arXiv subjects

D. Prabhakaran

Publications and source records attributed to D. Prabhakaran.

At least 109 records · Page 6Linked to original sources

arXiv2011

Femtosecond dynamics of the collinear-to-spiral antiferromagnetic phase transition in CuO

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic re-orientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.

cond-mat.str-el↗

arXiv2011

Evidence of Magnetic Correlations Among the Charge Stripe Electrons of Charge-Stripe Ordered La2NiO4.11

The magnetic excitations of charge-stripe ordered La2NiO4.11 where investigated using polarized- and unpolarized-neutron scattering to determine the magnetic excitations of the charge stripe electrons. We observed a magnetic excitation mode consistent with the gapped quasi-one-dimensional antiferromagnetic correlations of the charge stripe electrons previously observed in La(2-x)Sr(x)NiO(4) x = 1/3 and x = 0.275.

cond-mat.str-el↗

arXiv2011

Low-moment magnetism in the double perovskites Ba2MOsO6 (M=Li,Na)

The magnetic ground states of the isostructural double perovskites Ba$_2$NaOsO$_6$ and Ba$_2$LiOsO$_6$ are investigated with muon-spin rotation. In Ba$_2$NaOsO$_6$ long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below $T_{\mathrm{c}}=7.2\pm 0.2\;\mathrm{K}$, while in Ba$_2$LiOsO$_6$ a static but spatially-disordered internal field is found below 8 K. A novel probabilistic argument is used to show from the observed precession frequencies that the magnetic ground state in Ba$_2$NaOsO$_6$ is most likely to be low-moment ($\approx 0.2$\,$μ_{\rm B}$) ferromagnetism and not canted antiferromagnetism. Ba$_2$LiOsO$_6$ is antiferromagnetic and we find a spin-flop transition at 5.5\,T. A reduced osmium moment is common to both compounds, probably arising from a combination of spin-orbit coupling and frustration.

cond-mat.str-el↗

arXiv2011

Femtoscale magnetically induced lattice distortions in multiferroic TbMnO3

Magneto-electric multiferroics exemplified by TbMnO3 possess both magnetic and ferroelectric long-range order. The magnetic order is mostly understood, whereas the nature of the ferroelectricity has remained more elusive. Competing models proposed to explain the ferroelectricity are associated respectively with charge transfer and ionic displacements. Exploiting the magneto-electric coupling, we use an electric field to produce a single magnetic domain state, and a magnetic field to induce ionic displacements. Under these conditions, interference charge-magnetic X-ray scattering arises, encoding the amplitude and phase of the displacements. When combined with a theoretical analysis, our data allow us to resolve the ionic displacements at the femtoscale, and show that such displacements make a significant contribution to the zero-field ferroelectric moment.

cond-mat.str-el↗

arXiv2011

Cu3Nb2O8: a multiferroic with chiral coupling to the crystal structure

By combining bulk properties, neutron diffraction and non-resonant X-ray diffraction measurements, we demonstrate that the new multiferroic Cu$_3$Nb$_2$O$_8$ becomes polar simultaneously with the appearance of generalised helicoidal magnetic ordering. The electrical polarization is oriented perpendicularly to the common plane of rotation of the spins --- an observation that cannot be reconciled with the "conventional" theory developed for cycloidal multiferroics. Our results are consistent with coupling between a macroscopic structural rotation, which is allowed in the paramagnetic group, and magnetically-induced structural chirality.

cond-mat.str-el↗

arXiv2011

Probing magnetic order in LiMPO4, M = Ni, Co, Fe and lithium diffusion in LixFePO4

Muon spin relaxation measurements are reported on three members of the LixMPO4 series. The magnetic properties of stoichiometric samples with M = Ni, Co, Fe, were investigated at low-temperature. In LiNiPO4 we observe different forms of the muon decay asymmetry in the commensurate and incommensurate antiferromagnetic phases, accompanied by a change in the temperature dependence of the muon oscillation frequency. In LiCoPO4 the form of the muon decay asymmetry indicates that the correlation between layers decreases as the Neel temperature is approached from below. LiFePO4 shows more conventional behaviour, typical for an antiferromagnet. Measurements on LixFePO4 with x = 0.8, 0.9, and 1 show evidence for lithium diffusion below ~250 K and muon diffusion dominating the form of the relaxation at higher temperature. The thermally activated form of the observed hopping rate suggests an activation barrier for lithium diffusion of ~ 100 meV and a diffusion constant of D_Li ~ 10^-10 - 10^-9 cm2/s at room temperature.

