32 research outputs found
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
High Resolution Study of Spin Excitations in the Shastry-Sutherland Singlet Ground State of SrCu2(BO3)2
High resolution, inelastic neutron scattering measurements on SrCu2(BO3)2
reveal the dispersion of the three single triplet excitations continuously
across the (H,0) direction within its tetragonal basal plane. These
measurements also show distinct Q dependencies for the single and multiple
triplet excitations, and that these excitations are largely dispersionless
perpendicular to this plane. The temperature dependence of the intensities of
these excitations is well described as the complement of the dc-susceptibility
of SrCu2(BO3)2.Comment: 4 pages, 4 figures. Submitted to PR
Crystallization of spin superlattices with pressure and field in the layered magnet SrCu_2(BO_3)_2
An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu_2(BO_3)_2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. The magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems
Incommensurate two-dimensional checkerboard charge density wave in the low dimensional superconductor Ta4Pd3Te16
We report the observation of a two-dimensional (2D) checkerboard charge
density wave (CDW) in the low-dimensional superconductor Ta4Pd3Te16. By
determining its CDW properties across the temperature-pressure (T-P) phase
diagram and comparing with prototypical CDW materials, we conclude that
Ta4Pd3Te16 features: a) an incommensurate CDW with a mixed character of
dimensions (Q1D considering its needle-like shape along the b-axis, Q2D as the
CDW has checkerboard wavevectors, and 3D because of CDW projections along all
three axes); and b) one of the weakest CDWs compared to its superconductivity
(SC), i.e. enhanced SC with respect to CDW, suggesting an interesting interplay
of the two orders.Comment: Z.S. and S.J.K. contributed equally to this work / Accepted for
publication in Physical Review Research Rapid Communication
Co-existing Singlet and Ordered S=1/2 Moments in the Ground State of the Triclinic Quantum Magnet CuMoO4
CuMoO4 is a triclinic quantum magnet based on S = 1/2 moments at the Cu2+
site. It has recently attracted interest due to the remarkable changes in its
chromic and volumetric properties at high temperatures, and in its magnetic
properties at low temperatures. This material exhibits a first order structural
phase transition at T_C ~ 190 K as well as a magnetic phase transition at T_N ~
1.75 K. We report low temperature heat capacity measurements as well as
extensive elastic and inelastic neutron scattering measurements on powder
samples taken above and below T_N. We observe neutron diffraction consistent
with a simple (1/2, 0, 0) antiferromagnetic structure indicating a doubling of
the a-axis periodicity below T_N. In addition, inelastic neutron scattering
above a spin gap of ~ 2.3 meV is consistent with triplet excitations out of
paired S = 1/2 moments which form singlet dimers. Low lying spin wave
excitations are also observed and these originate from ordered S = 1/2 moments
below T_N. Taken together these measurements show the ground state of CuMoO4 to
display both non-magnetic singlets, and ferromagnetically-coupled spins
coexisting within an antiferromagnetic structure below T_N ~ 1.75 K.Comment: 7 pages, 7 figure
Emergence of long-range order in sheets of magnetic dimers
Quantum spins placed on the corners of a square lattice can dimerize and form singlets, which then can be transformed into a magnetic state as the interactions between dimers increase beyond threshold. This is a strictly 2D transition in theory, but real-world materials often need the third dimension to stabilize long-range order. We use high pressures to convert sheets of Cu^2+ spin 1/2 dimers from local singlets to global antiferromagnet in the model system SrCu_2(BO_3)_2. Single-crystal neutron diffraction measurements at pressures above 5 GPa provide a direct signature of the antiferromagnetic ordered state, whereas high-resolution neutron powder and X-ray diffraction at commensurate pressures reveal a tilting of the Cu spins out of the plane with a critical exponent characteristic of 3D transitions. The addition of anisotropic, interplane, spin–orbit terms in the venerable Shastry–Sutherland Hamiltonian accounts for the influence of the third dimension
Neutron and X-ray Scattering Studies of the Lightly-Doped Spin-Peierls System Cu(1-x)Cd(x)GeO3
Single crystals of the lightly-doped spin-Peierls system Cu(1-x)Cd(x)GeO3
have been studied using bulk susceptibility, x-ray diffraction, and inelastic
neutron scattering techniques. We investigate the triplet gap in the magnetic
excitation spectrum of this quasi-one dimensional quantum antiferromagnet, and
its relation to the spin-Peierls dimerisation order parameter. We employ two
different theoretical forms to model the inelastic neutron scattering cross
section and chi''(Q,omega), and show the sensitivity of the gap energy to the
choice of chi''(Q,omega). We find that a finite gap exists at the spin-Peierls
phase transition.Comment: 15 Pages, 7 Figures, Submitted to J. Phys. :Condensed Matte
Order parameter fluctuations at a buried quantum critical point
Quantum criticality is a central concept in condensed matter physics, but the
direct observation of quantum critical fluctuations has remained elusive. Here
we present an x-ray diffraction study of the charge density wave (CDW) in
2H-NbSe2 at high pressure and low temperature, where we observe a broad regime
of order parameter fluctuations that are controlled by proximity to a quantum
critical point. X-rays can track the CDW despite the fact that the quantum
critical regime is shrouded inside a superconducting phase, and, in contrast to
transport probes, allow direct measurement of the critical fluctuations of the
charge order. Concurrent measurements of the crystal lattice point to a
critical transition that is continuous in nature. Our results confirm the
longstanding expectations of enhanced quantum fluctuations in low dimensional
systems, and may help to constrain theories of the quantum critical Fermi
surface.Comment: to be published in PNA
Crystallization of spin superlattices with pressure and field in the layered magnet SrCu_2(BO_3)_2
An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu_2(BO_3)_2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. The magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems