Using high-resolution x-ray scattering, we have demonstrated the existence of quenched disordered charge stripes in a single crystal of La (5/3)Sr (1/3)NiO (4) at low temperatures. Above the second-order transition critical scattering was observed due to fluctuations into the charge stripe phase. The charge stripes are shown to be two dimensional in nature both by measurements of their correlation lengths (xi(a) approximately 185 A, xi(b) = 400 A, and xi(c) approximately 25 A) and by the critical exponents of the charge strip transition. The charge stripe ordering did not develop long-range order even at low temperatures, indicating that the charge stripes are disordered and that the length scale of the disorder is quenched.
Using x-ray scattering we have measured the response of the incommensurate
charge-density wave (CDW) modulation in the quasi-low-dimensional material
2H-NbSe2 to applied magnetic fields at low temperatures. The
application of a magnetic field, either perpendicular or parallel to the
layers of a single crystal of 2H-NbSe2, caused no significant change
to either the correlation length or the intensity of the CDW satellites for
magnetic fields up to 10 T. These results suggest that the enhancement of the
resistance observed in low-dimensional CDW materials exposed to the applied
magnetic field does not result from an appreciable conversion of carriers from
the normal state to the CDW state. In addition 2H-NbSe2 is a
superconductor at low temperatures (Tc = 7.2 K), whilst still within
the incommensurate CDW state. This material therefore affords an opportunity
to study any interaction between the CDW and superconducting condensates. High
magnetic fields can suppress the superconducting state yet no change in the
incommensurate CDW satellite correlation length or intensity was observed at
high applied magnetic fields. These results conflict with spectroscopic
measurements, which suggest a coupling between the superconducting gap
excitations and the CDW.
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Using magnetization, dielectric constant, and neutron diffraction measurements on a high quality single crystal of YBaCuFeO (YBCFO), we demonstrate that the crystal shows two antiferromagnetic transitions at [Formula: see text] K and [Formula: see text] K, and displays a giant dielectric constant with a characteristic of the dielectric relaxation at T . It does not show the evidence of the electric polarization for the crystal used for this study. The transition at T corresponds with a paramagnetic to antiferromagnetic transition with a magnetic propagation vector doubling the unit cell along three crystallographic axes. Upon cooling, at T , the commensurate spin ordering transforms to a spiral magnetic structure with a propagation vector of ([Formula: see text] [Formula: see text] [Formula: see text]), where [Formula: see text], [Formula: see text], and [Formula: see text] are odd, and the incommensurability δ is temperature dependent. Around the transition boundary at T, both commensurate and incommensurate spin ordering coexist.
Using magnetization, conductivity and x-ray scattering measurements, we demonstrate that the giant magnetoresistance of the oxygen-deficient ferrite SrFeO 2.875±0.02 is a consequence of the coupling between the charge and spin order parameters and the tetragonal to monoclinic structural distortion. Upon cooling the sample at T;120 K we find a shoulder in both field-cool and zero field cool magnetization data and the simultaneous appearance of incommensurate structural satellites observed using x-ray diffraction. These satellites are shown to be due to incommensurate charge ordering with the high temperature delocalized Fe + 3.5 ions becoming localized with a charge disproportion forming an incommensurate charge-ordered phase. Strong resonant enhancement of these satellites at the Fe L III absorption edge confirms that this charge ordering is occurring at the Fe(2) sites. Further cooling increases the charge order correlation until T;62 K where there is a full structural transition from the tetragonal phase to a mononclinic phase. This causes a jump in the charge order wavevector from an incommensurate value of 0.610 to a commensurate ground state position of 5/8. This first-order structural transition displays considerable hysteresis as well as dramatic reductions in the magnetization, resistivity and magnetoresistance. The transition also causes an antiferromagnetic spin-ordering with a doubled unit cell along the c-axis. Well as observing new commensurate magnetic reflections at the Fe III edge we also observed resonant enhancement at the oxygen K-edge showing considerable hybridization between the Fe 3d and oxygen 2p states at low temperatures. Our results show that the formation of a magnetic long-rage ordered ground state drives the charge ordering from an incommensurate ordering to a commensurate ground state. This is evidence of a strong coupling between the magnetic and charge order parameters which is the basis for the unusual magnetoresistive effects observed at the transition.
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