arXiv subjects

D. N. Argyriou

Publications and source records attributed to D. N. Argyriou.

52 records · Page 3Linked to original sources

arXiv2005

Quasiparticle-like peaks, kinks, and electron-phonon coupling at the ($π$,0) regions in the CMR oxide La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$

Using Angle-Resolved Photoemission (ARPES), we present the first observation of sharp quasiparticle-like peaks in a CMR manganite. We focus on the ($π$,0) regions of k-space and study their electronic scattering rates and dispersion kinks, uncovering the critical energy scales, momentum scales, and strengths of the interactions that renormalize the electrons. To identify these bosons we measured phonon dispersions in the energy range of the kink by inelastic neutron scattering (INS), finding a good match in both energy and momentum to the oxygen bond-stretching phonons.

cond-mat.str-el↗

arXiv2005

Field induced linear magneto-elastic coupling in multiferroic TbMnO3

We have used in-field neutron and X-ray single crystal diffraction to measure the incommensurability δ of the crystal and magnetic structure of multiferroic TbMnO3 . We show that the flop in the electric polarization at the critical field HC, for field H along the a− and b−axis coincides with a 1st order transition to a commensurate phase with propagation vector κ = (0, 1/4, 0). In-field X-ray diffraction measurements show that the quadratic magneto-elastic coupling breaks down with applied field as shown by the observation of the 1st harmonic lattice reflections above and below HC . This indicates that magnetic field induces a linear magneto-elastic coupling. We argue that the commensurate phase can be described by an ordering of Mn-O-Mn bond angles.

cond-mat.str-el↗

arXiv2005

Electronically competing phases and their magnetic field dependence in electron-doped nonsuperconducting and superconducting Pr0.88LaCe0.12CuO4

We present comprehensive neutron scattering studies of nonsuperconducting and superconducting electron-doped Pr0.88LaCe0.12CuO4(PLCCO). At zero field, the transition from antiferromagnetic (AF) as-grown PLCCO to superconductivity without static antiferromagnetism can be achieved by annealing the sample in pure Ar at different temperatures, which also induces an epitaxial (Pr,La,Ce)2O3 phase as an impurity. When the superconductivity first appears in PLCCO, a quasi-two-dimensional (2D) spin-density-wave (SDW) order is also induced, and both coexist with the residual three-dimensional (3D) AF state. A magnetic field applied along the [-1,1,0] direction parallel to the CuO2 plane induces a ``spin-flop'' transition, where the noncollinear AF spin structure of PLCCO is transformed into a collinear one. The spin-flop transition is continuous in semiconducting PLCCO, but gradually becomes sharp with increasing doping and the appearance of superconductivity. A c-axis aligned magnetic field that suppresses the superconductivity also enhances the quasi-2D SDW order at (0.5,0.5,0) for underdoped PLCCO. However, there is no effect on the 3D AF order in either superconducting or nonsuperconducting samples. Since the same field along the [-1,1,0] direction in the CuO2 plane has no (or little) effect on the superconductivity, (0.5,0.5,0) and (Pr,La,Ce)2O3 impurity positions, we conclude that the c-axis field-induced effect is intrinsic to PLCCO and arises from the suppression of superconductivity.

cond-mat.supr-con↗

arXiv2004

Revised superconducting phase diagram of hole doped Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}\cdot y$H$_{2}$O

We have studied the superconducting phase diagram of \NaH\space as a function of electronic doping, characterizing our samples both in terms of Na content $x$ and the Co valence state. Our findings are consistent with a recent report that intercalation of \oxp\space ions into Na$_{x}$CoO$_{2}$, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal \Tc\space is achieved through a Co valence range of 3.24 - 3.35, while \Tc\space decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO$_{2}$ sheets as Co valence is varied into the optimal \Tc\space region is further evidence for this criticality.

cond-mat.supr-con↗

arXiv2004

Field-induced avalanche to the ferromagnetic state in the phase-separated ground state of manganites

