arXiv subjects

S. E. Barnes

Publications and source records attributed to S. E. Barnes.

16 recordsLinked to original sources

arXiv2010

Ferromagnetic resonance with a magnetic Josephson junction

We show experimentally and theoretically that there is a coupling via the Aharonov-Bohm phase between the order parameter of a ferromagnet and a singlet, s-wave, Josephson supercurrent. We have investigated the possibility of measuring the dispersion of such spin waves by varying the magnetic field applied in the plane of the junction and demonstrated the electromagnetic nature of the coupling by the observation of magnetic resonance side-bands to microwave induced Shapiro steps.

cond-mat.supr-con↗

arXiv2009

Coupled Superconducting Phase and Ferromagnetic Order Parameter Dynamics

Via a direct coupling between the magnetic order parameter and the singlet Josephson supercurrent, we detect spin-wave resonances, and their dispersion, in ferromagnetic Josephson junctions in which the usual insulating or metallic barrier is replaced with a weak ferromagnet. The coupling arises within the Fraunhofer interferential description of the Josephson effect, because the magnetic layer acts as a time dependent phase plate. A spin-wave resonance at a frequency ws implies a dissipation that is reflected as a depression in the current-voltage curve of the Josephson junction when hbar ws = 2eV. We have thereby performed a resonance experiment on only 10^7 Ni atoms.

cond-mat.supr-con↗

arXiv2006

Magnetic memory and current amplification devices using moving domain walls

A moving magnetic domain wall produces an electromotive force (emf). It is therefore possible to read the state of a magnetic memory device via the emf it produces when subject to an interrogation pulse. It is also possible to amplify currents in pulse circuits, opening up the possibility of all magnetic logic circuits.

cond-mat.mtrl-sci↗

arXiv2004

Currents induced by domain wall motion in thin ferromagnetic wires

Historically the role of eddy currents in the motion of Bloch walls has been extensively studied. In contrast the direct interaction of currents with such a moving wall has received very little attention. However recently it has been shown that a wall can be displaced by the effects of a current alone, a fact which is important to the development of spintronics. We report here that such a moving wall within an itinerant ferromagnet constitutes a seat of electro-motive-force (or more accurately a spin-motive-force) which induces a current flow in an external circuit. While an external magnetic field is the most evident means by which to induce Bloch wall motion, it is not the only possibility and the forces intrinsic to certain geometries imply that the state of a spintronics memory device might be read via this mechanism using currents alone and that power amplification of spin polarized pulses is possible. An estimate of the induced current shows it to be technologically useful.

cond-mat.str-el↗

arXiv2004

Current-spin coupling for ferromagnetic domain walls in fine wires

The coupling between a current and a domain wall is examined. In the presence of a finite current and the absence of a potential which breaks the translational symmetry, there is a perfect transfer of angular momentum from the conduction electrons to the wall. As a result, the ground state is in uniform motion. This remains the case when relaxation is accounted for. This is described by, appropriately modified, Landau-Lifshitz-Gilbert equations.

cond-mat.str-el↗

arXiv2004

Current-driven domain wall motion in thin ferromagnetic wires

The coupling between a current and a Bloch wall is examined in the half-metal limit of the double exchange model. The conduction electrons transfer angular momentum to the Bloch wall with 100% efficiency in the absence of pinning. The wall is displaced without distortion with velocity proportional to the current. In the presence of a pinning potential either the angular momentum is destroyed by the perpendicular component of the anisotropy field or is converted to coherent magnons. A moving wall has its velocity reduced by pinning. The expression for the velocity agrees surprisingly well with experiment.

cond-mat.str-el↗

arXiv2004

Bi-layer splitting in overdoped high $T_{c}$ cuprates

Recent angle-resolved photoemission data for overdoped Bi2212 are explained. Of the peak-dip-hump structure, the peak corresponds the $\vec q =0$ component of a hole condensate which appears at $T_c$. The fluctuating part of this same condensate produces the hump. The bilayer splitting is large enough to produce a bonding hole and an electron antibonding quasiparticle Fermi surface. Smaller bilayer splittings observed in some experiments reflect the interaction of the peak structure with quasiparticle states near, but not at, the Fermi surface.

cond-mat.supr-con↗

arXiv2001

Hidden Symmetries in Nano-magnets

The hidden symmetry of certain nano-magnets leads to many of the levels being doubly degenerate for periodic values of the Zeeman energy. Corresponding to such a symmetry is an operator, $K_{n}$, related to the time reversal operator, and which commutes with the Hamiltonian. The degeneracies for whole integer spin values are often similar to the Kramer's degeneracy for half-integer spin. An all algebraic method for constructing Hamiltonians with such hidden symmetries and different crystal symmetries is described.

