370 research outputs found
Definitive Evidence for Order-by-Quantum-Disorder in Er2Ti2O7
Here we establish the systematic existence of a U(1) degeneracy of all
symmetry-allowed Hamiltonians quadratic in the spins on the pyrochlore lattice,
at the mean-field level. By extracting the Hamiltonian of Er2Ti2O7 from
inelastic neutron scattering measurements, we then show that the
U(1)-degenerate states of Er2Ti2O7 are its classical ground states, and
unambiguously show that quantum fluctuations break the degeneracy in a way
which is confirmed by experiment. This is the first definitive observation of
order-by-disorder in any material. We provide further verifiable consequences
of this phenomenon, and several additional comparisons between theory and
experiment.Comment: 4.5 pages, 3 figures, 7.5 pages of Supplemental Material, 8
supplemental figure
Quantum phase transitions and decoupling of magnetic sublattices in the quasi-two-dimensional Ising magnet Co3V2O8 in a transverse magnetic field
The application of a magnetic field transverse to the easy axis, Ising
direction in the quasi-two-dimensional Kagome staircase magnet, Co3V2O8,
induces three quantum phase transitions at low temperatures, ultimately
producing a novel high field polarized state, with two distinct sublattices.
New time-of-flight neutron scattering techniques, accompanied by large angular
access, high magnetic field infrastructure allow the mapping of a sequence of
ferromagnetic and incommensurate phases and their accompanying spin
excitations. At least one of the transitions to incommensurate phases at \mu
0Hc1~6.25 T and \mu 0Hc2~7 T is discontinuous, while the final quantum critical
point at \mu 0Hc3~13 T is continuous.Comment: 5 pages manuscript, 3 pages supplemental materia
Continuous and Discontinuous Quantum Phase Transitions in a Model Two-Dimensional Magnet
The Shastry-Sutherland model, which consists of a set of spin 1/2 dimers on a
2-dimensional square lattice, is simple and soluble, but captures a central
theme of condensed matter physics by sitting precariously on the quantum edge
between isolated, gapped excitations and collective, ordered ground states. We
compress the model Shastry-Sutherland material, SrCu2(BO3)2, in a diamond anvil
cell at cryogenic temperatures to continuously tune the coupling energies and
induce changes in state. High-resolution x-ray measurements exploit what
emerges as a remarkably strong spin-lattice coupling to both monitor the
magnetic behavior and the absence or presence of structural discontinuities. In
the low-pressure spin-singlet regime, the onset of magnetism results in an
expansion of the lattice with decreasing temperature, which permits a
determination of the pressure dependent energy gap and the almost isotropic
spin-lattice coupling energies. The singlet-triplet gap energy is suppressed
continuously with increasing pressure, vanishing completely by 2 GPa. This
continuous quantum phase transition is followed by a structural distortion at
higher pressure.Comment: 16 pages, 4 figures. Accepted for publication in PNA
Critical X-ray Scattering Studies of Jahn-Teller Phase Transitions in TbVAsO
The critical behaviour associated with cooperative Jahn-Teller phase
transitions in TbVAsO (where \textit{x} = 0, 0.17, 1)
single crystals have been studied using high resolution x-ray scattering. These
materials undergo continuous tetragonal orthorhombic structural phase
transitions driven by Jahn-Teller physics at T = 33.26(2) K, 30.32(2) K and
27.30(2) K for \textit{x} = 0, 0.17 and 1 respectively. The orthorhombic strain
was measured close to the phase transition and is shown to display mean field
behavior in all three samples. Pronounced fluctuation effects are manifest in
the longitudinal width of the Bragg scattering, which diverges as a power law,
with an exponent given by , on approaching the transition from
either above or below. All samples exhibited twinning; however the disordered x
= 0.17 sample showed a broad distribution of twins which were stable to
relatively low temperatures, well below T. This indicates that while the
orthorhombic strain continues to develop in a conventional mean field manner in
the presence of disorder, twin domains are easily pinned by the quenched
impurities and their associated random strains.Comment: 8 pages, 6 figure
Singlet-Triplet Excitations in the Unconventional Spin-Peierls System TiOBr
We have performed time-of-flight neutron scattering measurements on powder
samples of the unconventional spin-Peierls compound TiOBr using the
fine-resolution Fermi chopper spectrometer (SEQUOIA) at the SNS. These
measurements reveal two branches of magnetic excitations within the
commensurate and incommensurate spin-Peierls phases, which we associate with n
= 1 and n = 2 triplet excitations out of the singlet ground state. These
measurements represent the first direct measure of the singlet-triplet energy
gap in TiOBr, which is determined to be Eg = 21.2 +/- 1.0 meV.Comment: 5 pages, 4 figures, submitted for publicatio
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