arXiv subjects

Z. Islam

Publications and source records attributed to Z. Islam.

44 records · Page 3Linked to original sources

arXiv2008

Charge order, dynamics, and magneto-structural transition in multiferroic LuFe$_2$O$_4$

We investigated the series of temperature and field-driven transitions in LuFe$_2$O$_4$ by optical and Mössbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the "order by fluctuation" mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field.

cond-mat.str-el↗

arXiv2008

Chromium at High Pressures: Weak Coupling and Strong Fluctuations in an Itinerant Antiferromagnet

The spin- and charge-density-wave order parameters of the itinerant antiferromagnet chromium are measured directly with non-resonant x-ray diffraction as the system is driven towards its quantum critical point with high pressure using a diamond anvil cell. The exponential decrease of the spin and charge diffraction intensities with pressure confirms the harmonic scaling of spin and charge, while the evolution of the incommensurate ordering vector provides important insight into the difference between pressure and chemical doping as means of driving quantum phase transitions. Measurement of the charge density wave over more than two orders of magnitude of diffraction intensity provides the clearest demonstration to date of a weakly-coupled, BCS-like ground state. Evidence for the coexistence of this weakly-coupled ground state with high-energy excitations and pseudogap formation above the ordering temperature in chromium, the charge-ordered perovskite manganites, and the blue bronzes, among other such systems, raises fundamental questions about the distinctions between weak and strong coupling.

cond-mat.str-el↗

arXiv2007

Pressure-Tuned Spin and Charge Ordering in an Itinerant Antiferromagnet

Elemental chromium orders antiferromagnetically near room temperature, but the ordering temperature can be driven to zero by applying large pressures. We combine diamond anvil cell and synchrotron x-ray diffraction techniques to measure directly the spin and charge order in the pure metal at the approach to its quantum critical point. Both spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. By comparing the development of the order parameter with that of the magnetic wavevector, it is possible to ascribe the destruction of antiferromagnetism to the growth in electron kinetic energy relative to the underlying magnetic exchange interaction.

cond-mat.str-el↗

arXiv2006

Magnetic field induced orientation of superconducting MgB$_2$ crystallites determined by X-ray diffraction

X-ray diffraction studies of fine polycrystalline samples of MgB$_2$ in the superconducting state reveal that crystals orient with their \emph{c}-axis in a plane normal to the direction of the applied magnetic field. The MgB$_2$ samples were thoroughly ground to obtain average grain size 5 - 10 $μ$m in order to increase the population of free single crystal grains in the powder. By monitoring Bragg reflections in a plane normal to an applied magnetic field we find that the powder is textured with significantly stronger (\emph{0,0,l}) reflections in comparison to (\emph{h,k,0}), which remain essentially unchanged. The orientation of the crystals with the \emph{ab}-plane parallel to the magnetic field at all temperatures below $T_c$ demonstrates that the sign of the torque under magnetic field does not alter, in disagreement with current theoretical predictions.

cond-mat.supr-con↗

arXiv2005

Low temperature structural phase transition and incommensurate lattice modulation in the spin gap compound BaCuSi2O6

Results of high resolution x-ray diffraction experiments are presented for single crystals of the spin gap compound BaCuSi$_2$O$_6$ in the temperature range from 16 to 300 K. The data show clear evidence of a transition from the room temperature tetragonal phase into an incommensurately modulated orthorhombic structure below $\sim$100 K. This lattice modulation is characterized by a resolution limited wave vector {\bf q}$_{IC}$=(0,$\sim$0.13,0) and its 2$^{nd}$ and 3$^{rd}$ harmonics. The phase transition is first order and exhibits considerable hysteresis. This observation implies that the spin Hamiltonian representing the system is more complex than originally thought.

cond-mat.str-el↗

arXiv2005

Two and Three Dimensional Incommensurate Modulation in Optimally-Doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

X-ray scattering measurements on optimally-doped single crystal samples of the high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ reveal the presence of three distinct incommensurate charge modulations, each involving a roughly fivefold increase in the unit cell dimension along the {\bf b}-direction. The strongest scattering comes from the well known (H, K$\pm$ 0.21, L) modulation and its harmonics. However, we also observe broad diffraction which peak up at the L values complementary to those which characterize the known modulated structure. These diffraction features correspond to correlation lengths of roughly a unit cell dimension, $ξ_c$$\sim$20 $Å$ in the {\bf c} direction, and of $ξ_b$$\sim$ 185 $Å$ parallel to the incommensurate wavevector. We interpret these features as arising from three dimensional incommensurate domains and the interfaces between them, respectively. In addition we investigate the recently discovered incommensuate modulations which peak up at (1/2, K$\pm$ 0.21, L) and related wavevectors. Here we explicitly study the L-dependence of this scattering and see that these charge modulations are two dimensional in nature with weak correlations on the scale of a bilayer thickness, and that they correspond to short range, isotropic correlation lengths within the basal plane. We relate these new incommensurate modulations to the electronic nanostructure observed in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ using STM topography.

cond-mat.supr-con↗

arXiv2001

The Structure of Nanoscale Polaron Correlations in La1.2Sr1.8Mn2O7

A system of strongly-interacting electron-lattice polarons can exhibit charge and orbital order at sufficiently high polaron concentrations. In this study, the structure of short-range polaron correlations in the layered colossal magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by a crystallographic analysis of broad satellite maxima observed in diffuse X-ray and neutron scattering data. The resulting q=(0.3,0,1) modulation is a longitudinal octahedral-stretch mode, consistent with an incommensurate Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual orbital-stripe pattern parallel to the <100> directions.

cond-mat.mtrl-sci↗

arXiv2001

Momentum-Resolved Charge Excitations in a Prototype One Dimensional Mott Insulator

We report momentum resolved charge excitations in a one dimensional (1-D) Mott insulator studied using high resolution (~ 325 meV) inelastic x-ray scattering over the entire Brillouin zone for the first time. Excitations at the insulating gap edge are found to be highly dispersive (momentum dependent)compared to excitations observed in two dimensional Mott insulators. The observed dispersion in 1-D is consistent with charge excitations involving holons which is unique to spin-1/2 quantum chain systems. These results point to the potential utility of inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.

cond-mat.str-el↗