101 research outputs found
Magnetism, structure, and charge correlation at a pressure-induced Mott-Hubbard insulator-metal transition
We use synchrotron x-ray diffraction and electrical transport under pressure
to probe both the magnetism and the structure of single crystal NiS2 across its
Mott-Hubbard transition. In the insulator, the low-temperature
antiferromagnetic order results from superexchange among correlated electrons
and couples to a (1/2, 1/2, 1/2) superlattice distortion. Applying pressure
suppresses the insulating state, but enhances the magnetism as the
superexchange increases with decreasing lattice constant. By comparing our
results under pressure to previous studies of doped crystals we show that this
dependence of the magnetism on the lattice constant is consistent for both band
broadening and band filling. In the high pressure metallic phase the lattice
symmetry is reduced from cubic to monoclinic, pointing to the primary influence
of charge correlations at the transition. There exists a wide regime of phase
separation that may be a general characteristic of correlated quantum matter.Comment: 5 pages, 3 figure
Approaching the quantum critical point in a highly-correlated all-in-all-out antiferromagnet
Continuous quantum phase transition involving all-in–all-out (AIAO) antiferromagnetic order in strongly spin-orbit-coupled 5d compounds could give rise to various exotic electronic phases and strongly-coupled quantum critical phenomena. Here we experimentally trace the AIAO spin order in Sm₂Ir₂O₇ using direct resonant x-ray magnetic diffraction techniques under high pressure. The magnetic order is suppressed at a critical pressure P_c=6.30GPa, while the lattice symmetry remains in the cubic Fd−3m space group across the quantum critical point. Comparing pressure tuning and the chemical series R₂Ir₂O₇ reveals that the approach to the AIAO quantum phase transition is characterized by contrasting evolutions of the pyrochlore lattice constant a and the trigonal distortion surrounding individual Ir moments, which affects the 5d bandwidth and the Ising anisotropy, respectively. We posit that the opposite effects of pressure and chemical tuning lead to spin fluctuations with different Ising and Heisenberg character in the quantum critical region. Finally, the observed low pressure scale of the AIAO quantum phase transition in Sm₂Ir₂O₇ identifies a circumscribed region of P-T space for investigating the putative magnetic Weyl semimetal state
Diffraction line-shapes, Fermi surface nesting, and quantum criticality in antiferromagnetic chromium at high pressure (invited)
We explore the behavior of the nested bandstructure of chromium as a function of temperature and pressure to the point where magnetism disappears. X-ray diffraction measurements of the charge order parameter suggest that the nesting condition is maintained at high pressure, where the spin density wave ground state is destabilized by a continuous quantum phase transition. By comparing diffraction line-shapes measured throughout the temperature-pressure phase diagram we are able to identify and describe three regimes: thermal near-critical, weak coupling ground state, and quantum critical
Pressure-induced Spin-Peierls to Incommensurate Charge-Density-Wave Transition in the Ground State of TiOCl
The ground state of the spin-Peierls system TiOCl was probed using
synchrotron x-ray diffraction on a single-crystal sample at T = 6 K. We tracked
the evolution of the structural superlattice peaks associated with the
dimerized ground state as a function of pressure. The dimerization along the b
axis is rapidly suppressed in the vicinity of a first-order structural phase
transition at Pc = 13.1(1) GPa. The high-pressure phase is characterized by an
incommensurate charge density wave perpendicular to the original spin chain
direction. These results show that the electronic ground state undergoes a
fundamental change in symmetry, indicating a significant change in the
principal interactions.Comment: 5 pages, 4 figure
Strongly-coupled quantum critical point in an all-in-all-out antiferromagnet
Dimensionality and symmetry play deterministic roles in the laws of Nature.
