arXiv subjects

B. Lake

Publications and source records attributed to B. Lake.

At least 37 records · Page 2Linked to original sources

arXiv2020

Strongly coupled charge, orbital and spin order in TbTe$_{3}$

We report a ground state with strongly coupled magnetic and charge density wave orders mediated via orbital ordering in the layered compound \tbt. In addition to the commensurate antiferromagnetic (AFM) and charge density wave (CDW) orders, new magnetic peaks are observed whose propagation vector equals the sum of the AFM and CDW propagation vectors, revealing an intricate and highly entwined relationship. This is especially interesting given that the magnetic and charge orders lie in different layers of the crystal structure where the highly localized magnetic moments of the Tb$^{3+}$ ions are netted in the Tb-Te stacks, while the charge order is formed by the conduction electrons of the adjacent Te-Te layers. Our results, based on neutron diffraction and resonant x-ray scattering reveal that the charge and magnetic subsystems mutually influence each other via the orbital ordering of Tb$^{3+}$ ions.

cond-mat.str-el↗

arXiv2020

Enhanced spin correlations in the Bose-Einstein condensate compound Sr3Cr2O8

Combined experimental and modeling studies of the magnetocaloric effect, ultrasound, and magnetostriction were performed on single-crystal samples of the spin-dimer system Sr$_3$Cr$_2$O$_8$ in large magnetic fields, to probe the spin-correlated regime in the proximity of the field-induced XY-type antiferromagnetic order also referred to as a Bose-Einstein condensate of magnons. The magnetocaloric effect, measured under adiabatic conditions, reveals details of the field-temperature ($H,T$) phase diagram, a dome characterized by critical magnetic fields $H_{c1}$ = 30.4 T, $H_{c2}$ = 62 T, and a single maximum ordering temperature $T_{\rm max}(45~$T$)\simeq$8 K. The sample temperature was observed to drop significantly as the magnetic field is increased, even for initial temperatures above $T_{\rm max}$, indicating a significant magnetic entropy associated to the field-induced closure of the spin gap. The ultrasound and magnetostriction experiments probe the coupling between the lattice degrees of freedom and the magnetism in Sr$_3$Cr$_2$O$_8$. Our experimental results are qualitatively reproduced by a minimalistic phenomenological model of the exchange-striction by which sound waves renormalize the effective exchange couplings.

cond-mat.str-el↗

arXiv2020

Evidence for a three-dimensional quantum spin liquid in PbCuTe$_{2}$O$_{6}$

The quantum spin liquid (QSL) is a highly entangled magnetic state characterized by the absence of static magnetism in its ground state. Instead, the spins fluctuate in a highly correlated way down to the lowest temperatures. The QSL is very rare and is confined to a few specific cases where the interactions between the magnetic ions cannot be simultaneously satisfied (known as frustration). Lattices with magnetic ions in triangular or tetrahedral arrangements which interact via isotropic antiferromagnetic interactions can generate such a frustration. Three-dimensional isotropic spin liquids have mostly been sought in materials where the magnetic ions form pyrochlore or hyperkagome lattices. Here we present a three-dimensional lattice called the hyper-hyperkagome that enables spin liquid behaviour and manifests in the compound PbCuTe$_{2}$O$_{6}$. Using a combination of experiment and theory we show that this system exhibits signs of being a quantum spin liquid with no detectable static magnetism together with the presence of diffuse continua in the magnetic spectrum suggestive of fractional spinon excitations.

cond-mat.str-el↗

arXiv2020

Tensor network investigation of the double layer Kagome compound Ca$_{10}$Cr$_7$O$_{28}$

Quantum spin liquids are exotic quantum phases of matter that do not order even at zero temperature. While there are several toy models and simple Hamiltonians that could host a quantum spin liquid as their ground state, it is very rare to find actual, realistic materials that exhibits their properties. At the same time, the classical simulation of such instances of strongly correlated systems is intricate and reliable methods are scarce. In this work, we investigate the quantum magnet Ca$_{10}$Cr$_7$O$_{28}$ that has recently been discovered to exhibit properties of a quantum spin liquid in inelastic neutron scattering experiments. This compound has a distorted bilayer Kagome lattice crystal structure consisting of Cr$^{5+}$ ions with spin-$1/2$ moments. Coincidentally, the lattice structure renders a tensor network algorithm in 2D applicable that can be seen as a new variant of a projected entangled simplex state algorithm in the thermodynamic limit. In this first numerical investigation of this material that takes into account genuine quantum correlations, good agreement with the experimental findings is found. We argue that this is one of the very first studies of physical materials in the laboratory with tensor network methods, contributing to uplifting tensor networks from conceptual tools to methods to describe real two-dimensional quantum materials.

