47 research outputs found
Spin Liquid Regimes at Nonzero Temperature in Quantum Spin Ice
Quantum spin liquids are highly entangled ground states of quantum systems
with emergent gauge structure, fractionalized spinon excitations, and other
unusual properties. While these features clearly distinguish quantum spin
liquids from conventional, mean-field-like states at zero temperature (T),
their status at T>0 is less clear. Strictly speaking, it is known that most
quantum spin liquids lose their identity at non-zero temperature, being in that
case adiabatically transformable into a trivial paramagnet. This is the case
for the U(1) quantum spin liquid states recently proposed to occur in the
quantum spin ice pyrochlores. Here we propose, however, that in practical
terms, the latter quantum spin liquids can be regarded as distinct phases from
the high temperature paramagnet. Through a combination of gauge mean field
theory calculations and physical reasoning, we argue that these systems sustain
both quantum spin liquid and thermal spin liquid phases, dominated by quantum
fluctuations and entropy, respectively. These phases are separated by a first
order "thermal confinement" transition, such that for temperatures below the
transition, spinons and emergent photons are coherently propagating
excitations, and above it the dynamics is classical. Even for parameters for
which the ground state is magnetically ordered and not a quantum spin liquid,
this strong first order transition occurs, pre-empting conventional Landau-type
criticality. We argue that this picture explains the anomalously low
temperature phase transition observed in the quantum spin ice material
Yb2Ti2O7.Comment: 15 pages (including 7 pages of appendices), 3 figures, 1 tabl
Quantum Spin Liquids
Quantum spin liquids may be considered "quantum disordered" ground states of
spin systems, in which zero point fluctuations are so strong that they prevent
conventional magnetic long range order. More interestingly, quantum spin
liquids are prototypical examples of ground states with massive many-body
entanglement, of a degree sufficient to render these states distinct phases of
matter. Their highly entangled nature imbues quantum spin liquids with unique
physical aspects, such as non-local excitations, topological properties, and
more. In this review, we discuss the nature of such phases and their properties
based on paradigmatic models and general arguments, and introduce theoretical
technology such as gauge theory and partons that are conveniently used in the
study of quantum spin liquids. An overview is given of the different types of
quantum spin liquids and the models and theories used to describe them. We also
provide a guide to the current status of experiments to study quantum spin
liquids, and to the diverse probes used therein.Comment: 60 pages, 8 figures, 1 tabl
Disorder-Induced Entanglement in Spin Ice Pyrochlores
We propose that in a certain class of magnetic materials, known as
non-Kramers 'spin ice,' disorder induces quantum entanglement. Instead of
driving glassy behavior, disorder provokes quantum superpositions of spins
throughout the system, and engenders an associated emergent gauge structure and
set of fractional excitations. More precisely, disorder transforms a classical
phase governed by a large entropy, classical spin ice, into a quantum spin
liquid governed by entanglement. As the degree of disorder is increased, the
system transitions between (i) a "regular" Coulombic spin liquid, (ii) a phase
known as "Mott glass," which contains rare gapless regions in real space, but
whose behavior on long length scales is only modified quantitatively, and (iii)
a true glassy phase for random distributions with large width or large mean
amplitude.Comment: 6+2 pages, 2+1 figure
Coulombic Quantum Liquids in Spin-1/2 Pyrochlores
We develop a non-perturbative "gauge Mean Field Theory" (gMFT) method to
study a general effective spin-1/2 model for magnetism in rare earth
pyrochlores. gMFT is based on a novel exact slave-particle formulation, and
matches both the perturbative regime near the classical spin ice limit and the
semiclassical approximation far from it. We show that the full phase diagram
contains two exotic phases: a quantum spin liquid and a coulombic ferromagnet,
both of which support deconfined spinon excitations and emergent quantum
electrodynamics. Phenomenological properties of these phases are discussed.Comment: 4+ pages, 6+ pages of Supplementary Material, 4 figures, 1 tabl
Signatures of the Helical Phase in the Critical Fields at Twin Boundaries of Non-Centrosymmetric Superconductors