cond-mat.str-el↗

arXiv2011

Partial magnetization plateau emerging from a quantum spin ice state in Tb2Ti2O7

The quantum spin ice model applied to Tb2Ti2O7 predicts that magnetic fields applied along the [111] axis will induce a partial magnetization plateau [H. R. Molavian and M. J. P. Gingras, J. Phys.: Condens. Matter 21, 172201 (2009)]. We test this hypothesis using ac magnetic susceptibility and muon-spin relaxation measurements, finding features at 15 and 65 mT agreeing with the predicted boundaries of the magnetization plateau. This suggests that Tb2Ti2O7 is well described by a quantum spin ice model with an effective exchange constant of Jeff = 0.17(1) K.

cond-mat.str-el↗

arXiv2011

Dynamic fields in the partial magnetization plateau of Ca3Co2O6

Fluctuation dynamics in magnetization plateaux are a relatively poorly explored area in frustrated magnetism. Here we use muon spin relaxation to determine the fluctuation timescale and associated field distribution width in the partial magnetization plateau of Ca3Co2O6. The muon spin relaxation rate has a simple and characteristic field dependence which we model and by fitting to the data at 15 K extract a fluctuation timescale tau = 880(30) ps and a field distribution width Delta = 40.6(3) mT. Comparison with previous results on Ca3Co2O6 suggests that this fluctuation timescale can be associated with short-range, slowly fluctuating magnetic order.

cond-mat.str-el↗

arXiv2011

Bilayer manganites: polarons in the midst of a metallic breakdown

The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistant (CMR) effect, is still under heavy debate. By combining new photoemission and tunneling data, we show that in La{2-2x}Sr{1+2x}Mn2O7 the polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong electron-lattice interactions result in vanishing coherent quasi-particle spectral weight at the Fermi level for all locations in k-space. The incoherence of the charge carriers offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopic data. The stacking number N is the key factor for true metallic behavior, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in the bilayer system.

cond-mat.str-el↗

arXiv2011

Quantum criticality in an Ising chain: experimental evidence for emergent E8 symmetry

Quantum phase transitions take place between distinct phases of matter at zero temperature. Near the transition point, exotic quantum symmetries can emerge that govern the excitation spectrum of the system. A symmetry described by the E8 Lie group with a spectrum of 8 particles was long predicted to appear near the critical point of an Ising chain. We realize this system experimentally by tuning the quasi-one-dimensional Ising ferromagnet CoNb2O6 through its critical point using strong transverse magnetic fields. The spin excitations are observed to change character from pairs of kinks in the ordered phase to spin-flips in the paramagnetic phase. Just below the critical field, the spin dynamics shows a fine structure with two sharp modes at low energies, in a ratio that approaches the golden mean as predicted for the first two meson particles of the E8 spectrum. Our results demonstrate the power of symmetry to describe complex quantum behaviours.

cond-mat.str-el↗

arXiv2011

Hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet

Superconductivity in layered copper-oxide compounds emerges when charge carriers are added to antiferromagnetically-ordered CuO2 layers. The carriers destroy the antiferromagnetic order, but strong spin fluctuations persist throughout the superconducting phase and are intimately linked to super-conductivity. Neutron scattering measurements of spin fluctuations in hole-doped copper oxides have revealed an unusual `hour-glass' feature in the momentum-resolved magnetic spectrum, present in a wide range of superconducting and non-superconducting materials. There is no widely-accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes, an idea supported by measurements on stripe-ordered La1.875Ba0.125CuO4. However, many copper oxides without stripe order also exhibit an hour-glass spectrum$. Here we report the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. Our system has stripe correlations and is an insulator, which means its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper-oxide superconductors arises from fluctuating stripes.

cond-mat.str-el↗

arXiv2011

Probing photo-induced melting of antiferromagnetic order in La0.5Sr1.5MnO4 by ultrafast resonant soft X-ray diffraction

Photo-excitation in complex oxides1 transfers charge across semicovalent bonds, drastically perturbing spin and orbital orders2. Light may then be used in compounds like magnetoresistive manganites to control magnetism on nanometre lengthscales and ultrafast timescales. Here, we show how ultrafast resonant soft x-ray diffraction can separately probe the photo-induced dynamics of spin and orbital orders in La0.5Sr1.5MnO4. Ultrafast melting of CE antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand the (1/4,1/4,0) peak, reflecting orbital order, is only partially reduced. Cluster calculations aid our interpretation by considering different magnetically ordered states accessible after photo-excitation. Nonthermal coupling between light and magnetism emerges as a primary aspect of photo-induced phase transitions in manganites.

