyl semimetal TaAs far into its quantum limit (QL), where only the purely chiral 0th Landau levels (LLs) of the Weyl fermions are occupied. We find the electrical resistivity to be nearly independent of magnetic field up to 50 tesla: unusual for conventional metals but consistent with the chiral anomaly for Weyl fermions. Above 50 tesla we observe a two-order-of-magnitude increase in resistivity, indicating that a gap opens in the chiral LLs. Above 80 tesla we observe strong ultrasonic attenuation below 2 kelvin, suggesting a mesoscopically-textured state of matter. These results point the way to inducing new correlated states of matter in the QL of Weyl semimetals.

cond-mat.str-el↗

arXiv2018

Anisotropic magneto-crystalline coupling of the skyrmion lattice in MnSi

We investigate the anisotropic nature of magnetocrystalline coupling between the crystallographic and skyrmion crystal (SKX) lattices in the chiral magnet MnSi by magnetic field-angle resolved resonant ultrasound spectroscopy. Abrupt changes are observed in the elastic moduli and attenuation when the magnetic field is parallel to the [011] crystallographic direction. These observations are interpreted in a phenomenological Ginzburg-Landau theory that identifies switching of the SKX orientation to be the result of an anisotropic magnetocrystalline coupling potential. Our paper sheds new light on the nature of magnetocrystalline coupling potential relevant to future spintronic applications.

cond-mat.str-el↗

arXiv2018

Resonant torsion magnetometry in anisotropic quantum materials

Unusual behavior of quantum materials commonly arises from their effective low-dimensional physics, which reflects the underlying anisotropy in the spin and charge degrees of freedom. Torque magnetometry is a highly sensitive technique to directly quantify the anisotropy in quantum materials, such as the layered high-T$_c$ superconductors, anisotropic quantum spin-liquids, and the surface states of topological insulators. Here we introduce the magnetotropic coefficient $k=\partial^2 F/\partial θ^2$, the second derivative of the free energy F with respect to the angle $θ$ between the sample and the applied magnetic field, and report a simple and effective method to experimentally detect it. A sub-$μ$g crystallite is placed at the tip of a commercially available atomic force microscopy cantilever, and we show that $k$ can be quantitatively inferred from a shift in the resonant frequency under magnetic field. While related to the magnetic torque $τ=\partial F/\partial θ$, $k$ takes the role of torque susceptibility, and thus provides distinct insights into anisotropic materials akin to the difference between magnetization and magnetic susceptibility. The thermodynamic coefficient $k$ is discontinuous at second-order phase transitions and subject to Ehrenfest relations with the specific heat and magnetic susceptibility. We apply this simple yet quantitative method on the exemplary cases of the Weyl-semimetal NbP and the spin-liquid candidate RuCl$_3$, yet it is broadly applicable in quantum materials research.

cond-mat.str-el↗

arXiv2017

Elasticity in the skyrmion phase unveils depinning at ultra-low current densities

Controlled movement of nano-scale stable magnetic objects has been proposed as the foundation for a new generation of magnetic storage devices. Magnetic skyrmions, vortex-like spin textures stabilized by their topology are particularly promising candidates for this technology. Their nanometric size and ability to be displaced in response to an electrical current density several orders of magnitude lower than required to induce motion of magnetic domain walls suggest their potential for high-density memory devices that can be operated at low power. However, to achieve this, skyrmion movement needs to be controlled, where a key question concerns the coupling of skyrmions with the underlying atomic lattice and disorder (pinning). Here, we use Resonant Ultrasound Spectroscopy (RUS), a probe highly sensitive to changes in the elastic properties, to shed new light on skyrmion elasticity and depinning in the archetypal skyrmion material MnSi. In MnSi, skyrmions form a lattice that leads to pronounced changes in the elastic properties of the atomic lattice as a result of magneto-crystalline coupling. Without an applied current, the shear and compressional moduli of the underlying crystal lattice exhibit an abrupt change in the field-temperature range where skyrmions form. For current densities exceeding $j_c^*$ the changes of elastic properties vanish, signaling the decoupling of skyrmion and atomic lattices. Interestingly, $j_c^*$, which we identify as the onset of skyrmion depinning, is about 20 times smaller than $j_c$ previously measured via non-linear Hall effect. Our results suggest the presence of a previously-undetected intermediate dynamic regime possibly dominated by skyrmion-creep motion with important consequences for potential applications.