Domains in non-centrosymmetric materials represent regions of different
crystal structure and spin-orbit coupling. Twin boundaries separating such
domains display unusual properties in non-centrosymmetric superconductors
(NCS), where magneto-electric effects influence the local lower and upper
critical magnetic fields. As a model system, we investigate NCS with tetragonal
crystal structure and Rashba spin-orbit coupling (RSOC), and with twin
boundaries parallel to their basal planes. There, we report that there are two
types of such twin boundaries which separate domains of opposite RSOC. In a
magnetic field parallel to the basal plane, magneto-electric coupling between
the spin polarization and supercurrents induces an effective magnetic field at
these twin boundaries. We show this leads to unusual effects in such
superconductors, and in particular to the modification of the upper and lower
critical fields, in ways that depend on the type of twin boundary, as analyzed
in detail, both analytically and numerically. Experimental implications of
these effects are discussed.Comment: 10 pages, 6 figure
A New Type of Quantum Criticality in the Pyrochlore Iridates
Magnetic fluctuations and electrons couple in intriguing ways in the vicinity
of zero temperature phase transitions - quantum critical points - in conducting
materials. Quantum criticality is implicated in non-Fermi liquid behavior of
diverse materials, and in the formation of unconventional superconductors. Here
we uncover an entirely new type of quantum critical point describing the onset
of antiferromagnetism in a nodal semimetal engendered by the combination of
strong spin-orbit coupling and electron correlations, and which is predicted to
occur in the iridium oxide pyrochlores. We formulate and solve a field theory
for this quantum critical point by renormalization group techniques, show that
electrons and antiferromagnetic fluctuations are strongly coupled, and that
both these excitations are modified in an essential way. This quantum critical
point has many novel features, including strong emergent spatial anisotropy, a
vital role for Coulomb interactions, and highly unconventional critical
exponents. Our theory motivates and informs experiments on pyrochlore iridates,
and constitutes a singular realistic example of a non-trivial quantum critical
point with gapless fermions in three dimensions.Comment: 5 pages + 8 pages of Supplementary Material, 3 figures + 1
supplementary figur
Phonon Thermal Hall Conductivity from Scattering with Collective Fluctuations
Because electrons and ions form a coupled system, it is a priori clear that
the dynamics of the lattice should reflect symmetry breaking within the
electronic degrees of freedom. This has been recently clearly evidenced for the
case of time-reversal and mirror symmetry breakings by observations of a large
phononic thermal Hall effect in many strongly correlated electronic materials.
The mechanism by which time-reversal breaking and chirality is communicated to
the lattice is, however, far from evident. In this paper we discuss how this
occurs via many-body scattering of phonons by collective modes: a consequence
of non-Gaussian correlations of the latter modes. We derive fundamental new
results for such skew (i.e. chiral) scattering and the consequent thermal Hall
conductivity. From this we also obtain general formulae for these quantities
for ordered antiferromagnets. From the latter we obtain the scaling behavior of
the phonon thermal Hall effect in clean antiferromagnets. The calculations show
several different regimes and give quantitative estimates of similar order to
that seen in recent experiments.Comment: 14 pages, including 4 pages of appendices, 4 figures. Companion
paper: arXiv:2202.1036
Impurity Effects in Highly Frustrated Diamond Lattice Antiferromagnets
We consider the effects of local impurities in highly frustrated diamond
lattice antiferromagnets, which exhibit large but non-extensive ground state
degeneracies. Such models are appropriate to many A-site magnetic spinels. We
argue very generally that sufficiently dilute impurities induce an ordered
magnetic ground state, and provide a mechanism of degeneracy breaking. The
states which are selected can be determined by a "swiss cheese model" analysis,
which we demonstrate numerically for a particular impurity model in this case.
Moreover, we present criteria for estimating the stability of the resulting
ordered phase to a competing frozen (spin glass) one. The results may explain
the contrasting finding of frozen and ordered ground states in CoAl2O4 and
MnSc2S4, respectively.Comment: 13 pages, 7 figure