Perovskite manganite compounds such as Pr_(1-x)(Ca_(1-y)Sr_y)_xMn3 can be tuned to exhibit a metastable ground state where two magnetic/crystallographic phases coexist in zero magnetic field. Field-dependent neutron diffraction measurements on both poly- and single-crystal samples with a range of Pr, Ca, and Sr dopings(0.3<x<0.35 and y<0.30) reveal that the charge-ordered, antiferromagnetic phase of the ground state suddenly and irreversibly jumps to the ferromagnetic state. The transition occurs spontaneously at some time after the field is set above a threshold field, indicating that once the transition is initiated an avalanche occurs that drives it to completion.

cond-mat.str-el↗

arXiv2004

The structure of intercalated water in superconducting Na$_{0.35}$CoO$_{2}\cdot$1.37D$_{2}$O: Implications for the superconducting phase diagram

We have used electron and neutron powder diffraction to elucidate the structural properties of superconducting \NaD. Our measurements show that our superconducting sample exhbits a number of supercells ranging from ${1/3}a^{*}$ to ${1/15}a^{*}$, but the most predominant one, observed also in the neutron data, is a double hexagonal cell with dimensions \dhx. Rietveld analysis reveals that \deut\space is inserted between CoO$_{2}$ sheets as to form a layered network of NaO$_{6}$ triangular prisms. Our model removes the need to invoke a 5K superconducting point compound and suggests that a solid solution of Na is possible within a constant amount of water $y$.

cond-mat.supr-con↗

arXiv2003

Reply to the comment cond-mat/0308067

Mang et al. report the observation of a cubic (Nd,Ce)2O3 impurity phase grown epitaxially in annealed samples of electron-doped Nd2-xCexCuO4 (NCCO) and argue that our observed magnetic scattering in NCCO [Nature 423,522(2003)]is due entirely to (Nd,Ce)2O3. Here we show why this is incorrect.

cond-mat.supr-con↗

arXiv2003

Effect of a magnetic field on the long-range magnetic order in insulating Nd2CuO4, nonsuperconducting and superconducting Nd1.85Ce0.15CuO4

We have measured the effect of a c-axis aligned magnetic field on the long-range magnetic order of insulating Nd2CuO4, as-grown nonsuperconducting and superconducting Nd1.85Ce0.15CuO4. On cooling from room temperature, Nd2CuO4 goes through a series of antiferromagnetic (AF) phase transitions with different noncollinear spin structures. In all phases of Nd2CuO4, we find that the applied c-axis field induces a canting of the AF order but does not alter the basic zero-field noncollinear spin structures. Similar behavior is also found in as-grown nonsuperconducting Nd1.85Ce0.15CuO4. These results contrast dramatically with those of superconducting Nd1.85Ce0.15CuO4, where a c-axis aligned magnetic field induces a static, anomalously conducting, long-range ordered AF state. We confirm that the annealing process necessary to make superconducting Nd1.85Ce0.15CuO4 also induces epitaxial, three-dimensional long-range ordered cubic (Nd,Ce)2O3 as an impurity phase. In addition, the annealing process makes a series of quasi two-dimensional superlattice reflections associated with lattice distortions of Nd1.85Ce0.15CuO4 in the CuO2 plane. While the application of a magnetic field will induce a net moment in the impurity phase, we determine its magnitude and eliminate this as a possibility for the observed magnetic field-induced effect in superconducting Nd1.85Ce0.15CuO4.

cond-mat.str-el↗

arXiv2003

Inhomogeneous Magnetism in La-doped CaMnO3. (II) Mesoscopic Phase Separation due to Lattice-coupled FM Interactions

A detailed investigation of mesoscopic magnetic and crystallographic phase separation in Ca(1-x)La(x)MnO3, 0.00<=x<=0.20, is reported. Neutron powder diffraction and DC-magnetization techniques have been used to isolate the different roles played by electrons doped into the eg level as a function of their concentration x. The presence of multiple low-temperature magnetic and crystallographic phases within individual polycrystalline samples is argued to be an intrinsic feature of the system that follows from the shifting balance between competing FM and AFM interactions as a function of temperature. FM double-exchange interactions associated with doped eg electrons are favored over competing AFM interactions at higher temperatures, and couple more strongly with the lattice via orbital polarization. These FM interactions thereby play a privileged role, even at low eg electron concentrations, by virtue of structural modifications induced above the AFM transition temperatures.

cond-mat.mtrl-sci↗

arXiv2002

Structural and magnetic ordering in Pr0.65(CaySr1-y)0.35MnO3: "quantum critical point" versus phase segregation scenarios

