5 research outputs found
Spin density wave, Fermi liquid, and fractionalized phases in a theory of antiferromagnetic metals using paramagnons and bosonic spinons
The pseudogap metal phase of the hole-doped cuprates can be described by
small Fermi surfaces of electron-like quasiparticles, which enclose a volume
violating the Luttinger relation. This violation requires the existence of
additional fractionalized excitations which can be viewed as fractionalized
remnants of the paramagnon. We fractionalize the paramagnon into bosonic
spinons, and present a gauge theory of bosonic spinons, a Higgs field, and an
ancilla layer of fermions coupled to the original electrons. Along with the
small Fermi surface metal, this theory displays conventional phases: the Fermi
liquid with a low-energy paramagnon mode, and phases with spin density wave
order. We follow the evolution of the electronic photoemission spectrum across
these quantum phase transitions. We consider both the two-sublattice N\'eel and
incommensurate spin density wave phases.Comment: 36 pages, 16 figures; (v5) Improved figures and added clarification
Impurity-induced pairing in two-dimensional Fermi gases
We study induced pairing between two identical fermions mediated by an
attractively interacting quantum impurity in two-dimensional systems. Based on
a Stochastic Variational Method (SVM), we investigate the influence of
confinement and finite interaction range effects on the mass ratio beyond which
the ground state of the quantum three-body problem undergoes a transition from
a composite bosonic trimer to an unbound dimer-fermion state. We find that
confinement as well as a finite interaction range can greatly enhance trimer
stability, bringing it within reach of experimental implementations such as
found in ultracold atom systems. In the context of solid-state physics our
solution of the confined three-body problem shows that exciton-mediated
interactions can become so dominant that they can even overcome detrimental
Coulomb repulsion between electrons in atomically-thin semiconductors. Our work
thus paves the way towards a universal understanding of boson-induced pairing
across various fermionic systems at finite density, and opens perspectives
towards realizing novel forms of electron pairing beyond the conventional
paradigm of Cooper pair formation.Comment: 13 pages, 9 figure
Observation of a smooth polaron-molecule transition in a degenerate Fermi gas
Understanding the behavior of an impurity strongly interacting with a Fermi
sea is a long-standing challenge in many-body physics. When the interactions
are short-ranged, two vastly different ground states exist: a polaron
quasiparticle and a molecule dressed by the majority atoms. In the
single-impurity limit, it is predicted that at a critical interaction strength,
a first-order transition occurs between these two states. Experiments, however,
are always conducted in the finite temperature and impurity density regime. The
fate of the polaron-to-molecule transition under these conditions, where the
statistics of quantum impurities and thermal effects become relevant, is still
unknown. Here, we address this question experimentally and theoretically. Our
experiments are performed with a spin-imbalanced ultracold Fermi gas with
tunable interactions. Utilizing a novel Raman spectroscopy combined with a
high-sensitivity fluorescence detection technique, we isolate the quasiparticle
contribution and extract the polaron energy, spectral weight, and the contact
parameter. As the interaction strength is increased, we observe a continuous
variation of all observables, in particular a smooth reduction of the
quasiparticle weight as it goes to zero beyond the transition point. Our
observation is in good agreement with a theoretical model where polaron and
molecule quasiparticle states are thermally occupied according to their quantum
statistics. At the experimental conditions, polaron states are hence populated
even at interactions where the molecule is the ground state and vice versa. The
emerging physical picture is thus that of a smooth transition between polarons
and molecules and a coexistence of both in the region around the expected
transition.Comment: 15 (main text) + 5 (appendices) pages, 13 figures; minor changes and
addition
Transition from a polaronic condensate to a degenerate Fermi gas of heteronuclear molecules
The interplay of quantum statistics and interactions in atomic Bose--Fermi
mixtures leads to a phase diagram markedly different from pure fermionic or
bosonic systems. However, investigating this phase diagram remains challenging
when bosons condense. Here, we observe evidence for a quantum phase transition
from a polaronic to a molecular phase in a density-matched degenerate
Bose--Fermi mixture. The condensate fraction, representing the order parameter
of the transition, is depleted by interactions and the build-up of strong
correlations results in the emergence of a molecular Fermi gas. By driving
through the transition, we ultimately produce a quantum-degenerate sample of
sodium-potassium molecules exhibiting a large molecule-frame dipole moment of
2.7 Debye. The observed phase transition represents a new phenomenon
complementary to the paradigmatic BEC-BCS crossover observed in Fermi systems.Comment: 12 pages, 9 figure
Dispersion forces between weakly disordered Van der Waals crystals
We describe a many-body theory for interlayer dispersion forces between
weakly disordered atomically thin crystals and numerically investigate the role
of disorder for different layer-separation distances and for different
densities of induced electrons and holes. In contrast to the common wisdom that
disorder tends to enhance the importance of Coulomb interactions in Fermi
liquids, we find that short range disorder tends to {\it weaken} interlayer
dispersion forces. This is in line with previous findings that suggest that
transitioning from metallic to insulating propagation weakens interlayer
dispersion forces. We demonstrate that disorder alters the scaling laws of
dispersion forces and we comment on the role of the maximally crossed
vertex-correction diagrams responsible for logarithmic divergences in the
resistivity of two-dimensional metals.Comment: 10 pages, 8 figures. Added references and revised interpretation of
result