cond-mat.str-el↗

arXiv2011

Low energy, quasi-one-dimensional, spin dynamics in charge-ordered La(2-x)Sr(x)NiO(4)

The low energy spin excitations of La(2-x)Sr(x)NiO(4), x = 0.275 and 1/3, have been investigated by unpolarized- and polarized-inelastic neutron scattering from single crystals. A pattern of magnetic diffuse scattering is observed in both compositions, and is consistent with quasi-one-dimensional AFM spin correlations along the charge stripes. Analysis of the energy lineshape for x = 1/3 indicates that the diffuse scattering is inelastic with a characteristic energy of 1.40 +/- 0.07 meV. There is no discernible difference between the diffuse scattering from x = 0.275 and x = 1/3, suggesting that it is an intrinsic property of the charge stripes.

cond-mat.str-el↗

arXiv2010

Magnetic spectrum of the two-dimensional antiferromagnet La2CoO4 studied by inelastic neutron scattering

We report measurements of the magnetic excitation spectrum of the layered antiferromagnet La2CoO4 by time-of-flight neutron inelastic scattering. In the energy range probed in our experiments (0-250 meV) the magnetic spectrum consists of spin-wave modes with strong in-plane dispersion extending up to 60 meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a small (~1 meV) dispersion along the magnetic zone boundary. We show that the magnetic spectrum can be described very well by a model of a Heisenberg antiferromagnet that includes the full spin and orbital degrees of freedom of Co2+ in an axially-distorted crystal field. The collective magnetic dynamics are found to be controlled by dominant nearest-neighbour exchange interactions, strong XY-like single-ion anisotropy and a substantial unquenched orbital angular momentum.

cond-mat.str-el↗

arXiv2010

Magnetic excitations in multiferroic LuMnO3 studied by inelastic neutron scattering

We present data on the magnetic and magneto-elastic coupling in the hexagonal multiferroic manganite LuMnO3 from inelastic neutron scattering, magnetization and thermal expansion measurements. We measured the magnon dispersion along the main symmetry directions and used this data to determine the principal exchange parameters from a spin-wave model. An analysis of the magnetic anisotropy in terms of the crystal field acting on the Mn is presented. We compare the results for LuMnO3 with data on other hexagonal RMnO3 compounds.

cond-mat.str-el↗

arXiv2010

Ferromagnetic Excitations in La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ Observed Using Neutron Inelastic Scattering

Polarized neutron inelastic scattering has been used to measure spin excitations in ferromagnetic La$_{0.82}$Sr$_{0.18}$CoO$_{3}$. The magnon spectrum of these spin excitations is well defined at low energies but becomes heavily damped at higher energies, and can be modeled using a quadratic dispersion. We determined a spin wave stiffness constant of $D=94\pm 3$\,meV\,Å$^{2}$. Assuming a nearest-neighbor Heisenberg model we find reasonable agreement between the exchange determined from D and the bulk Curie temperature. Several possible mechanisms to account for the observed spin-wave damping are discussed.

cond-mat.str-el↗

arXiv2010

Antiferromagnetically spin polarized oxygen observed in magneto-electric TbMn2O5

We report the direct measurement of antiferromagnetic spin polarization at the oxygen sites in the multiferroic TbMn2O5, through resonant soft x-ray magnetic scattering. This supports recent theoretical models suggesting that the oxygen spin polarization is key to the magnetoelectric coupling mechanism. The spin polarization is observed through a resonantly enhanced diffraction signal at the oxygen K edge at the commensurate antiferromagnetic wavevector. Using the fdmnes code we have accurately reproduced the experimental data. We have established that the resonance arises through the spin polarization on the oxygen sites hybridized with the square based pyramid Mn3+ ions. Furthermore we have discovered that the position of the Mn3+ ion directly influences the oxygen spin polarization.

cond-mat.str-el↗

arXiv2010

Magnetic excitations of spin and orbital moments in cobalt oxide

Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched orbital angular momentum are studied by neutron scattering. Results of energy scans in several Brillouin zones in the (HHL) plane for energy transfers up to 16 THz are presented. The measurements were performed in the antiferromagnetic ordered state at 6 K (well below TN~290 K) as well as in the paramagnetic state at 450 K. Several magnetic excitation modes are identified from the dependence of their intensity on wavevector and temperature. Within a Hund's rule model the excitations correspond to fluctuations of coupled orbital and spin degrees of freedom whose bandwidth is controlled by interionic superexchange. The different <111> ordering domains give rise to several magnetic peaks at each wavevector transfer.

cond-mat.str-el↗