505 research outputs found
Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet
We report magnetic susceptibility, specific heat, and neutron scattering
measurements as a function of applied magnetic field and temperature to
characterize the quasi-two-dimensional frustrated magnet piperazinium
hexachlorodicuprate (PHCC). The experiments reveal four distinct phases. At low
temperatures and fields the material forms a quantum paramagnet with a 1 meV
singlet triplet gap and a magnon bandwidth of 1.7 meV. The singlet state
involves multiple spin pairs some of which have negative ground state bond
energies. Increasing the field at low temperatures induces three dimensional
long range antiferromagnetic order at 7.5 Tesla through a continuous phase
transition that can be described as magnon Bose-Einstein condensation. The
phase transition to a fully polarized ferromagnetic state occurs at 37 Tesla.
The ordered antiferromagnetic phase is surrounded by a renormalized classical
regime. The crossover to this phase from the quantum paramagnet is marked by a
distinct anomaly in the magnetic susceptibility which coincides with closure of
the finite temperature singlet-triplet pseudo gap. The phase boundary between
the quantum paramagnet and the Bose-Einstein condensate features a finite
temperature minimum at K, which may be associated with coupling to
nuclear spin or lattice degrees of freedom close to quantum criticality.Comment: Submitted to New Journal of Physic
Spin resonance in the d-wave superconductor CeCoIn5
Neutron scattering is used to probe antiferromagnetic spin fluctuations in
the d-wave heavy fermion superconductor CeCoIn (T=2.3 K).
Superconductivity develops from a state with slow (=0.3 0.15
meV) commensurate (=(1/2,1/2,1/2)) antiferromagnetic spin
fluctuations and nearly isotropic spin correlations. The characteristic
wavevector in CeCoIn is the same as CeIn but differs from the
incommensurate wavevector measured in antiferromagnetically ordered
CeRhIn. A sharp spin resonance ( meV) at
= 0.60 0.03 meV develops in the superconducting state removing spectral
weight from low-energy transfers. The presence of a resonance peak is
indicative of strong coupling between f-electron magnetism and
superconductivity and consistent with a d-wave gap order parameter satisfying
.Comment: (5 pages, 4 figures, to be published in Phys. Rev. Lett.
Magnetic field splitting of the spin-resonance in CeCoIn5
Neutron scattering in strong magnetic fields is used to show the
spin-resonance in superconducting CeCoIn5 (Tc=2.3 K) is a doublet. The
underdamped resonance (\hbar \Gamma=0.069 \pm 0.019 meV) Zeeman splits into two
modes at E_{\pm}=\hbar \Omega_{0}\pm g\mu_{B} \mu_{0}H with g=0.96 \pm 0.05. A
linear extrapolation of the lower peak reaches zero energy at 11.2 \pm 0.5 T,
near the critical field for the incommensurate "Q-phase" indicating that the
Q-phase is a bose condensate of spin excitons.Comment: 5 pages, 4 figure
Spin-lattice order in frustrated ZnCr2O4
Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice
order in highly frustrated ZnCrO where magnetic chromium ions occupy
the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of
anti-aligning spins surrounding the triangular faces of tetrahedra is resolved
by establishing weak interactions on each triangle through an intricate lattice
distortion. The resulting spin order is however, not simply a N\'{e}el state on
strong bonds. A complex co-planar spin structure indicates that antisymmetric
and/or further neighbor exchange interactions also play a role as ZnCrO
resolves conflicting magnetic interactions
The S=1/2 chain in a staggered field: High-energy bound-spinon state and the effects of a discrete lattice
We report an experimental and theoretical study of the antiferromagnetic
S=1/2 chain subject to uniform and staggered fields. Using inelastic neutron
scattering, we observe a novel bound-spinon state at high energies in the
linear chain compound CuCl2 * 2((CD3)2SO). The excitation is explained with a
mean-field theory of interacting S=1/2 fermions and arises from the opening of
a gap at the Fermi surface due to confining spinon interactions. The mean-field
model also describes the wave-vector dependence of the bound-spinon states,
particularly in regions where effects of the discrete lattice are important. We
calculate the dynamic structure factor using exact diagonalization of finite
length chains, obtaining excellent agreement with the experiments.Comment: 16 pages, 7 figures, accepted by Phys. Rev.
Phase diagram and spin Hamiltonian of weakly-coupled anisotropic S=1/2 chains in CuCl2*2((CD3)2SO)
Field-dependent specific heat and neutron scattering measurements were used
to explore the antiferromagnetic S=1/2 chain compound CuCl2 * 2((CD3)2SO). At
zero field the system acquires magnetic long-range order below TN=0.93K with an
ordered moment of 0.44muB. An external field along the b-axis strengthens the
zero-field magnetic order, while fields along the a- and c-axes lead to a
collapse of the exchange stabilized order at mu0 Hc=6T and mu0 Hc=3.5T,
respectively (for T=0.65K) and the formation of an energy gap in the excitation
spectrum. We relate the field-induced gap to the presence of a staggered
g-tensor and Dzyaloshinskii-Moriya interactions, which lead to effective
staggered fields for magnetic fields applied along the a- and c-axes.
Competition between anisotropy, inter-chain interactions and staggered fields
leads to a succession of three phases as a function of field applied along the
c-axis. For fields greater than mu0 Hc, we find a magnetic structure that
reflects the symmetry of the staggered fields. The critical exponent, beta, of
the temperature driven phase transitions are indistinguishable from those of
the three-dimensional Heisenberg magnet, while measurements for transitions
driven by quantum fluctuations produce larger values of beta.Comment: revtex 12 pages, 11 figure
From incommensurate correlations to mesoscopic spin resonance in YbRh2Si2
Spin fluctuations are reported near the magnetic field driven quantum
critical point in YbRh2Si2. On cooling, ferromagnetic fluctuations evolve into
incommensurate correlations located at q0=+/- (delta,delta) with delta=0.14 +/-
0.04 r.l.u. At low temperatures, an in plane magnetic field induces a sharp
intra doublet resonant excitation at an energy E0=g muB mu0 H with g=3.8 +/-
0.2. The intensity is localized at the zone center indicating precession of
spin density extending xi=6 +/- 2 A beyond the 4f site.Comment: (main text - 4 pages, 4 figures; supplementary information - 3 pages,
3 figures; to be published in Physical Review Letters
The Lorenz number in CeCoIn inferred from the thermal and charge Hall currents
The thermal Hall conductivity and Hall conductivity
in CeCoIn are used to determine the Lorenz number at low temperature . This enables the separation of the observed
thermal conductivity into its electronic and non-electronic parts. We uncover
evidence for a charge-neutral, field-dependent thermal conductivity, which we
identify with spin excitations. At low , these excitations dominate the
scattering of charge carriers. We show that suppression of the spin excitations
in high fields leads to a steep enhancement of the electron mean-free-path,
which leads to an interesting scaling relation between the magnetoresistance,
thermal conductivity and .Comment: 6 pages, 7 figures Intro para slightly lengthened. Added 2 new re
Quantum Criticality in an Organic Magnet
Exchange interactions between sites in piperazinium
hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground
state. We have determined the field-temperature phase diagram by high field
magnetization and neutron scattering experiments. There are two quantum
critical points: T separates a quantum paramagnet phase from a
three dimensional, antiferromagnetically-ordered state while T
marks the onset of a fully polarized state. The ordered phase, which we
describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum
critical regime with short range correlations. A low temperature anomaly in the
BEC phase boundary indicates that additional low energy features of the
material become important near .Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. Replaced original
text with additional conten
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