cond-mat.str-el↗

arXiv2019

Order out of a Coulomb phase and Higgs transtion: frustrated transverse interactions of Nd2Zr2O7

The pyrochlore material Nd$_2$Zr$_2$O$_7$ with an "all-in-all-out" (AIAO) magnetic order shows novel quantum moment fragmentation with gapped flat dynamical spin ice modes. The parameterized spin Hamiltonian with a dominant frustrated ferromagnetic transverse term reveals a proximity to a U(1) spin liquid. Here we study magnetic excitations of Nd$_2$Zr$_2$O$_7$ above the ordering temperature ($T_\text{N}$) using high-energy-resolution inelastic neutron scattering. We find strong spin ice correlations at zero energy with the disappearance of gapped magnon excitations of the AIAO order. It seems that the gap to the dynamical spin ice closes above $T_\text{N}$ and the system enters a quantum spin ice state competing with and suppressing the AIAO order. Classical Monte Carlo, molecular dynamics and quantum boson calculations support the existence of a Coulombic phase above $T_\text{N}$. Our findings relate the magnetic ordering of Nd$_2$Zr$_2$O$_7$ with the Higgs mechanism and provide explanations for several previously reported experimental features.

cond-mat.mtrl-sci↗

arXiv2019

Anisotropic exchange Hamiltonian, magnetic phase diagram and domain inversion of Nd$_2$Zr$_2$O$_7$

We present thermodynamic and neutron scattering measurements on the quantum spin ice candidate Nd$_2$Zr$_2$O$_7$. The parameterization of the anisotropic exchange Hamiltonian is refined based on high-energy-resolution inelastic neutron scattering data together with thermodynamic data using linear spin wave theory and numerical linked cluster expansion. Magnetic phase diagrams are calculated using classical Monte Carlo simulations with fields along \mbox{[100]}, \mbox{[110]} and \mbox{[111]} crystallographic directions which agree qualitatively with the experiment. Large hysteresis and irreversibility for \mbox{[111]} is reproduced and the microscopic mechanism is revealed by mean field calculations to be the existence of metastable states and domain inversion. Our results shed light on the explanations of the recently observed dynamical kagome ice in Nd$_2$Zr$_2$O$_7$ in \mbox{[111]} fields.

cond-mat.mtrl-sci↗

arXiv2018

Optimization of single crystal growth of candidate quantum spin-ice Pr2Hf2O7 by optical floating-zone method

We report the optimized conditions for growing the high quality single crystals of candidate quantum spin-ice Pr2Hf2O7 using the optical floating-zone method. Large single crystals of Pr2Hf2O7 have been grown under different growth conditions using a four-mirror type optical floating-zone furnace and their microscopic structural differences have been probed by high-resolution synchrotron x-ray diffraction (SXRD). The SXRD data reveal that the crystals grown under fowing argon (~ 2 L/h) atmosphere with slightly off-stoichiometric (optimized) starting composition yields the highest quality crystals. The magnetic susceptibility, isothermal magnetization and heat capacity data of optimally grown crystals are presented.

cond-mat.str-el↗

arXiv2018

Superconductivity in Ru0.55Rh0.45P and Ru0.75Rh0.25As probed by muon spin relaxation and rotation measurements

Superconductivity in the pseudo-binary pnictides Ru0.55Rh0.45P and Ru0.75Rh0.25As is probed by muon spin relaxation and rotation (muSR) measurements in conjuction with magnetic susceptibility, heat capacity and electrical resistivity measurements. Powder x-ray diffraction confirmed the MnP-type orthorhombic structure (space group Pnma) and showed a nearly single phase nature with small impurity phase(s) of about 5% for both the samples. The occurence of bulk superconductivity is confirmed with Tc = 3.7 K for Ru0.55Rh0.45P and T = 1.6 K for Ru0.75Rh0.25As. The superconducting state electronic heat capacity data reveal weak-coupling single-band isotropic s-wave gap BCS superconductivity. Various normal and superconducting state parameters are determined which reveal a weak-coupling electron-phonon driven type-II dirty-limit superconductivity for both the compounds. The upper critical field shows a linear temperature dependence down to the lowest measured temperatures which is quite unusual for a single-band superconductor. The muSR data confirm the conventional type-II behavior, and show evidence for a single-band s-wave singlet pairing superconductivity with a preserved time reversal symmetry for both the compounds.

