30 research outputs found
Interfacial Spin Seebeck effect in noncollinear magnetic systems
The interplay between spin and heat currents at magnetic
insulator|nonmagnetic metal interfaces has been a subject of much scrutiny
because of both fundamental physics and the promise for technological
applications. While ferrimagnetic and, more recently, antiferromagnetic systems
have been extensively investigated, a theory generalizing the heat-to-spin
interconversion in noncollinear magnets is still lacking. Here, we establish a
general framework for thermally-driven spin transport at the interface between
a noncollinear magnet and a normal metal. Modeling the interfacial coupling
between localized and itinerant magnetic moments via an exchange Hamiltonian,
we derive an expression for the spin current, driven by a temperature
difference, for an arbitrary noncollinear magnetic order. Our theory reproduces
previously obtained results for ferromagnetic and antiferromagnet systems
Longitudinal Spin Seebeck effect in Pyrochlore Iridates with Bulk and Interfacial Dzyaloshinskii-Moriya interaction
The longitudinal spin-Seebeck effect (SSE) in magnetic
insulatornon-magnetic metal heterostructures has been theoretically studied
primarily with the assumption of an isotropic interfacial exchange coupling.
Here, we present a general theory of the SSE in the case of an antisymmetric
Dzyaloshinskii-Moriya interaction (DMI) at the interface, in addition to the
usual Heisenberg form. We numerically evaluate the dependence of the spin
current on the temperature and bulk DMI using a pyrochlore iridate as a model
insulator with all-in all-out (AIAO) ground state configuration. We also
compare the results of different crystalline surfaces arising from different
crystalline orientations and conclude that the relative angles between the
interfacial moments and Dzyaloshinskii-Moriya vectors play a significant role
in the spin transfer. Our work extends the theory of the SSE by including the
anisotropic nature of the interfacial Dzyaloshinskii-Moriya exchange
interaction in magnetic insulatornon-magnetic metal heterostructures and can
suggest possible materials to optimize the interfacial spin transfer in
spintronic devices
Even-Odd-Layer-Dependent Symmetry Breaking in Synthetic Antiferromagnets
In this work we examine synthetic antiferromagnetic structures consisting of
two, three, and four antiferromagnetic coupled layers, i.e., bilayers,
trilayers, and tetralayers. We vary the thickness of the ferromagnetic layers
across all structures and, using a macrospin formalism, find that the nearest
neighbor exchange interaction between layers is consistent across all
structures for a given thickness. Our model and experimental results
demonstrate significant differences in how the magnetostatic equilibrium states
of even and odd-layered structures evolve as a function of the external field.
Even layered structures continuously evolve from a collinear antiferromagnetic
state to a spin canted non-collinear magnetic configuration that is
mirror-symmetric about the external field. In contrast, odd-layered structures
begin with a ferrimagnetic ground state; at a critical field, the ferrimagnetic
ground state evolves into a non-collinear state with broken symmetry.
Specifically, the magnetic moments found in the odd-layered samples possess
stable static equilibrium states that are no longer mirror-symmetric about the
external field after a critical field is reached. Our results reveal the rich
behavior of synthetic antiferromagnets
A solid-state platform for cooperative quantum phenomena
The dissipation resulting from the coupling of a system with its environment
is commonly viewed as a foe for quantum technologies. Nonetheless, recent
developments at light-matter interfaces have shown that correlated dissipation
can be leveraged to engineer novel dynamical states of matter and entanglement
in many-body quantum systems. Inspired by this progress, here we set the stage
for the -- yet uncharted -- exploration of cooperative quantum phenomena in
quantum hybrid solid-state platforms. We develop a comprehensive formalism for
the quantum many-body dynamics of an ensemble of solid-state spin defects
interacting via the magnetic field fluctuations of a common solid-state
reservoir. Our general framework captures effective qubit-qubit interactions
mediated by correlated dissipation and naturally extends the theory of quantum
sensing of local magnetic noise via single solid-state spin defects to sensing
of nonlocal temporal and spatial correlations. To understand whether
dissipative correlations can play a relevant role in a realistic experimental
setup, we apply our model to a qubit array interacting via the spin
fluctuations of a ferromagnetic reservoir. Our results show that collective
relaxation rates of an ensemble of solid-state spin defects placed nearby a
common ferromagnetic thin film can display clear signatures of superradiance
and subradiance in the appropriate parameter regime. Furthermore, we find that
the cooperative quantum behavior exhibits remarkable robustness against spatial
disorder and thermal fluctuations. Our work lays the foundation for merging
spintronics and quantum optics towards a common research horizon in the
incoming future
Non-Hermitian topology of one-dimensional spin-torque oscillator arrays
Magnetic systems have been extensively studied both from a fundamental physics perspective and as building blocks for a variety of applications. Their topological properties, in particular those of excitations, remain relatively unexplored due to their inherently dissipative nature. The recent introduction of non-Hermitian topological classifications opens up new opportunities for engineering topological phases in dissipative systems. Here, we propose a magnonic realization of a non-Hermitian topological system. A crucial ingredient of our proposal is the injection of spin current into the magnetic system, which alters and can even change the sign of terms describing dissipation. We show that the magnetic dynamics of an array of spin-torque oscillators can be mapped onto a non-Hermitian Su-Schrieffer-Heeger model exhibiting topologically protected edge states. Using exact diagonalization of the linearized dynamics and numerical solutions of the non-linear equations of motion, we find that a topological magnonic phase can be accessed by tuning the spin current injected into the array. In the topologically nontrivial regime, a single spin-torque oscillator on the edge of the array is driven into auto-oscillation and emits a microwave signal, while the bulk oscillators remain inactive. Our findings have practical utility for memory devices and spintronics neural networks relying on spin-torque oscillators as constituent units
Floquet-engineering topological transitions in a twisted transition metal dichalcogenide homobilayer
Motivated by the recent experimental realization of twisted transition metal
dichalcogenide bilayers, we study a simplified model driven by different forms
of monochromatic light. As a concrete and representative example we use
parameters that correspond to a twisted MoTe homobilayer. First, we
consider irradiation with circularly polarized light in free space and
demonstrate that the corresponding Floquet Hamiltonian takes the same form as
the static Hamiltonian, only with a constant overall shift in quasi-energy.
This is in stark contrast to twisted bilayer graphene, where new terms are
typically generated under an analagous drive. Longitudinal light, on the other
hand, which can be generated from the transverse magnetic mode in a waveguide,
has a much more dramatic effect--it renormalizes the tunneling strength between
the layers, which effectively permits the tuning of the twist angle {\em
in-situ}. We find that, by varying the frequency and amplitude of the drive,
one can induce a topological transition that cannot be obtained with the
traditional form of the Floquet drive in free space. Furthermore, we find that
strong drives can have a profound effect on the layer pseudospin texture of the
twisted system, which coincides with multiple simultaneous band gap closings in
the infinite-frequency limit. Surprisingly, these bandgap closings are not
associated with topological transitions. For high but finite drive frequencies
near eV, the infinite-frequency band crossings become band gap minima of
the order of eV or smaller