cond-mat.str-el↗

arXiv2016

Landau levels and shallow donor states in GaAs/AlGaAs multiple quantum wells at mega-gauss magnetic fields

Landau levels and shallow donor states in multiple GaAs/AlGaAs quantum wells (MQWs) are investigated by means of the cyclotron resonance at mega-gauss magnetic fields. Measurements of magneto-optical transitions were performed in pulsed fields up to 140 T and temperatures from 6 to 300 K. The $14\times14$ \textbf{P}$\cdot$\textbf{p} band model for GaAs is used to interpret free-electron transitions in a magnetic field. Temperature behavior of the observed resonant structure indicates, in addition to the free-electron Landau states, contributions of magneto-donor states in the GaAs wells and possibly in the AlGaAs barriers. The magneto-donor energies are calculated using a variational procedure suitable for high magnetic fields and accounting for conduction band nonparabolicity in GaAs. It is shown that the above states, including their spin splitting, allow one to interpret the observed magneto-optical transitions in MQWs in the middle infrared region. Our experimental and theoretical results at very high magnetic fields are consistent with the picture used previously for GaAs/AlGaAs MQWs at lower magnetic fields.

cond-mat.str-el↗

arXiv2015

Thermodynamic signature of a magnetic-field-driven phase transition within the superconducting state of an underdoped high-temperature superconductor

More than a quarter century after the discovery of the high temperature superconductor (HTS) YBa$_2$Cu$_3$O$_{6+δ}$ (YBCO) studies continue to uncover complexity in its phase diagram. In addition to HTS and the pseudogap there is growing evidence for multiple phases with boundaries which are functions of temperature ($T$), doping (p) and magnetic field. Here we report the low temperature electronic specific heat (C$_{elec}$) of YBCO6.47 (p=0.08) up to a magnetic field (H) of 34.5 teslas (T), a poorly understood region of the underdoped H-$T$-p phase space. We observe two regimes: below a characteristic magnetic field H'$\approx$10 T, C$_{elec}/T$ obeys an expected H$^{1/2}$ behavior, however, near H' there is a sharp inflection followed by a linear-in-H behavior. H' rests deep within the superconducting phase and the linear-in-H behavior is observed in the zero resistance regime. In the limit of zero temperature, C$_{elec}/T$ is proportional to the zero-energy electronic density of states. Thus this inflection is evidence of a magnetic-field-driven quantum phase transition.

cond-mat.supr-con↗

arXiv2015

Quasiparticle mass enhancement approaching optimal doping in a high-Tc superconductor

In the quest for superconductors with high transition temperatures (T$_\mathrm{c}$s), one emerging motif is that unconventional superconductivity is enhanced by fluctuations of a broken-symmetry phase near a quantum-critical point. While recent experiments have suggested the existence of the requisite broken symmetry phase in the high-T$_\mathrm{c}$ cuprates, the signature of quantum-critical fluctuations in the electronic structure has thus far remained elusive, leaving their importance for high-T$_\mathrm{c}$ superconductivity in question. We use magnetic fields exceeding 90 tesla to access the underlying metallic state of the cuprate YBa2Cu3O6+$_δ$ over an unprecedented range of doping, and magnetic quantum oscillations reveal a strong enhancement in the quasiparticle effective mass toward optimal doping. This mass enhancement is a characteristic signature of quantum criticality, and identifies a quantum-critical point at p$_{crit}$ $\approx$ 0.18. This point also represents the juncture of the vanishing pseudogap energy scale and the disappearance of Kerr rotation, the negative Hall coefficient, and the recently observed charge order, suggesting a mechanism of high-T$_\mathrm{c}$ that is strongest when these definitive experimental signatures of the underdoped cuprates converge at a quantum critical point.

