1,450 research outputs found
Role of p-f Hybridization in the Metal-Non-Metal Transition of PrRu4P12
Electronic state evolution in the metal-non-metal transition of PrRu4P12 has
been studied by X-ray and polarized neutron diffraction experiments. It has
been revealed that, in the low-temperature non-metallic phase, two inequivalent
crystal-field (CF) schemes of Pr3+ 4f^2 electrons with Gamma_1 and Gamma_4^(2)
ground states are located at Pr1 and Pr2 sites forming the bcc unit cell
surrounded by the smaller and larger cubic Ru-ion sublattices, respectively.
This modulated electronic state can be explained by the p-f hybridization
mechanism taking two intermediate states of 4f^1 and 4f^3. The p-f
hybridization effect plays an important role for the electronic energy gain in
the metal-non-metal transition originated from the Fermi surface nesting.Comment: 5 pages, 5 figures. Accepted by J. Phys. Soc. Jp
Microscopic Mechanism for Staggered Scalar Order in PrFe4P12
A microscopic model is proposed for the scalar order in PrFe4P12 where f2
crystalline electric field (CEF) singlet and triplet states interact with two
conduction bands. By combining the dynamical mean-field theory and the
continuous-time quantum Monte Carlo, we obtain an electronic order with
staggered Kondo and CEF singlets with the total conduction number being unity
per site. The ground state becomes semimetallic provided that the two
conduction bands have different occupation numbers. This model naturally
explains experimentally observed properties in the ordered phase of PrFe4P12
such as the scalar order parameter, temperature dependence of the resistivity,
field-induced staggered moment, and inelastic features in neutron scattering.
The Kondo effect plays an essential role for ordering, in strong contrast with
ordinary magnetic orders by the RKKY interaction.Comment: 4 pages, 4figure
Solution of reduced equations derived with singular perturbation methods
For singular perturbation problems in dynamical systems, various appropriate
singular perturbation methods have been proposed to eliminate secular terms
appearing in the naive expansion. For example, the method of multiple time
scales, the normal form method, center manifold theory, the renormalization
group method are well known. In this paper, it is shown that all of the
solutions of the reduced equations constructed with those methods are exactly
equal to sum of the most divergent secular terms appearing in the naive
expansion. For the proof, a method to construct a perturbation solution which
differs from the conventional one is presented, where we make use of the theory
of Lie symmetry group.Comment: To be published in Phys. Rev.
A Field Effect Transitor based on the Mott Transition in a Molecular Layer
Here we propose and analyze the behavior of a FET--like switching device, the
Mott transition field effect transistor, operating on a novel principle, the
Mott metal--insulator transition. The device has FET-like characteristics with
a low ``ON'' impedance and high ``OFF'' impedance. Function of the device is
feasible down to nanoscale dimensions. Implementation with a class of organic
charge transfer complexes is proposed.Comment: Revtex 11pages, Figures available upon reques
Electronic Orders Induced by Kondo Effect in Non-Kramers f-Electron Systems
This paper clarifies the microscopic nature of the staggered scalar order,
which is specific to even number of f electrons per site. In such systems,
crystalline electric field (CEF) can make a singlet ground state. As exchange
interaction with conduction electrons increases, the CEF singlet at each site
gives way to Kondo singlets. The collective Kondo singlets are identified with
itinerant states that form energy bands. Near the boundary of itinerant and
localized states, a new type of electronic order appears with staggered Kondo
and CEF singlets. We present a phenomenological three-state model that
qualitatively reproduces the characteristic phase diagram, which have been
obtained numerically with use of the continuous-time quantum Monte Carlo
combined with the dynamical mean-field theory. The scalar order observed in
PrFe_4P_{12} is ascribed to this staggered order accompanying charge density
wave (CDW) of conduction electrons. Accurate photoemission and tunneling
spectroscopy should be able to probe sharp peaks below and above the Fermi
level in the ordered phase.Comment: 7 pages, 8 figure
Social evolution leads to persistent corruption
