102 research outputs found

### Nonlocal Magnetoresistance Mediated by Spin Superfluidity

The electrical response of two diffusive metals is studied when they are
linked by a magnetic insulator hosting a topologically stable (superfluid) spin
current. We discuss how charge currents in the metals induce a spin
supercurrent state, which in turn generates a magnetoresistance that depends on
the topology of the electrical circuit. This magnetoresistance relies on phase
coherence over the entire magnet and gives direct evidence for spin
superfluidity. We show that driving the magnet with an ac current allows
coherent spin transport even in the presence of U(1)-breaking magnetic
anisotropy that can preclude dc superfluid transport. Spin transmission in the
ac regime shows a series of resonance peaks as a function of frequency. The
peak locations, heights, and widths can be used to extract static interfacial
properties, e.g., the spin-mixing conductance and effective spin Hall angle,
and to probe dynamic properties such as the spin-wave dispersion. Thus, ac
transport may provide a simpler route to realizing nonequilbrium coherent spin
transport and a useful way to characterize the magnetic system, serving as a
precursor to the realization of dc superfluid spin transport.Comment: 4 pages; 3 figure

### Noise in tunneling spin current across coupled quantum spin chains

We theoretically study the spin current and its dc noise generated between
two spin-1/2 spin chains weakly coupled at a single site in the presence of an
over-population of spin excitations and a temperature elevation in one
subsystem relative to the other, and compare the corresponding transport
quantities across two weakly coupled magnetic insulators hosting magnons. In
the spin chain scenario, we find that applying a temperature bias exclusively
leads to a vanishing spin current and a concomitant divergence in the spin Fano
factor, defined as the spin current noise-to-signal ratio. This divergence is
shown to have an exact analogy to the physics of electron scattering between
fractional quantum Hall edge states and not to arise in the magnon scenario. We
also reveal a suppression in the spin current noise that exclusively arises in
the spin chain scenario due to the fermion nature of the spin-1/2 operators. We
discuss how the spin Fano factor may be extracted experimentally via the
inverse spin Hall effect used extensively in spintronics.Comment: 12 pages, 8 figure

### Quantum control of topological defects in magnetic systems

Energy-efficient classical information processing and storage based on
topological defects in magnetic systems have been studied over past decade. In
this work, we introduce a class of macroscopic quantum devices in which a
quantum state is stored in a topological defect of a magnetic insulator. We
propose non-invasive methods to coherently control and readout the quantum
state using ac magnetic fields and magnetic force microscopy, respectively.
This macroscopic quantum spintronic device realizes the magnetic analog of the
three-level rf-SQUID qubit and is built fully out of electrical insulators with
no mobile electrons, thus eliminating decoherence due to the coupling of the
quantum variable to an electronic continuum and energy dissipation due to Joule
heating. For a domain wall sizes of $10-100$~nm and reasonable material
parameters, we estimate qubit operating temperatures in the range of $0.1-1$~K,
a decoherence time of about $0.01-1$~$\mu$s, and the number of Rabi flops
within the coherence time scale in the range of $10^{2}-10^{4}$.Comment: 8 pages, 4 figure

### Superfluid Spin Transport through Easy-Plane Ferromagnetic Insulators

Superfluid spin transport $-$ dissipationless transport of spin $-$ is
theoretically studied in a ferromagnetic insulator with easy-plane anisotropy.
We consider an open geometry where spin current is injected into the
ferromagnet from one side by a metallic reservoir with a nonequilibrium spin
accumulation, and ejected into another metallic reservoir located downstream.
Spin transport through the device is studied using a combination of
magnetoelectric circuit theory, Landau-Lifshitz-Gilbert phenomenology, and
microscopic linear-response theory. We discuss how spin superfluidity can be
probed using a magnetically-mediated electron-drag experiment.Comment: 5 pages, 2 figures (version published on Phys. Rev. Lett. and
supplementary material added

### Detecting Spin Transport in Quantum Magnets with Photons

A minimally invasive technique is proposed for detecting the differential
spin conductance and spin current noise across a junction between two quantum
magnets using a high-quality microwave resonator coupled to a transmission line
which is impedance matched to a photon detector downstream. Photons in the
microwave resonator couple inductively to the spins in the spin subsystem, and
the noise in the junction spin current imprints itself into the output photons
propagating along the transmission line. The technique is capable of extracting
both the dc and finite frequency noise via the output photon flux and of
measuring the junction spin conductance by driving the electromagnetic
environment into a different temperature regime.Comment: 5 pages, 2 figure

