595 research outputs found
Evidence for the existence in human serum of large molecular weight nonsuppressible insulin-like activity (NSILA) different from the small molecular weight forms
The monoclinic crystal structure of -RuCl and the zigzag antiferromagnetic ground state
The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to
realize a Kitaev spin model with strongly frustrated, bond-dependent,
anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic
moments. Here we report a detailed study of the three-dimensional crystal
structure using x-ray diffraction on untwinned crystals combined with
structural relaxation calculations. We consider several models for the stacking
of honeycomb layers and find evidence for a crystal structure with a monoclinic
unit cell corresponding to a stacking of layers with a unidirectional in-plane
offset, with occasional in-plane sliding stacking faults, in contrast with the
currently-assumed trigonal 3-layer stacking periodicity. We report electronic
band structure calculations for the monoclinic structure, which find support
for the applicability of the jeff=1/2 picture once spin orbit coupling and
electron correlations are included. We propose that differences in the
magnitude of anisotropic exchange along symmetry inequivalent bonds in the
monoclinic cell could provide a natural mechanism to explain the spin gap
observed in powder inelastic neutron scattering, in contrast to spin models
based on the three-fold symmetric trigonal structure, which predict a gapless
spectrum within linear spin wave theory. Our susceptibility measurements on
both powders and stacked crystals, as well as neutron powder diffraction show a
single magnetic transition at TN ~ 13K. The analysis of the neutron data
provides evidence for zigzag magnetic order in the honeycomb layers with an
antiferromagnetic stacking between layers. Magnetization measurements on
stacked single crystals in pulsed field up to 60T show a single transition
around 8T for in-plane fields followed by a gradual, asymptotic approach to
magnetization saturation, as characteristic of strongly anisotropic exchange
interactions.Comment: 13 pages, 9 figures, published in Physical Review
Experimental investigation of the competing orders and quantum criticality in hole- and electron-doped cuprate superconductors
We investigate the issues of competing orders and quantum criticality in cuprate superconductors via experimental studies of the high-field thermodynamic phase diagrams and the quasiparticle tunneling spectroscopy. Substantial field-induced quantum fluctuations are found in all cuprates investigated, and the corresponding correlation with quasiparticle spectra suggest that both electron- (n-type) and hole-doped (p-type) cuprate superconductors are in close proximity to a quantum critical point that separates a pure superconducting (SC) phase from a phase consisting of coexisting SC and a competing order. We further suggests that the relevant competing order is likely a spin-density wave (SDW) or a charge density wave (CDW), which can couple efficiently to an in-plane Cu-O bond stretching longitudinal optical (LO) phonon mode in the p-type cuprates but not in the n-type cuprates. This cooperative interaction may account for the pseudogap phenomenon above T, only in the p-type cuprate superconductors
Macroscopic evidence for quantum criticality and field-induced quantum fluctuations in cuprate superconductors
We present macroscopic experimental evidence for field-induced microscopic
quantum fluctuations in different hole- and electron-type cuprate
superconductors with varying doping levels and numbers of CuO layers per
unit cell. The significant suppression of the zero-temperature in-plane
magnetic irreversibility field relative to the paramagnetic field in all
cuprate superconductors suggests strong quantum fluctuations due to the
proximity of the cuprates to quantum criticality.Comment: 3 figures. To appear in Phys. Rev. B, Rapid Communications (2007).
For correspondence, contact: Nai-Chang Yeh (e-mail: [email protected]
Persistent detwinning of iron pnictides by small magnetic fields
Our comprehensive study on EuFeAs reveals a dramatic reduction of
magnetic detwinning fields compared to other AFeAs (A = Ba, Sr, Ca)
iron pnictides by indirect magneto-elastic coupling of the Eu ions. We
find that only 0.1T are sufficient for persistent detwinning below the local
Eu ordering; above = 19K, higher fields are necessary.
Even after the field is switched off, a significant imbalance of twin domains
remains constant up to the structural and electronic phase transition (190K).
This persistent detwinning provides the unique possibility to study the low
temperature electronic in-plane anisotropy of iron pnictides without applying
any symmetrybreaking external force.Comment: accepted by Physical Review Letter
Quasiparticle spectroscopy and high-field phase diagrams of cuprate superconductors -- An investigation of competing orders and quantum criticality
We present scanning tunneling spectroscopic and high-field thermodynamic
studies of hole- and electron-doped (p- and n-type) cuprate superconductors.
Our experimental results are consistent with the notion that the ground state
of cuprates is in proximity to a quantum critical point (QCP) that separates a
pure superconducting (SC) phase from a phase comprised of coexisting SC and a
competing order, and the competing order is likely a spin-density wave (SDW).
The effect of applied magnetic field, tunneling current, and disorder on the
revelation of competing orders and on the low-energy excitations of the
cuprates is discussed.Comment: 10 pages, 5 figures. Accepted for publication in the International
Journal of Modern Physics B. (Correspondence author: Nai-Chang Yeh, e-mail:
[email protected]
Spin Dynamics of in the Field-Induced Ordered Phase
- (known as DTN) is a spin-1 material with a strong
single-ion anisotropy that is regarded as a new candidate for Bose-Einstein
condensation (BEC) of spin degrees of freedom. We present a systematic study of
the low-energy excitation spectrum of DTN in the field-induced magnetically
ordered phase by means of high-field electron spin resonance measurements at
temperatures down to 0.45 K. We argue that two gapped modes observed in the
experiment can be consistently interpreted within a four-sublattice
antiferromagnet model with a finite interaction between two tetragonal
subsystems and unbroken axial symmetry. The latter is crucial for the
interpretation of the field-induced ordering in DTN in terms of BEC.Comment: 4 pages, 3 figure
Dimensionality of superconductivity in the infinite-layer high-temperature cuprate Sr0.9M0.1CuO2 (M = La, Gd)
The high magnetic field phase diagram of the electron-doped infinite layer
high-temperature superconducting (high-T_c) compound Sr_{0.9}La_{0.1}CuO_2 was
probed by means of penetration depth and magnetization measurements in pulsed
fields to 60 T. An anisotropy ratio of 8 was detected for the upper critical
fields with H parallel (H_{c2}^{ab}) and perpendicular (H_{c2}^c) to the CuO_2
planes, with H_{c2}^{ab} extrapolating to near the Pauli paramagnetic limit of
160 T. The longer superconducting coherence length than the lattice constant
along the c-axis indicates that the orbital degrees of freedom of the pairing
wavefunction are three dimensional. By contrast, low-field magnetization and
specific heat measurements of Sr_{0.9}Gd_{0.1}CuO_2 indicate a coexistence of
bulk s-wave superconductivity with large moment Gd paramagnetism close to the
CuO_2 planes, suggesting a strong confinement of the spin degrees of freedom of
the Cooper pair to the CuO_2 planes. The region between H_{c2}^{ab} and the
irreversibility line in the magnetization, H_{irr}^{ab}, is anomalously large
for an electron-doped high-T_c cuprate, suggesting the existence of additional
quantum fluctuations perhaps due to a competing spin-density wave order.Comment: 4 pages, 4 figures, submitted to Phys. Rev. B, Rapid Communications
(2004). Corresponding author: Nai-Chang Yeh (E-mail: [email protected]
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