Cooperation can be sustained by institutions that punish free-riders. Such institutions, however, tend to be subverted by corruption if they are not closely watched. Monitoring can uphold the enforcement of binding agreements ensuring cooperation, but this usually comes at a price. The temptation to skip monitoring and take the institution’s integrity for granted leads to outbreaks of corruption and the breakdown of cooperation. We model the corresponding mechanism by means of evolutionary game theory, using analytical methods and numerical simulations, and find that it leads to sustained or damped oscillations. The results confirm the view that corruption is endemic and transparency a major factor in reducing it
Games of corruption: How to suppress illegal logging
Corruption is one of the most serious obstacles for ecosystem management and biodiversity conservation. In particular, more than half of the loss of forested area in many tropical countries is due to illegal logging, with corruption implicated in a lack of enforcement. Here we study an evolutionary game model to analyze the illegal harvesting of forest trees, coupled with the corruption of rule enforcers. We consider several types of harvesters, who may or may not be committed towards supporting an enforcer service, and who may cooperate (log legally) or defect (log illegally). We also consider two types of rule enforcers, honest and corrupt: while honest enforcers fulfill their function, corrupt enforcers accept bribes from defecting harvesters and refrain from fining them. We report three key findings. First, in the absence of strategy exploration, the harvester-enforcer dynamics are bistable: one continuum of equilibria consists of defecting harvesters and a low fraction of honest enforcers, while another consists of cooperating harvesters and a high fraction of honest enforcers. Both continua attract nearby strategy mixtures. Second, even a small rate of strategy exploration removes this bistability, rendering one of the outcomes globally stable. It is the relative rate of exploration among enforcers that then determines whether most harvesters cooperate or defect and most enforcers are honest or corrupt, respectively. This suggests that the education of enforcers, causing their more frequent trialing of honest conduct, can be a potent means of curbing corruption. Third, if information on corrupt enforcers is available, and players react opportunistically to it, the domain of attraction of cooperative outcomes widens considerably. We conclude by discussing policy implications of our results
Evidence for Insulating Behavior in the Electric Conduction of (NH)KC Systems
Microwave study using cavity perturbation technique revealed that the
conductivity of antiferromagnet (NH)KRbC at 200K is
already 3-4 orders of magnitude smaller than those of superconductors,
KC and (NH)NaRbC, and that the antiferromagnetic
compounds are {\it insulators} below 250K without metal-insulator transitions.
The striking difference in the magnitude of the conductivity between these
materials strongly suggests that the Mott-Hubbard transition in the ammoniated
alkali fullerides is driven by a reduction of lattice symmetry from
face-centered-cubic to face-centered-orthorhombic, rather than by the magnetic
ordering.Comment: accepted for publication in PR
On the origin of multiple ordered phases in PrFe4P12
The nature of multiple electronic orders in skutterudite PrFe_4P_{12} is
discussed on the basis of a model with antiferro-quadrupole (AFQ) interaction
of \Gamma_3 symmetry. The high-field phase can be reproduced qualitatively
provided (i) ferro-type interactions are introduced between the dipoles as well
as between the octupoles of localized f-electrons, and (ii) separation is
vanishingly small between the \Gamma_1-\Gamma_4^{(1)} crystalline electric
field (CEF) levels. The high-field phase can have either the same ordering
vector q=(1,0,0) as in the low-field phase, or a different one q=0 depending on
the parameters. In the latter case, distortion of the crystal perpendicular to
the (111) axis is predicted. The corresponding anomaly in elastic constants
should also appear. The electrical resistivity is calculated with account of
scattering within the CEF quasi-quartet. It is found that the resistivity as a
function of the direction of magnetic field shows a sharp maximum around the
(111) axis at low temperatures because of the level crossing.Comment: 16 pages, 5 figure
- …