The phase diagram of Pr0.65(CaySr1-y)0.35MnO3, 0.6 <= y <= 0.8, has been determined by neutron diffraction, magnetization and electrical conductivity measurements in order to investigate the nature of the transition between ferromagnetic metallic and charge-ordered insulating states near y=0.75. Two possible scenarios for this transition have been proposed: a "quantum critical point"-like feature, near which an associated charge-disordered paramagnetic phase is present, or a phase coexistence region. We demonstrate that the latter case is true, phase segregation occurring on a mesoscopic/macroscopic length-scale (several hundred Angstroms to several microns). Our results show that no significant amount of the charge-disordered paramagnetic phase is present at low temperatures. Our data also indicate that the charge-ordered insulator to ferromagnetic metal phase boundary is temperature as well as composition-dependent.

cond-mat.str-el↗

arXiv2002

Magnetization Distribution in the layered CMR Manganite La1.2Sr1.8Mn2O7 from Polarized Neutron Diffraction

In the ferromagnetic metallic state of the colossal magnetoresistive (CMR) manganite La1.2Sr1.8Mn2O7, the spin density distribution is essentially in agreement with the standard picture in which the unpaired electrons occupy the three t2g orbitals, dxy, dyz, and dxz. However we find a small spin density ~4% of the total Mn spin) on the apical O(2) oxygen atom at both 100 K and 220 K and we suggest that this is due to covalency effects. Surprisingly we find no evidence of spin on the other apical oxygen O(1) suggesting that the Mn eg electron distribution along the c-axis is highly anisotropic.

cond-mat.soft↗

arXiv2002

Glass Transition in the Polaron Dynamics of CMR Manganites

Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polarons correlations are completely dynamic at high T, but then freeze upon cooling to a temperature T* 310 K. This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (Tc=114 K), where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.

cond-mat.str-el↗

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↗

arXiv2000

Mesoscopic and Microscopic Phase Segregation in Manganese Perovskites

Mesoscopic (500-2000 Angstrom) and microscopic (5-20 Angstrom) phase segregation with temperature and magnetic field was studied in the model manganite Pr0.7Ca0.3MnO3 by high-resolution neutron diffraction and inelastic neutron scattering. Intra-granular strain-driven mesoscopic segregation between two insulating phases, one of which is charge ordered (CO), sets in below the CO temperature in zero field. The CO phase orders antiferromagnetically, while the other insulating phase shows spin-glass behavior. After field-induced metallization, the CO phase coexists with a ferromagnetic metallic phase.

cond-mat.mtrl-sci↗

arXiv1999

Lattice Displacements Above Tc in the Layered Manganite La1.2Sr1.8Mn2O7

Neutron diffraction data presented in this paper demonstrates the relevance of lattice displacement above TC, in our understanding of the evolution of the crystal structure with temperature in the layered CMR manganite La1.2Sr1.8Mn2O7. The anomalous temperature behavior of thermal diffuse scattering (TDS) in La1.2Sr1.8Mn2O7 strongly suggests that it arises from lattice displacements and correlates directly with anomalies in the displacement parameters of the O- and Mn-atoms and Mn-O bond lengths. From our measurements, the insulator - metal transition can be described as a transition from a high temperature state with disordered Mn-O bond lengths to a low temperature state with a more uniform distribution on Mn-O bonds. These observations are in agreement with polaronic charge transport above TC in the perovskite manganites; as electron hopping is responsible for bond disorder above TC, below the transition where eg carries are delocalized, any lattice displacements are uniformly averaged.

cond-mat.mtrl-sci↗

arXiv1997

Neutron scattering investigation of magnetic bilayer correlations in $La_{1.2}Sr_{1.8}Mn_2O_7$

Neutron scattering investigations of the paramagnetic correlations in the layered manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$, which exhibits colossal magnetoresistance above the Curie transition at T$_C$ = 112 K, show that spins in neighboring layers within each bilayer are strongly canted at an average angle that is dependent on both the magnetic field and temperature, as predicted by de Gennes. The in-plane correlation length does not diverge at T$_C$, although the magnetic Bragg intensity obeys critical scaling below T$_C$, with the same temperature dependence as the zero-field electrical conductance.

cond-mat.str-el↗