Spin current and dc noise generated across biased quantum spin chains
Joshua Aftergood and So Takei
Department of Physics, Queens College of the City University of New York, Queens, NY 11367 USA andThe Physics PhD Program, The Graduate Center of the City University of New York, New York, NY 10016
USA
stakei@qc.cuny.edu
Spin current refers to a net flow of spin angular momentum through various condensed matter media.
While electrical insulators preclude charge currents, spin currents can flow through insulators insofar as
the latter support mobile spin excitations. A series of recent experiments have achieved two-terminal spintransport through magnetic insulators [1-4], in which mobile spin excitations, i.e., magnons, carry the spincurrents in the insulator bulk, and the injection and detection of spin currents using electrical signals are
facilitated by the so-called spin Hall phenomena [5]. These experiments open doors to the fascinating
possibility of performing two-terminal spin transport measurements on general quantum spin systems.
Subsequent research efforts have mainly focused on magnetically ordered insulators, whose ground states
are conventional classical spin configurations with quantum fluctuations generating merely perturbative
effects. However, there are a rich plethora of quantum paramagnets, which emerge in systems withquantum fluctuations strong enough to destroy any conventional magnetic order and often possess exotic
ground states and excitations above them that defy any conventional understanding based on the Landau
symmetry-breaking principle. One-dimensional quantum magnets, or quantum spin chains (QSCs), is agreat paradigm to study such unconventional quantum magnetism, and the physics of spin transport
through these QSCs will be the topic of this talk.
The focus will be on spin transport across two coupled spin-1/2 xxz antiferromagnet chains, with aparticular emphasis on the spin current and the dc noise of this current. We consider spin flow induced bysimultaneously over-populating spin excitations and elevating the temperature in one sub-system relative
to the other. We find a vanishing spin current and a diverging spin Fano factor, defined as the spin current
noise-to-signal ratio, even in the presence of a temperature bias, as long as an over-population of spin
excitations in one sub-system relative to other is absent. We attribute these effects to the underlying
fermionic nature of the spin excitations, and compare these results to the case of spin transport across atwo coupled magnetic insulators that harness conventional (bosonic) magnon excitations. We address how
the spin Fano factor has strikingly different behaviors between the two cases as a function of thepopulation and thermal biases. We finally discuss how the spin Fano factor may be used to extract the socalled spin Hall angle in certain regimes, which can be used to supplement existing methods ofcharacterizing the quantity via ferromagnetic resonance techniques that make use of spin transfer torques
acting on the magnetization of a ferromagnetic insulator [5].

[1] L. Cornelissen et al., Nature Phys. 11, 1022 (2015).
[2] S. B. Gönnenwein et al., App. Phys. Lett. 107, 172405 (2015).
[3] B. L. Giles et al., Phys. Rev. B 92, 224415 (2015).
[4] J. Li et al., Nature Comm. 7, 10858 (2016).
[5] J. Sinova et al., Rev. Mod. Phys. 87, 1213 (2015).