cond-mat.mes-hall↗

arXiv2001

Zone Edge Softening and Relaxation in the Double Exchange Model

The $J\to \infty$ double exchange model is formulated in terms of three auxiliary particles. A slow true bosonic magnon propagates by admixture with a fast fermionic pseudo-magnon. This process involves the absorption of a conduction electron which, for this half-metal, carries only charge degrees of freedom. The magnon dispersion becomes much weaker and the relaxation rate increases rapidly upon approaching the zone boundary. That the magnons relax for all wave vector values implies the existence of a low energy spin continuum.

cond-mat.str-el↗

arXiv2001

A Jordan-Wigner transformation for the $t-J$ and Hubbard models with holes

A Jordan-Wigner (JW) transformation for the $t-J$ and Hubbard models is described. Introduced are holon and doublon particles for hole and double occupied sites. There is only a single spin sector particle. Flux tubes occur in a naturel fashion, within a specific gauge, when the method is adapted to two dimensions. In order to accommodate three dimensions ``flux sheets'' are defined. The adaptation of the method to the $t-J$-model in the context of high $T_{c}$ is described.

cond-mat.str-el↗

arXiv2000

Intermediate spin approach to the tunneling of nano-magnets

The theory for the quantum tunneling of nano-magnets is developed within the intermediate spin framework. Periodic magnetic effects are seen to reflect that associated with a change of flux by a single flux quantum $Φ_{0}$. Essential are Schrödinger cat wave functions which involve superpositions of different magnitudes of the applied magnetic field. For systems in which the tunneling paths are not co-planar the theory leads to essentially complex magnetic fields although the expectation values of the fields remain real. In general the ground states correspond to minimal uncertainty squeezed states. The degree of squeezing depends on the anisotropy parameters.

cond-mat.mes-hall↗

arXiv2000

Intermediate spin, Schrödinger cat states and nano-magnets

Quantum tunneling of nano-magnets finds a natural description in terms of intermediate spin. Periodic magnetic effects correspond to a change of flux by the flux quantum $Φ_{0}$. Schrödinger cat states with different superpositions of the applied magnetic field occur. The molecular magnet Fe$_{8}$ is discussed and oscillations are predicted for Mn$_{12}$.

cond-mat.mes-hall↗

arXiv1999

Manifestation of intermediate spin for Fe$_{8}$

Intermediate spin, which occurs in the theory of anyons, can also be exhibited by mesoscopic magnetic particles. The necessary broken time reversal symmetry is due to a suitably directed magnetic field. As a function of this field a system passes periodically through points which correspond to whole or half-integer spin. Intermediate spin is defined by fields which lie between these points. Since the tunnel splitting in the ground doublet vanishes for half-integer spin, this splitting becomes periodic. The doubly periodic oscillations observed in the magnetic molecular cluster Fe$_{8}$ represent the first unequivocal observation of this phenomena. Here the whole or half-integer nature of the system, i.e., the parity, is periodic for a field which is along either the easy or hard axis or a suitable combination of the two. A detailed theory is presented.

cond-mat.mes-hall↗

arXiv1999

Intermediate spin and quantum critical points, etc

Unlike that of SO(3) or SU(2), the Lie algebra for SO(2), which defines intermediate spin, comprises only $S_{z}$ and implies $S^{\pm}$ commute. In general, $S_{z}$ has a continuous spectrum. This intermediate spin scheme can realized for the low energy excitations of a wide class of large spin magnets. A magnetic field provides the necessary time reversal symmetry breaking and controls the effective value of the spin $\tilde S$. Physical quantities are periodic in the equilibrium magnetization component induced by this field. In particular for one dimensional antiferromagnets there are periodic regions on the field axis for which the model is quantum critical while in two or three dimensions criticality is reduced to points.

cond-mat.stat-mech↗

arXiv1998

Efficient quantum computing on low temperature spin ensembles

A new scheme is proposed which will permit electron spin resonance pulse techniques to be used to realize a quantum computer with a 100 qbits, or more. The computation is performed on effective pure states which correspond to off-diagonal blocks of the density matrix. Described is a scheme which very efficiently performs the preparation stage and which permits ``pseudo-projective measurement'' to be made on the output. With such measurements all members of the ensemble remain coherent.

quant-ph↗

arXiv1997

Field-induced quasi-particle coherence effects in certain small magnets

Small ferromagnets and anti-ferromagnets with an easy-plane anisotropy have a ground to first excited state (tunnel) splitting which is quasi-periodic in the magnitude of a field applied perpendicular to a principal anisotropy axis. The associated oscillations in thermodynamic quantities might be used to prove the existence of a coherent ground state even when the tunnel splitting itself cannot be directly detected.

cond-mat↗