cond-mat.supr-con↗

arXiv2018

Orbital- and spin-driven lattice instabilities in quasi-one-dimensional CaV$_2$O$_4$

Calcium vanadate CaV$_2$O$_4$ has a crystal structure of quasi-one-dimensional zigzag chains composed of orbital-active V$^{3+}$ ions and undergoes successive structural and antiferromagnetic phase transitions at $T_s\sim 140$ K and $T_N \sim 70$ K, respectively. We perform ultrasound velocity measurements on a single crystal of CaV$_2$O$_4$. The temperature dependence of its shear elastic moduli exhibits huge Curie-type softening upon cooling that emerges above and below $T_s$ depending on the elastic mode. The softening above $T_s$ suggests the presence of either onsite Jahn-Teller-type or intersite ferro-type orbital fluctuations in the two inequivalent V$^{3+}$ zigzag chains. The softening below $T_s$ suggests the occurrence of a dimensional spin-state crossover, from quasi-one to three, that is driven by the spin-lattice coupling along the inter-zigzag-chain orthogonal direction. The successive emergence of the orbital- and spin-driven lattice instabilities above and below $T_s$, respectively, is unique to the orbital-spin zigzag chain system of CaV$_2$O$_4$.

cond-mat.str-el↗

arXiv2018

Evidence for a dynamical ground state in a frustrated pyrohafnate Tb2Hf2O7

We report the physical properties of Tb2Hf2O7 based on ac magnetic susceptibility χ_ac(T), dc magnetic susceptibility χ(T), isothermal magnetization M(H), and heat capacity C_p(T) measurements combined with muon spin relaxation (\muSR) and neutron powder diffraction measurements. No evidence for long-range magnetic order is found down to 0.1 K. However, χ_ac(T) data present a frequency-dependent broad peak (near 0.9 K at 16 Hz) indicating slow spin dynamics. The slow spin dynamics is further evidenced from the \muSR data (characterized by a stretched exponential behavior) which show persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering) but no magnetic Bragg peaks. The analysis of the diffuse scattering data reveals a dominant antiferromagnetic interaction in agreement with the negative Weiss temperature. The absence of long-range magnetic order and the presence of slow spin dynamics and persistent spin fluctuations together reflect a dynamical ground state in Tb2Hf2O7.

cond-mat.str-el↗

arXiv2018

Inverted hysteresis and negative remanence in a homogeneous antiferromagnet

Magnetic remanence - found in bar magnets or magnetic storage devices - is probably the oldest and most ubiquitous phenomenon underpinning technological applications of magnetism. It is a macroscopic non-equilibrium phenomenon: a remanent magnetisation appears when a magnetic field is applied to an initially unmagnetised ferromagnet, and then taken away. Here, we present an inverted magnetic hysteresis loop in the pyrochlore compound Nd$_2$Hf$_2$O$_7$: the remanent magnetisation points in a direction opposite to the applied field. This phenomenon is exquisitely tunable as a function of the protocol in field and temperature, and it is reproducible as in a quasi-equilibrium setting. We account for this phenomenon in considerable detail in terms of the properties of non-equilibrium population of domain walls which exhibit a magnetic moment between domains of an ordered antiferromagnetic state which itself has zero net magnetisation. Properties and (non-equilibrium) dynamics of topological defects play an important role in modern spintronics, and our study adds an instance where a uniform field couples selectively to domain walls rather than the bulk.

cond-mat.str-el↗

arXiv2018

Investigation of the spin-1 honeycomb antiferromagnet BaNi$_2$V$_2$O$_8$ with easy plane anisotropy

