10 research outputs found
Ordering in Two-Dimensional Ising Models with Competing Interactions
We study the 2D Ising model on a square lattice with additional non-equal
diagonal next-nearest neighbor interactions. The cases of classical and quantum
(transverse) models are considered. Possible phases and their locations in the
space of three Ising couplings are analyzed. In particular, incommensurate
phases occurring only at non-equal diagonal couplings, are predicted. We also
analyze a spin-pseudospin model comprised of the quantum Ising model coupled to
XY spin chains in a particular region of interactions, corresponding to the
Ising sector's super-antiferromagnetic (SAF) ground state. The spin-SAF
transition in the coupled Ising-XY model into a phase with co-existent SAF
Ising (pseudospin) long-range order and a spin gap is considered. Along with
destruction of the quantum critical point of the Ising sector, the phase digram
of the Ising-XY model can also demonstrate a re-entrance of the spin-SAF phase.
A detailed study of the latter is presented. The mechanism of the re-entrance,
due to interplay of interactions in the coupled model, and the conditions of
its appearance are established. Applications of the spin-SAF theory for the
transition in the quarter-filled ladder compound NaV2O5 are discussed.Comment: Minor revisions and refs. added; published version of the invited
paper in a special issue of "Low Temp. Physics
Universal corrections to the Fermi-liquid theory
We show that the singularities in the dynamical bosonic response functions of
a generic 2D Fermi liquid give rise to universal, non-analytic corrections to
the Fermi-liquid theory. These corrections yield a term in the specific
heat, terms in the effective mass and the uniform spin susceptibility
, and term in . The existence of these
terms has been the subject of recent controversy, which is resolved in this
paper. We present exact expressions for all non-analytic terms to second order
in a generic interaction and show that only U(0) and matter.Comment: references added, a typo correcte
Local versus Nonlocal Order Parameter Field Theories for Quantum Phase Transitions
General conditions are formulated that allow to determine which quantum phase
transitions in itinerant electron systems can be described by a local
Landau-Ginzburg-Wilson or LGW theory solely in terms of the order parameter. A
crucial question is the degree to which the order parameter fluctuations couple
to other soft modes. Three general classes of zero-wavenumber order parameters,
in the particle-hole spin-singlet and spin-triplet channels, and in the
particle-particle channel, respectively, are considered. It is shown that the
particle-hole spin-singlet class does allow for a local LGW theory, while the
other two classes do not. The implications of this result for the critical
behavior at various quantum phase transitions are discussed, as is the
connection with nonanalyticities in the wavenumber dependence of order
parameter susceptibilities in the disordered phase.Comment: 9 pp., LaTeX, no figs, final version as publishe
Dynamical symmetry breaking in the Nambu-Jona-Lasino model with external gravitational and constant electric fields
An investigation of the Nambu-Jona-Lasino model with external constant
electric and weak gravitational fields is carried out in three- and four-
dimensional spacetimes. The effective potential of the composite bifermionic
fields is calculated keeping terms linear in the curvature, while the electric
field effect is treated exactly by means of the proper- time formalism.
A rich dynamical symmetry breaking pattern, accompanied by phase transitions
which are ruled, independently, by both the curvature and the electric field
strength is found. Numerical simulations of the transitions are presented.Comment: 20 pages, LaTeX, 6 .ps-figures, Final version published in "Classical
and Quantum Gravity
Horizon Scan of the Belt and Road Initiative
We present 11 frontier issues identified in the first BRI Horizon Scan.
Issues were submitted by 14 researchers after consulting approximately 250 people in their networks, and scoring, based on impact and novelty, was conducted using the Delphi process.
Frontier issues include: threats to karst-based ecosystems via extraction for cement, BRI expansion to the Arctic, and the impacts of geopolitical rivalry on environmental and social standards and on building in conflict zones.
Other issues include: the potential spread of microbial species, the growth of TCM in BRI countries, and opportunities arising from more inclusive governance and partnership with local and indigenous communities.
We highlight the challenges faced in building the Belt and Road, and the compromises that may be made between sustainability and developmental goals.
The Belt and Road Initiative (BRI) represents the largest infrastructure and development project in human history, and presents risks and opportunities for ecosystems, economies, and communities. Some risks (habitat fragmentation, roadkill) are obvious, however, many of the BRI’s largest challenges for development and conservation are not obvious and require extensive consideration to identify. In this first BRI Horizon Scan, we identify 11 frontier issues that may have large environmental and social impacts but are not yet recognised. More generally, the BRI will increase China’s participation in international environmental governance. Thus, new cooperative modes of governance are needed to balance geopolitical, societal, and environmental interests. Upgrading and standardising global environmental standards is essential to safeguard ecological systems and human societies