cond-mat.supr-con↗

arXiv2014

Avoided Valence Transition in a Plutonium Superconductor

Some of the most remarkable phenomena---and greatest theoretical challenges---in condensed matter physics arise when $d$ or $f$ electrons are neither fully localized around their host nuclei, nor fully itinerant. This localized/itinerant "duality" underlies the correlated electronic states of the high-$T_c$ cuprate superconductors and the heavy-fermion intermetallics, and is nowhere more apparent than in the $5f$ valence electrons of plutonium. Here we report the full set of symmetry-resolved elastic moduli of $PuCoGa_5$---the highest $T_c$ superconductor of the heavy fermions ($T_c$=18.5 K)---and find that the bulk modulus softens anomalously over a wide range in temperature above $T_c$. Because the bulk modulus is known to couple strongly to the valence state, we propose that plutonium valence fluctuations drive this elastic softening. This elastic softening is observed to disappear when the superconducting gap opens at $T_c$, suggesting that plutonium valence fluctuations have a strong footprint on the Fermi surface, and that $PuCoGa_5$ avoids a valence-transition by entering the superconducting state. These measurements provide direct evidence of a valence instability in a plutonium compound, and suggest that the unusually high-$T_c$ in this system is driven by valence fluctuations.

cond-mat.supr-con↗

arXiv2012

A cascade of magnetic field induced spin transitions in LaCoO3

We present magnetization and magnetostriction studies of the insulating perovskite LaCoO3 in magnetic fields approaching 100 T. In marked contrast with expectations from single-ion models, the data reveal two distinct first-order spin transitions and well-defined magnetization plateaux. The magnetization at the higher plateau is only about half the saturation value expected for spin-1 Co3+ ions. These findings strongly suggest collective behavior induced by strong interactions between different electronic -- and therefore spin -- configurations of Co3+ ions. We propose a model of these interactions that predicts crystalline spin textures and a cascade of four magnetic phase transitions at high fields, of which the first two account for the experimental data.

cond-mat.str-el↗

arXiv2010

Planes, Chains, and Orbits: Quantum Oscillations and High Magnetic Field Heat Capacity in Underdoped YBCO

The underlying physics of the magnetic-field-induced resistive state in high temperature cuprate superconductors remains a mystery. One interpretation is that the application of magnetic field destroys the d-wave superconducting gap to uncover a Fermi surface that behaves like a conventional (i.e.Fermi Liquid) metal (1). Another view is that an applied magnetic field destroys long range superconducting phase coherence, but the superconducting gap amplitude survives (2, 3). By measuring the specific heat of ultra-clean YBa2Cu3O6.56 (YBCO 6.56), we obtain a measure of the quasi-particle density of states from the superconducting state well into the magnetic-field-induced resistive state. We have found that at very high magnetic fields the specific heat exhibits both the conventional temperature dependence and quantum oscillations expected for a Fermi Liquid. On the other hand, the magnetic field dependence of the quasi-particle density of states follows a \sqrt{H} behavior that persists right through the zero-resistance transition, evidencing the fully developed d-wave superconducting gap over the entire magnetic field range measured. The coexistence of these two phenomena pose a rigorous thermodynamic constraint on theories of high-magnetic-field resistive state in the cuprates.

cond-mat.supr-con↗

arXiv2008

Quantum Phase Transition in the Normal State of High-Tc Cuprates at Optimum Doping

By using a 60 T magnetic field to suppress superconductivity in La2-pSrpCuO4, (LSCO) we reveal an anomalous peak in the Hall number, located at optimum doping and developing at temperatures below the zero-field superconducting transition temperature, Tc. The anomaly bears a striking resemblance to observations in Bi2Sr2-xLaxCuO6+delta (BSLCO) [F. F. Balakirev et al., Nature (London) 424, 912 (2003)], suggesting a normal state phenomenology common to the cuprates that underlies the high-temperature superconducting phase. The peak is ascribed to the transformation of the "Fermi arcs" into a conventional FS, the signature of a Fermi surface reconstruction associated with a quantum phase transition (QPT) near optimum doping and co-incident with the collapse of the pseudogap state.

cond-mat.supr-con↗