80 research outputs found
Topological defects of N\'eel order and Kondo singlet formation for Kondo-Heisenberg model on a honeycomb lattice
Heavy fermion systems represent a prototypical setting to study magnetic
quantum phase transitions. A particular focus has been on the physics of Kondo
destruction, which captures quantum criticality beyond the Landau framework of
order-parameter fluctuations. In this context, we study the spin one-half
Kondo-Heisenberg model on a honeycomb lattice at half filling. The problem is
approached from the Kondo destroyed, antiferromagnetically ordered insulating
phase. We describe the local moments in terms of a coarse grained quantum
non-linear sigma model, and show that the skyrmion defects of the
antiferromagnetic order parameter host a number of competing order parameters.
In addition to the spin Peierls, charge and current density wave order
parameters, we identify for the first time Kondo singlets as the competing
orders of the antiferromagnetism. We show that the antiferromagnetism and
various competing singlet orders can be related to each other via generalized
chiral transformations of the underlying fermions. We also show that the
conduction electrons acquire a Berry phase through their coupling to the
hedgehog configurations of the N\'eel order, which cancels the Berry phase of
the local moments. Our results demonstrate the competition between the
Kondo-singlet formation and spin-Peierls order when the antiferromagnetic order
is suppressed, thereby shedding new light on the global phase diagram of heavy
fermion systems at zero temperature.Comment: 14 pages, 4 figure
Topological Weyl Superconductor to Diffusive Thermal Hall Metal Crossover in the B-Phase of UPt
The recent phase sensitive measurements in the superconducting -phase of
UPt provide strong evidence for the triplet, chiral
pairing symmetries, which endow the Cooper pairs with orbital angular momentum
projections along the -axis. In the absence of disorder such
pairing can support both line and point nodes, and both types of nodal
quasiparticles exhibit nontrivial topology in the momentum space. The point
nodes, located at the intersections of the closed Fermi surfaces with the
-axis, act as the double monopoles and the antimonopoles of the Berry
curvature, and generalize the notion of Weyl quasiparticles. Consequently, the
phase should support an anomalous thermal Hall effect, the polar Kerr
effect, in addition to the protected Fermi arcs on the (1,0,0) and the (0,1,0)
surfaces. The line node at the Fermi surface equator acts as a vortex loop in
the momentum space and gives rise to the zero energy, dispersionless Andreev
bound states on the (0,0,1) surface. At the transition from the -phase to
the -phase, the time reversal symmetry is restored, and only the line node
survives inside the -phase. As both line and double-Weyl point nodes possess
linearly vanishing density of states, we show that weak disorder acts as a
marginally relevant perturbation. Consequently, an infinitesimal amount of
disorder destroys the ballistic quasiparticle pole, while giving rise to a
diffusive phase with a finite density of states at the zero energy. The
resulting diffusive phase exhibits -linear specific heat, and an anomalous
thermal Hall effect. We predict that the low temperature thermodynamic and
transport properties display a crossover between a ballistic thermal Hall
semimetal and a diffusive thermal Hall metal.Comment: 8 pages, 1 figure; replaced by the version accepted by Phys. Rev.
Itinerant quantum multi-criticality of two dimensional Dirac fermions
We analyze emergent quantum multi-criticality for strongly interacting,
massless Dirac fermions in two spatial dimensions () within the framework
of Gross-Neveu-Yukawa models, by considering the competing order parameters
that give rise to fully gapped (insulating or superconducting) ground states.
We focus only on those competing orders, which can be rotated into each other
by generators of an exact or emergent chiral symmetry of massless Dirac
fermions, and break and symmetries in the ordered phase.
Performing a renormalization group analysis by using the
expansion scheme, we show that all the coupling constants in the critical
hyperplane flow toward a new attractive fixed point, supporting an
\emph{enlarged} chiral symmetry. Such a fixed point acts as an
exotic quantum multi-critical point (MCP), governing the \emph{continuous}
semimetal-insulator as well as insulator-insulator (for example antiferromagnet
to valence bond solid) quantum phase transitions. In comparison with the lower
symmetric semimetal-insulator quantum critical points, possessing either
or chiral symmetry, the MCP displays enhanced correlation
length exponents, and anomalous scaling dimensions for both fermionic and
bosonic fields. We discuss the scaling properties of the ratio of bosonic and
fermionic masses, and the increased dc resistivity at the MCP. By computing the
scaling dimensions of different local fermion bilinears in the particle-hole
channel, we establish that most of the four fermion operators or generalized
density-density correlation functions display faster power law decays at the
MCP compared to the free fermion and lower symmetric itinerant quantum critical
points. Possible generalization of this scenario to higher dimensional Dirac
fermions is also outlined.Comment: 15 pages, 9 figures; replaced by the version published in Phys. Rev.
Skyrmion defects and competing singlet orders in a half-filled antiferromagnetic Kondo-Heisenberg model on the honeycomb lattice
Due to the interaction between topological defects of an order parameter and
underlying fermions, the defects can possess induced fermion numbers, leading
to several exotic phenomena of fundamental importance to both condensed matter
and high energy physics. One of the intriguing outcome of induced fermion
number is the presence of fluctuating competing orders inside the core of
topological defect. In this regard, the interaction between fermions and
skyrmion excitations of antiferromagnetic phase can have important consequence
for understanding the global phase diagrams of many condensed matter systems
where antiferromagnetism and several singlet orders compete. We critically
investigate the relation between fluctuating competing orders and skyrmion
excitations of the antiferromagnetic insulating phase of a half-filled
Kondo-Heisenberg model on honeycomb lattice. By combining analytical and
numerical methods we obtain exact eigenstates of underlying Dirac fermions in
the presence of a single skyrmion configuration, which are used for computing
induced chiral charge. Additionally, by employing this nonperturbative
eigenbasis we calculate the susceptibilities of different translational
symmetry breaking charge, bond and current density wave orders and
translational symmetry preserving Kondo singlet formation. Based on the
computed susceptibilities we establish spin Peierls and Kondo singlets as
dominant competing orders of antiferromagnetism. We show favorable agreement
between our findings and field theoretic predictions based on perturbative
gradient expansion scheme which crucially relies on adiabatic principle and
plane wave eigenstates for Dirac fermions. The methodology developed here can
be applied to many other correlated systems supporting competition between
spin-triplet and spin-singlet orders in both lower and higher spatial
dimensions.Comment: 15 pages, 11 figure
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