The magnetic properties of the two-dimensional, S=1 honeycomb antiferromagnet BaNi$_2$V$_2$O$_8$ have been comprehensively studied using DC susceptibility measurements and inelastic neutron scattering techniques. The magnetic excitation spectrum is found to be dispersionless within experimental resolution between the honeycomb layers, while it disperses strongly within the honeycomb plane where it consists of two gapped spin-wave modes. The magnetic excitations are compared to linear spin-wave theory allowing the Hamiltonian to be determined. The first- and second-neighbour magnetic exchange interactions are antiferromagnetic and lie within the ranges 10.90meV$\le$J$_n$$\le$13.35 meV and 0.85meV$\le$J$_{nn}$$\le$1.65 meV respectively. The interplane coupling J$_{out}$ is four orders of magnitude weaker than the intraplane interactions, confirming the highly two-dimensional magnetic behaviour of this compound. The sizes of the energy gaps are used to extract the magnetic anisotropies and reveal substantial easy-plane anisotropy and a very weak in-plane easy-axis anisotropy. Together these results reveal that BaNi$_2$V$_2$O$_8$ is a candidate compound for the investigation of vortex excitations and Berezinsky-Kosterliz-Thouless phenomenona.

cond-mat.str-el↗

arXiv2017

Muon spin relaxation and inelastic neutron scattering investigations of all-in/all-out antiferromagnet Nd2Hf2O7

Nd2Hf2O7, belonging to the family of geometrically frustrated cubic rare earth pyrochlore oxides, was recently identified to order antiferromagnetically below T_N = 0.55 K with an all-in/all-out arrangement of Nd3+ moments, however with a much reduced ordered state moment. Herein we investigate the spin dynamics and crystal field states of Nd2Hf2O7 using muon spin relaxation (muSR) and inelastic neutron scattering (INS) measurements. Our muSR study confirms the long range magnetic ordering and shows evidence for coexisting persistent dynamic spin fluctuations deep inside the ordered state down to 42 mK. The INS data show the crytal electric field (CEF) excitations due to the transitions both within the ground state multiplet and to the first excited state multiplet. The INS data are analyzed by a model based on CEF and crystal field states are determined. Strong Ising-type anisotropy is inferred from the ground state wavefunction. The CEF parameters indicate the CEF-split Kramers doublet ground state of Nd3+ to be consistent with the dipolar-octupolar character.

cond-mat.str-el↗

arXiv2017

Spinon confinement in a quasi one dimensional anisotropic Heisenberg magnet

Confinement is a process by which particles with fractional quantum numbers bind together to form quasiparticles with integer quantum numbers. The constituent particles are confined by an attractive interaction whose strength increases with increasing particle separation and as a consequence, individual particles are not found in isolation. This phenomenon is well known in particle physics where quarks are confined in baryons and mesons. An analogous phenomenon occurs in certain magnetic insulators; weakly coupled chains of spins S=1/2. The collective excitations in these systems is spinons (S=1/2). At low temperatures weak coupling between chains can induce an attractive interaction between pairs of spinons that increases with their separation and thus leads to confinement. In this paper, we employ inelastic neutron scattering to investigate the spinon confinement in the quasi-1D S=1/2 XXZ antiferromagnet SrCo2V2O8. Spinon excitations are observed above TN in quantitative agreement with established theory. Below TN the pairs of spinons are confined and two sequences of meson-like bound states with longitudinal and transverse polarizations are observed. Several theoretical approaches are used to explain the data. A new theoretical technique based on Tangent-space Matrix Product States gives a very complete description of the data and provides good agreement not only with the energies of the bound modes but also with their intensities. We also successfully explained the effect of temperature on the excitations including the experimentally observed thermally induced resonance between longitudinal modes below TN ,and the transitions between thermally excited spinon states above TN. In summary, our work establishes SrCo2V2O8 as a beautiful paradigm for spinon confinement in a quasi-1D quantum magnet and provides a comprehensive picture of this process.

cond-mat.str-el↗

arXiv2017

Evolution of antiferromagnetic domains in the all-in-all-out ordered pyrochlore Nd$_2$Zr$_2$O$_7$

We report the observation of magnetic domains in the exotic, antiferromagnetically ordered all-in-all-out state of Nd$_2$Zr$_2$O$_7$, induced by spin canting. The all-in-all-out state can be realized by Ising-like spins on a pyrochlore lattice and is established in Nd$_2$Zr$_2$O$_7$ below 0.31 K for external magnetic fields up to 0.14 T. Two different spin arrangements can fulfill this configuration which leads to the possibility of magnetic domains. The all-in-all-out domain structure can be controlled by an external magnetic field applied parallel to the [111] direction. This is a result of different spin canting mechanism for the two all-in-all-out configurations for such a direction of the magnetic field. The change of the domain structure is observed through a hysteresis in the magnetic susceptibility. No hysteresis occurs, however, in case the external magnetic field is applied along [100].

