arXiv subjects

N. Shannon

Publications and source records attributed to N. Shannon.

16 recordsLinked to original sources

arXiv2019

Negative thermal expansion in the plateau state of a magnetically-frustrated spinel

We report on negative thermal expansion (NTE) in the high-field, half-magnetization plateau phase of the frustrated magnetic insulator CdCr2O4. Using dilatometry, we precisely map the phase diagram at fields of up to 30T, and identify a strong NTE associated with the collinear half-magnetization plateau for B > 27T. The resulting phase diagram is compared with a microscopic theory for spin-lattice coupling, and the origin of the NTE is identified as a large negative change in magnetization with temperature, coming from a nearly-localised band of spin excitations in the plateau phase. These results provide useful guidelines for the discovery of new NTE materials.

cond-mat.str-el↗

arXiv2019

Putative spin-nematic phase in BaCdVO(PO$_{4}$)$_{2}$

We report neutron scattering and AC magnetic susceptibility measurements of the 2D spin-1/2 frustrated magnet BaCdVO(PO$_{4}$)$_{2}$. At temperatures well below $T_{\sf N}\approx 1K$, we show that only 34 % of the spin moment orders in an up-up-down-down strip structure. Dominant magnetic diffuse scattering and comparison to published $μ$sr measurements indicates that the remaining 66 % is fluctuating. This demonstrates the presence of strong frustration, associated with competing ferromagnetic and antiferromagnetic interactions, and points to a subtle ordering mechanism driven by magnon interactions. On applying magnetic field, we find that at $T=0.1$ K the magnetic order vanishes at 3.8 T, whereas magnetic saturation is reached only above 4.5 T. We argue that the putative high-field phase is a realization of the long-sought bond-spin-nematic state.

cond-mat.str-el↗

arXiv2014

Cascade of field-induced magnetic transitions in a frustrated antiferromagnetic metal

Frustrated magnets can exhibit many novel forms of order when exposed to high magnetic fields, however, much less is known about materials where frustration occurs in the presence of itinerant electrons. Here we report thermodynamic and transport measurements on micron-sized single crystals of the triangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase transitions at 13.5 20, 28 and 39T in fields applied along the easy axis, and we combine magnetic torque, specific heat and transport data to construct the field-temperature phase diagram. The results are discussed in the context of a frustrated easy-axis Heisenberg model for the localized moments where intermediate applied magnetic fields are predicted to stabilize a magnetic supersolid phase. Deviations in the measured phase diagram from this model predictions are attributed to the role played by the itinerant electrons.

cond-mat.str-el↗

arXiv2009

A quantum liquid with deconfined fractional excitations in three dimensions

Excitations which carry "fractional" quantum numbers are known to exist in one dimension in polyacetylene, and in two dimensions, in the fractional quantum Hall effect. Fractional excitations have also been invoked to explain the breakdown of the conventional theory of metals in a wide range of three-dimensional materials. However the existence of fractional excitations in three dimensions remains highly controversial. In this Letter we report direct numerical evidence for the existence of a quantum liquid phase supporting fractional excitations in a concrete, three-dimensional microscopic model - the quantum dimer model on a diamond lattice. We demonstrate explicitly that the energy cost of separating fractional monomer excitations vanishes in this liquid phase, and that its energy spectrum matches that of the Coulomb phase in (3+1) dimensional quantum electrodynamics.

cond-mat.str-el↗

arXiv2009

Spin correlations and exchange in square lattice frustrated ferromagnets

The J1-J2 model on a square lattice exhibits a rich variety of different forms of magnetic order that depend sensitively on the ratio of exchange constants J2/J1. We use bulk magnetometry and polarized neutron scattering to determine J1 and J2 unambiguously for two materials in a new family of vanadium phosphates, Pb2VO(PO4)2 and SrZnVO(PO4)2, and we find that they have ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground state is reduced, and the diffuse magnetic scattering is enhanced, as the predicted bond-nematic region of the phase diagram is approached.

cond-mat.str-el↗

arXiv2009

Directional field-induced metallization of quasi-one-dimensional Li$_{0.9}$Mo$_6$O$_{17}$

