68 research outputs found
Origin of anomalous breakdown of Bloch's rule in the Mott-Hubbard insulator MnTe
We reinvestigate the pressure dependence of the crystal structure and
antiferromagnetic phase transition in MnTe by the rigorous and reliable
tool of high pressure neutron powder diffraction. First-principles density
functional theory calculations are carried out in order to gain microscopic
insight. The measured N\'eel temperature of MnTe is found to show unusually
large pressure dependence of K GPa. This gives rise to large
violation of Bloch's rule given by , to a value of -6.0 0.1 for
MnTe. The ab-initio calculation of the electronic structure and the
magnetic exchange interactions in MnTe, for the measured crystal structures
at different pressures, gives the pressure dependence of the Ne\'el
temperature, to be -5.61, in close agreement with experimental
finding. The microscopic origin of this behavior turns to be dictated by the
distance dependence of the cation-anion hopping interaction strength
Pressure tuning of structure, superconductivity and novel magnetic order in the Ce-underdoped electron-doped cuprate T'-Pr_1.3-xLa_0.7Ce_xCuO_4 (x = 0.1)
High-pressure neutron powder diffraction, muon-spin rotation and
magnetization studies of the structural, magnetic and the superconducting
properties of the Ce-underdoped superconducting (SC) electron-doped cuprate
system T'-Pr_1.3-xLa_0.7Ce_xCuO_4 with x = 0.1 are reported. A strong reduction
of the lattice constants a and c is observed under pressure. However, no
indication of any pressure induced phase transition from T' to T structure is
observed up to the maximum applied pressure of p = 11 GPa. Large and non-linear
increase of the short-range magnetic order temperature T_so in
T'-Pr_1.3-xLa_0.7Ce_xCuO_4 (x = 0.1) was observed under pressure.
Simultaneously pressure causes a non-linear decrease of the SC transition
temperature T_c. All these experiments establish the short-range magnetic order
as an intrinsic and a new competing phase in SC T'-Pr_1.2La_0.7Ce_0.1CuO_4. The
observed pressure effects may be interpreted in terms of the improved nesting
conditions through the reduction of the in-plane and out-of-plane lattice
constants upon hydrostatic pressure.Comment: 11 pages, 10 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
Pressure induced topological quantum phase transition in Weyl semimetal T_d-MoTe_2
We report the pressure (p_max = 1.5 GPa) evolution of the crystal structure
of the Weyl semimetal T_d-MoTe_2 by means of neutron diffraction experiments.
We find that the fundamental non-centrosymmetric structure T_d is fully
suppressed and transforms into a centrosymmertic 1T' structure at a critical
pressure of p_cr = 1.2 GPa. This is strong evidence for a pressure induced
quantum phase transition (QPT) between topological to a trivial electronic
state. Although the topological QPT has strong effect on magnetoresistance, it
is interesting that the superconducting critical temperature T_c, the
superfluid density, and the SC gap all change smoothly and continuously across
p_cr and no sudden effects are seen concomitantly with the suppression of the
T_d structure. This implies that the T_c, and thus the SC pairing strength, is
unaffected by the topological QPT. However, the QPT requires the change in the
SC gap symmetry from non-trivial s+- to a trivial s++ state, which we discuss
in this work. Our systematic characterizations of the structure and
superconducting properties associated with the topological QPT provide deep
insight into the pressure induced phase diagram in this topological quantum
material.Comment: 9 pages, 4 figure
Decoupling lattice and magnetic instabilities in frustrated CuMnO2
Funding: This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Oak Ridge National Laboratory is managed by UT-Batelle, LLC, for the DOE under contract DE-AC05-1008 00OR22725. This research was sponsored in part by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Co-operative Agreement DE-NA0001982. Ce travail a été soutenu par le programme “Investissements d’Avenir”, projet ISITE-BFC (contrat ANR-15-IDEX-0003).The AMnO2 delafossites (A = Na, Cu) are model frustrated antiferromagnets, with triangular layers of Mn3+ spins. At low temperatures (TN = 65 K), a C2/m → P1̅ transition is found in CuMnO2, which breaks frustration and establishes magnetic order. In contrast to this clean transition, A = Na only shows short-range distortions at TN . Here, we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO2. We show that, even in stoichiometric samples, nonzero anisotropic Cu displacements coexist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures act to decouple these degrees of freedom. This manifests as an isostuctural phase transition at ∼10 GPa, with a reversible collapse of the c-axis. This is shown to be the high-pressure analogue of the c-axis negative thermal expansion seen at ambient pressure. Density functional theory (DFT) simulations confirm that dynamical instabilities of the Cu+ cations and edge-shared MnO6 layers are intertwined at ambient pressure. However, high pressure selectively activates the former, before an eventual predicted reemergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO2 are quantitatively different from nonmagnetic Cu delafossites and raise questions about the role of intrinsic inhomogeneity in frustrated antiferromagnets.PostprintPeer reviewe
Reply to Zayed: Interplay of magnetism and structure in the Shastry–Sutherland model
The connection between electronic and structural degrees of freedom—whether successive, coincident, or causal—suffuses the study of phase transitions. The Shastry–Sutherland model of a planar network of coupled spin dimers (1) and its physical realization in SrCu_2(BO_3)_2 (SCBO) provide a fundamental quantum mechanical test of this connection at the onset of antiferromagnetic order. We summarize in Fig. 1 the current understanding of SCBO’s phase diagram for T < 200 K and an intermediate pressure range of 3.5–6 GPa (2–4). At pressures below ∼4–5 GPa, SCBO has a tetragonal structure that hosts several low-temperature magnetic phases. Above this pressure, monoclinic distortions reduce the symmetry of the lattice. In ref. 2, we performed full structural refinements of X-ray and neutron scattering measurements to identify a change in space group at 5.5 GPa as a function of temperature (red circles in Fig. 1). This structural change coincides with the onset of antiferromagnetic ordering as a function of temperature, and we argue that this is not a coincidence but instead represents a cooperative effect between distortions of the lattice, the dimers tilting out of the plane, and the emergence of long-range magnetic order. In his comment on our work, Zayed (5) proposes an alternative scenario, in which the antiferromagnetic ordering onsets at lower pressure, within the tetragonal phase, and is then stabilized by the structural distortion associated with the monoclinic phase. He further speculates that this earlier onset may be associated with the dome in the phase boundary reported in ref. 4 (dark red region, Fig. 1)
On single-crystal neutron-diffraction in DACs: quantitative structure refinement of light elements on SNAP and TOPAZ
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