### Noise due to neutral modes in the v=2/3 fractional quantum Hall state

We theoretically study charge noise generated by excited neutral modes, which
impinge on the quantum point contact of a quantum Hall bar with filling
fraction v=2/3. The noise is computed for thermally excited neutral modes as
well as for biased neutral modes with dipole-fermion excitations. Within the
dipole-fermion picture, we show that the noise arising from two colliding modes
can be suppressed due to Pauli-blocking and be non-universal due to random edge
disorder, but becomes universal upon disorder-averaging. The ratio of noise due
to two colliding neutral modes and noise due to only one such mode is smaller
for dipole-fermions than for thermal excitations, thus providing evidence for
the different quantum statistics of the two types of excitations.Comment: 4 pages, 2 figure

### Spin transport in an electrically-driven magnon gas near Bose-Einstein condensation: Hartree-Fock-Keldysh theory

An easy-plane ferromagnetic insulator in a uniform external magnetic field
and in contact with a phonon bath and a normal metal bath is studied
theoretically in the presence of dc spin current injection via the spin Hall
effect in the metal. The Keldysh path integral formalism is used to model the
magnon gas driven into a nonequilibrium steady state by mismatched bath
temperatures and/or electrical injection, and we analyze the magnon system in
the normal (uncondensed) state, but close to the instability to Bose-Einstein
condensation (BEC), within the self-consistent Hartree-Fock approximation. We
find that the steady state magnon distribution function generally has a
non-thermal form that cannot be described by a single effective chemical
potential and effective temperature. We also show that the BEC instability in
the electrically-driven magnon system is signaled by a sign change in the
imaginary part of the poles for long-wavelength magnon modes and by the
divergence of the nonequilibrium magnon distribution function. In the presence
of two bath temperatures, we find that the correlation length of the superfluid
order parameter fluctuations exhibits nontrivial finite temperature crossover
behaviors that are richer than the standard crossover behaviors obtained for
the vacuum-superfluid transition in an equilibrium dilute Bose gas. We study
the consequences of these thermal crossovers on the magnon spin conductivity
and obtain an inverse square-root divergence in the spin conductivity in the
vicinity of the electrically-induced BEC instability. A spintronics device
capable of testing our spin transport predictions is discussed.Comment: 14 pages, 8 figures, appendice

### Topological spin transport by Brownian diffusion of domain walls

We propose thermally-populated domain walls (DWs) in an easy-plane
ferromagnetic insulator as robust spin carriers between two metals. The
chirality of a DW, which serves as a topological charge, couples to the metal
spin accumulation via spin-transfer torque and results in the
chirality-dependent thermal nucleation rates of DWs at the interface. After
overpopulated DWs of a particular (net) chirality diffuse and leave the
ferromagnet at the other interface, they reemit the spin current by spin
pumping. The conservation of the topological charge supports an algebraic decay
of spin transport as the length of the ferromagnet increases; this is analogous
to the decaying behavior of superfluid spin transport but contrasts with the
exponential decay of magnon spin transport. We envision that similar spin
transport with algebraic decay may be implemented in materials with exotic spin
phases by exploiting topological characteristics and the associated conserved
quantities of their excitations.Comment: 5 pages + references, 4 figure

### Thermally-Activated Phase Slips in Superfluid Spin Transport in Magnetic Wires

We theoretically study thermally-activated phase slips in superfluid spin
transport in easy-plane magnetic wires within the stochastic
Landau-Lifshitz-Gilbert phenomenology, which runs parallel to the
Langer-Ambegaokar-McCumber-Halperin theory for thermal resistances in
superconducting wires. To that end, we start by obtaining the exact solutions
for free-energy minima and saddle points. We provide an analytical expression
for the phase-slip rate in the zero spin-current limit, which involves detailed
analysis of spin fluctuations at extrema of the free energy. An experimental
setup of a magnetoeletric circuit is proposed, in which thermal phase slips can
be inferred by measuring nonlocal magnetoresistance.Comment: 4 pages, 2 figures, and a supplemental materia

### Squeezed noise due to two-level system defects in superconducting resonator circuits

Motivated by recent surprising experimental results for the noise output of
superconducting microfabricated resonators used in quantum computing
applications and astronomy, we develop a fully quantum theoretical model to
describe quantum dynamics of these circuits. Building on theoretical techniques
from quantum optics, we calculate the noise in the output voltage due to
two-level system (TLS) defects. The theory predicts squeezing for the noise in
the amplitude quadrature with respect to the input noise, which qualitatively
reproduces the noise ellipse observed in experiment. We show that noise
enhancement along the phase direction persists for pump frequencies away from
resonance. Our results also suggest that intrinsic TLS fluctuations must be
incorporated in the model in order to describe the experimentally observed
dependence of the phase noise on input power.Comment: 4 pages, 4 figure

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