arXiv subjects

A. Berger

Publications and source records attributed to A. Berger.

22 records · Page 2Linked to original sources

arXiv2006

Spin Precession and Avalanches

In many magnetic materials, spin dynamics at short times are dominated by precessional motion as damping is relatively small. In the limit of no damping and no thermal noise, we show that for a large enough initial instability, an avalanche can transition to an ergodic phase where the state is equivalent to one at finite temperature, often above that for ferromagnetic ordering. This dynamical nucleation phenomenon is analyzed theoretically. For small finite damping the high temperature growth front becomes spread out over a large region. The implications for real materials are discussed.

cond-mat.mtrl-sci↗

arXiv1999

Direct Imaging of the First Order Spin Flop Transition in the Layered Manganite La1.4Sr1.6Mn2O7

Magnetic field induced transitions in the antiferromagnetic layered manganite La1.4Sr1.6Mn2O7 were studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the first direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not by domain wall motion. A small magnetic hysteresis is caused by the nucleation and annihilation of domains in the mixed state. These results establish a direct link betweeen the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.

cond-mat↗

arXiv1999

Evidence for a Purely Double-Exchange Mechanism for the Anisotropic Conductivities in Layered La-Sr-Mn-O Single Crystals Below TC

Experimental evidence supports the double-exchange (DE) mechanism for both in-plane and c-axis conductivity in the colossal-magnetoresistive (CMR) layered manganite La1.4Sr1.6Mn2O7. Below TC, the data determine both the DE and antiferromagnetic (AF) superexchange between bilayers. These agree with recent spin-wave data, such that the ratio of intra- to inter-bilayer DE constants is very close to the spin-independent conductance anisotropy of ~300. The conductivity is shown to be proportional to the square of the measured magnetization over a wide range of fields and for temperatures below the in-plane ferromagnetic ordering temperature, TC. This dependence is shown to be consistent with DE coupling and earlier zero-field studies of La1.2Sr1.8Mn2O7 near TC, which used neutron scattering for the local magnetization. A mixed AF and spin-flop state (similar to the intermediate state of type-I superconductors) is a rigorous prediction of the data and modeling.

cond-mat↗

arXiv1997

Optical Response of Grating-Coupler-Induced Intersubband Resonances: The Role of Wood's Anomalies

Grating-coupler-induced collective intersubband transitions in a quasi-two-dimensional electron system are investigated both experimentally and theoretically. Far-infrared transmission experiments are performed on samples containing a quasi-two-dimensional electron gas quantum-confined in a parabolic quantum well. For rectangular shaped grating couplers of different periods we observe a strong dependence of the transmission line shape and peak height on the period of the grating, i.e. on the wave vector transfer from the diffracted beams to the collective intersubband resonance. It is shown that the line shape transforms with increasing grating period from a Lorentzian into a strongly asymmetric line shape. Theoretically, we treat the problem by using the transfer-matrix method of local optics and apply the modal-expansion method to calculate the influence of the grating. The optically uniaxial quasi-two-dimensional electron gas is described in the long-wavelength limit of the random-phase approximation by a local dielectric tensor, which includes size quantization effects. Our theory reproduces excellently the experimental line shapes. The deformation of the transmission line shapes we explain by the occurrence of both types of Wood's anomalies.

cond-mat.mes-hall↗