cond-mat.str-el↗

arXiv2017

Kondo lattice heavy fermion behavior in CeRh2Ga2

The physical properties of an intermetallic compound CeRh2Ga2 have been investigated by magnetic susceptibility χ(T), isothermal magnetization M(H), heat capacity C_p(T), electrical resistivity ρ(T), thermal conductivity κ(T) and thermopower S(T) measurements. CeRh2Ga2 is found to crystallize with CaBe2Ge2-type primitive tetragonal structure (space group P4/nmm). No evidence of long range magnetic order is seen down to 1.8 K. The χ(T) data show paramagnetic behavior with an effective moment μ_eff ~ 2.5 μ_B/Ce indicating Ce^3+ valence state of Ce ions. The ρ(T) data exhibit Kondo lattice behavior with a metallic ground state. The low-T C_p(T) data yield an enhanced Sommerfeld coefficient γ= 130(2) mJ/mol K^2 characterizing CeRh2Ga2 as a moderate heavy fermion system. The high-T C_p(T) and ρ(T) show an anomaly near 255 K, reflecting a phase transition. The κ(T) suggests phonon dominated thermal transport with considerably higher values of Lorenz number L(T) compared to the theoretical Sommerfeld value L_0.

cond-mat.str-el↗

arXiv2017

Physical properties of a candidate quantum spin-ice system Pr2Hf2O7

Physical properties of a pyrohafnate compound Pr2Hf2O7 have been investigated by ac magnetic susceptibility χ_ ac(T), dc magnetic susceptibility χ(T), isothermal magnetization M(H) and heat capacity C_p(T) measurements on polycrystalline as well as single crystal samples combined with high-resolution synchrotron x-ray diffraction (XRD) for structural characterization and inelastic neutron scattering (INS) to determine the crystal field energy level scheme and wave functions. Synchrotron XRD data confirm the ordered cubic pyrochlore (Fd-3m) structure without any noticeable site mixing or oxygen deficiency. No clear evidence of long range magnetic ordering is observed down to 90 mK, however the χ_ac(T) evinces slow spin dynamics revealed by a frequency dependent broad peak associated with spin freezing. The INS data reveal the expected five well defined magnetic excitations due to crystal field splitting of the J = 4 ground state multiplet of the Pr^3+. The crystal field parameters and ground state wavefunction of Pr^3+ have been determined. The Ising anisotropic nature of magnetic ground state is inferred from the INS as well as χ(T) and M(H) data. Together these properties make Pr2Hf2O7 a candidate compound for quantum spin-ice behavior.

cond-mat.str-el↗

arXiv2016

Spin dynamics of the ordered dipolar-octupolar pseudospin-$^1/_2$ pyrochlore Nd$_2$Zr$_2$O$_7$ probed by muon spin relaxation

We present a muon spin relaxation study on the Ising pyrochlore Nd$_2$Zr$_2$O$_7$ which develops an "all-in-all-out" magnetic order below 0.4~K. At 20~mK far below the ordering transition temperature, the zero-field muon spin relaxation spectra show no static features and can be well described by a dynamical Gaussian-broadened Gaussian Kubo-Toyabe function indicating strong fluctuations of the ordered state. The spectra of the paramagnetic state (below 4.2~K) reveal anomalously slow paramagnetic spin dynamics and show only small difference with the spectra of the ordered state. We find that the fluctuation rate decreases with decreasing temperature and becomes nearly temperature independent below the transition temperature indicating persistent slow spin dynamics in the ground state. The field distribution width shows a small but sudden increase at the transition temperature and then becomes almost constant. The spectra in applied longitudinal fields are well fitted by the conventional dynamical Gaussian Kubo-Toyabe function, which further supports the dynamical nature of the ground state. The fluctuation rate shows a peak as a function of external field which is associated with a field-induced spin-flip transition. The strong dynamics in the ordered state are attributed to the transverse coupling of the Ising spins introduced by the multipole interactions.

cond-mat.mtrl-sci↗