They are important tools to characterize and understand quantum phase
transitions, especially in the limit of strong correlations between spin,
orbit, charge, and structural degrees of freedom. Using newly-developed,
high-pressure resonant x-ray magnetic and charge diffraction techniques, we
have discovered a quantum critical point in Cd2Os2O7 as the all-in-all-out
(AIAO) antiferromagnetic order is continuously suppressed to zero temperature
and, concomitantly, the cubic lattice structure continuously changes from space
group Fd-3m to F-43m. Surrounded by three phases of different time reversal and
spatial inversion symmetries, the quantum critical region anchors two phase
lines of opposite curvature, with striking departures from a mean-field form at
high pressure. As spin fluctuations, lattice breathing modes, and quasiparticle
excitations interact in the quantum critical region, we argue that they present
the necessary components for strongly-coupled quantum criticality in this
three-dimensional compound
Invited Article: High-pressure techniques for condensed matter physics at low temperature
Condensed matter experiments at high pressure accentuate the need for accurate pressure scales over a broad range of temperatures, as well as placing a premium on a homogeneous pressure environment. However, challenges remain in diamond anvil cell technology, including both the quality of various pressure transmitting media and the accuracy of secondary pressure scales at low temperature. We directly calibrate the ruby fluorescence R1 line shift with pressure at T=4.5 K using high-resolution x-ray powder diffraction measurements of the silver lattice constant and its known equation of state up to P=16 GPa. Our results reveal a ruby pressure scale at low temperatures that differs by 6% from the best available ruby scale at room T. We also use ruby fluorescence to characterize the pressure inhomogeneity and anisotropy in two representative and commonly used pressure media, helium and methanol:ethanol 4:1, under the same preparation conditions for pressures up to 20 GPa at T=5 K. Contrary to the accepted wisdom, both media show equal levels of pressure inhomogeneity measured over the same area, with a consistent Delta P/P per unit area of +/- 1.8 %/(10^(4) µm^(2)) from 0 to 20 GPa. The helium medium shows an essentially constant deviatoric stress of 0.021 +/- 0.011 GPa up to 16 GPa, while the methanol:ethanol mixture shows a similar level of anisotropy up to 10 GPa, above which the anisotropy increases. The quality of both pressure media is further examined under the more stringent requirements of single crystal x-ray diffraction at cryogenic temperature. For such experiments we conclude that the ratio of sample-to-pressure chamber volume is a critical parameter in maintaining sample quality at high pressure, and may affect the choice of pressure medium
Direct probe of Fermi surface evolution across a pressure-induced quantum phase transition
The nature of a material's Fermi surface is crucial to understanding its electronic, magnetic, optical, and thermal characteristics. Traditional measurements such as angle-resolved photoemission spectroscopy and de Haas–van Alphen quantum oscillations can be difficult to perform in the vicinity of a pressure-driven quantum phase transition, although the evolution of the Fermi surface may be tied to the emergence of exotic phenomena. We demonstrate here that magnetic x-ray diffraction in combination with Hall effect measurements in a diamond anvil cell can provide valuable insight into the Fermi surface evolution in spin- and charge-density-wave systems near quantum phase transitions. In particular, we track the gradual evolution of the Fermi surface in elemental chromium and delineate the critical pressure and absence of Fermi surface reconstruction at the spin-flip transition
Approaching the quantum critical point in a highly-correlated all-in-all-out antiferromagnet
Continuous quantum phase transitions involving all-in-all-out (AIAO)
antiferromagnetic order in strongly spin-orbit-coupled 5d compounds could give
rise to various exotic electronic phases and strongly-coupled quantum critical
phenomena. Here we experimentally trace the AIAO spin order in Sm2Ir2O7 using
direct resonant x-ray magnetic diffraction techniques under high pressure. The
magnetic order is suppressed at a critical pressure Pc=6.30 GPa, while the
lattice symmetry remains in the cubic Fd-3m space group across the quantum
critical point. Comparing pressure tuning and the chemical series R2Ir2O7
reveals that the suppression of the AIAO order and the approach to the
spin-disordered state is characterized by contrasting evolutions of both the
pyrochlore lattice constant a and the trigonal distortion x. The former affects
the 5d bandwidth, the latter the Ising anisotropy, and as such we posit that
the opposite effects of pressure and chemical tuning lead to spin fluctuations
with different Ising and Heisenberg character in the quantum critical region.
Finally, the observed low-pressure scale of the AIAO quantum phase transition
in Sm2Ir2O7 identifies a circumscribed region of P-T space for investigating
the putative magnetic Weyl-semimetal state
Quantum interference in superposed lattices
Charge transport in solids at low temperature reveals a material's mesoscopic
properties and structure. Under a magnetic field, Shubnikov-de Haas (SdH)
oscillations inform complex quantum transport phenomena that are not limited by
the ground state characteristics. Here, in elemental metal Cr with two
incommensurately superposed lattices of ions and a spin-density-wave ground
state, we reveal that the phases of several low-frequency SdH oscillations in
sigma_xx (rho_xx) and sigma_yy (rho_yy) are opposite, contrast with
oscillations from normal cyclotron orbits that maintain identical phases. We
trace the origin of the low frequency SdH oscillations to quantum interference
effects arising from the incommensurate orbits of Cr's superposed reciprocal
lattices, and explain the observed pi-phase shift by the reconnection of
anisotropic joint open and closed orbits
Approaching the quantum critical point in a highly-correlated all-in-all-out antiferromagnet
Continuous quantum phase transition involving all-in–all-out (AIAO) antiferromagnetic order in strongly spin-orbit-coupled 5d compounds could give rise to various exotic electronic phases and strongly-coupled quantum critical phenomena. Here we experimentally trace the AIAO spin order in Sm₂Ir₂O₇ using direct resonant x-ray magnetic diffraction techniques under high pressure. The magnetic order is suppressed at a critical pressure P_c=6.30GPa, while the lattice symmetry remains in the cubic Fd−3m space group across the quantum critical point. Comparing pressure tuning and the chemical series R₂Ir₂O₇ reveals that the approach to the AIAO quantum phase transition is characterized by contrasting evolutions of the pyrochlore lattice constant a and the trigonal distortion surrounding individual Ir moments, which affects the 5d bandwidth and the Ising anisotropy, respectively. We posit that the opposite effects of pressure and chemical tuning lead to spin fluctuations with different Ising and Heisenberg character in the quantum critical region. Finally, the observed low pressure scale of the AIAO quantum phase transition in Sm₂Ir₂O₇ identifies a circumscribed region of P-T space for investigating the putative magnetic Weyl semimetal state
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