We report a detailed magnetotransport study of the highly anisotropic quasi-one-dimensional oxide Li$_{0.9}$Mo$_6$O$_{17}$ whose in-chain electrical resistivity diverges below a temperature $T_{\rm min} \sim$ 25 K. For $T < T_{\rm min}$, a magnetic field applied parallel to the conducting chain induces a large negative magnetoresistance and ultimately, the recovery of a metallic state. We show evidence that this insulator/metal crossover is a consequence of field-induced suppression of a density-wave gap in a highly one-dimensional conductor. At the highest fields studied, there is evidence for the possible emergence of a novel superconducting state with an onset temperature $T_c >$ 10 K.

cond-mat.str-el↗

arXiv2009

Two dimensional frustrated magnets in high magnetic field

Frustrated magnets in high magnetic field have a long history of offering beautiful surprises to the patient investigator. Here we present the results of extensive classical Monte Carlo simulations of a variety of models of two dimensional magnets in magnetic field, together with complementary spin wave analysis. Striking results include (i) a massively enhanced magnetocaloric effect in antiferromagnets bordering on ferromagnetic order, (ii) a route to an $m=1/3$ magnetization plateau on a square lattice, and (iii) a cascade of phase transitions in a simple model of AgNiO$_2$.

cond-mat.str-el↗

arXiv2009

Spin dynamics of frustrated easy-axis triangular antiferromagnet 2H-AgNiO2 explored by inelastic neutron scattering

We report inelastic neutron scattering measurements of the spin dynamics in the layered hexagonal magnet 2H-AgNiO2 which has stacked triangular layers of antiferromagnetically-coupled Ni2+ spins (S=1) ordered in a collinear alternating stripe pattern. We observe a broad band of magnetic excitations above a small gap of 1.8 meV and extending up to 7.5 meV, indicating strongly dispersive excitations. The measured dispersions of the boundaries of the powder-averaged spectrum can be quantitatively explained by a linear spin-wave dispersion for triangular layers with antiferromagnetic nearest- and weak next-nearest neighbor couplings, a strong easy-axis anisotropy and additional weak inter-layer couplings. The resulting dispersion relation has global minima not at magnetic Bragg wavevectors but at symmetry-related soft points and we attribute this anomalous feature to the strong competition between the easy-axis anisotropy and the frustrated antiferromagnetic couplings. We have also calculated the quantum corrections to the dispersion relation to order 1/S in spin-wave theory by extending the work of Chubukov and Jolicoeur [Phys. Rev. B v46, 11137 (1992)] and find that the presence of easy-axis anisotropy significantly reduces the quantum renormalizations predicted for the isotropic model.

cond-mat.str-el↗

arXiv2008

MuSR studies of RE(O,F)FeAs (RE = La, Nd, Ce) and LaOFeP systems: possible incommensurate/stripe magnetism and superfluid density

Muon spin relaxation (MuSR) measurements in iron oxy-pnictide systems have revealed: (1) commensurate long-range order in undoped LaOFeAs; (2) Bessel function line shape in La(O0.97F0.03)FeAs which indicates possible incommensurate or stripe magnetism; (3) anomalous weak magnetism existing in superconducting LaOFeP, Ce(O0.84F0.16)FeAs, and Nd(O0.88F0.12)FeAs but absent in superconducting La(O0.92F0.08)FeAs; and (4) scaling of superfluid density and Tc in the Ce, La, and Nd-FeAs superconductors following a nearly linear relationship found in cuprates.

cond-mat.supr-con↗

arXiv2007

Quantum fluctuations in high field magnetization of 2D square lattice J1-J2 antiferromagnets

The J1-J2 square lattice Heisenberg model with spin S=1/2 has three phases with long-range magnetic order and two unconventionally ordered phases depending on the ratio of exchange constants. It describes a number of recently found layered vanadium oxide compounds. A simple means of investigating the ground state is the study of the magnetization curve and high-field susceptibility. We discuss these quantities by using the spin-wave theory and the exact diagonalization in the whole J1-J2 plane. We compare both results and find good overall agreement in the sectors of the phase diagram with magnetic order. Close to the disordered regions the magnetization curve shows strong deviations from the classical linear behaviour caused by large quantum fluctuations and spin-wave approximation breaks down. On the FM side (J1<0) where one approaches the quantum gapless spin nematic ground state this region is surprisingly large. We find that inclusion of second order spin-wave corrections does not lead to fundamental improvement. Quantum corrections to the tilting angle of the ordered moments are also calculated. They may have both signs, contrary to the always negative first order quantum corrections to the magnetization. Finally we investigate the effect of the interlayer coupling and find that the quasi-2D picture remains valid up to |J_\perp/J1| ~ 0.3.

cond-mat.str-el↗

arXiv2007

Magnetocaloric effect in the frustrated square lattice J1-J2 model

We investigate the magnetocaloric properties of the two-dimensional frustrated J1-J2 model on a square lattice. This model describes well the magnetic behavior of two classes of quasi-two-dimensional S = 1/2 vanadates, namely the Li2VOXO4 (X = Si, Ge) and AA'VO(PO4)2 (A, A' = Pb, Zn, Sr, Ba) compounds. The magnetocaloric effect (MCE) consists in the adiabatic temperature change upon changing the external magnetic field. In frustrated systems, the MCE can be enhanced close to the saturation field because of massive degeneracies among low lying excitations. We discuss results for the MCE in the two distinct antiferromagnetic regimes of the phase diagram. Numerical finite temperature Lanczos as well as analytical methods based on the spin wave expansion are employed and results are compared. We give explicit values for the saturation fields of the vanadium compounds. We predict that at subcritical fields there is first a (positive) maximum followed by sign change of the MCE, characteristic of all magnetically ordered phases.

cond-mat.str-el↗

arXiv2007

Local Tunneling Spectroscopy across a Metamagnetic Critical Point in the Bi-layer Ruthenate Sr3Ru2O7

The local spectroscopic signatures of metamagnetic criticality in Sr3Ru2O7 were explored using scanning tunneling microscopy (STM). Singular features in the tunneling spectrum were found close to the Fermi level, as would be expected in a Stoner picture of itinerant electron metamagnetism. These features showed a pronounced magnetic field dependence across the metamagnetic critical point, which cannot be understood in terms of a naive Stoner theory. In addition, a pseudo-gap structure was observed over several tens of meV, accompanied by a c(2x2) superstructure in STM images. This result represents a new electronic ordering at the surface in the absence of any measurable surface reconstruction.

cond-mat.str-el↗

arXiv2007

Patterning of sodium ions and the control of electrons in sodium cobaltate

NaxCoO2 has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the high-Tc cuprates. The density, x, of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard Hamiltonian taking into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy. As a consequence, holes occupy preferentially the lowest potential regions and, therefore, the Na+ ion patterning plays a decisive role in the transport and magnetic properties.

cond-mat.str-el↗

arXiv2005

On the Ordering of Na$^+$ Ions in Na$_x$CoO$_2$

The influence of electrostatic interactions on the ordering of sodium ions in Na$_x$CoO$_2$ is studied theoretically through Monte-Carlo simulations. For large $x$ small di- or tri-vacancy clusters are stable with respect to isolated Na vacancies. At commensurate fillings these small clusters order in triangular superstructures. These results agree with recent electron diffraction data at $x=1/2$ and 3/4. We have performed neutron Laüe diffraction experiments at higher $x$, which confirm the predictions of this simple model. The consequences on the properties of the electronic charges in the Co layers are discussed.

cond-mat.str-el↗

arXiv2005

Spin correlations in the paramagnetic phase and ring exchange in lanthanum cuprate

Spin correlations in the paramagnetic phase of lanthanum cuprate have been studied using polarized neutron scattering, with two important results. First, the temperature dependence of the characteristic energy scale of the fluctuations and the amplitude of the neutron structure factor are shown to be in quantitative agreement with the predictions of the quantum non-linear sigma model. Secondly, comparison of a high-temperature series expansion of the equal-time spin correlations with the diffuse neutron intensity provides definitive experimental evidence for ring exchange.

cond-mat.str-el↗

arXiv2004

Half-magnetization plateau stabilized by structural distortion in the antiferromagnetic Heisenberg model on a pyrochlore lattice

Magnetization plateaus, visible as anomalies in magnetic susceptibility at low temperatures, are one of the hallmarks of frustrated magnetism. We show how an extremely robust half-magnetization plateau can arise from coupling between spin and lattice degrees of freedom in a pyrochlore antiferromagnet, and develop a detailed symmetry of analysis of the simplest possible scenario for such a plateau state. The application of this theory to the spinel oxides CdCr2O4 and HgCr2O4, where a robust half magnetization plateau has been observed, is discussed.

cond-mat.str-el↗