Superconductivity in the repulsive Hubbard model

The two-dimensional repulsive Hubbard model has been investigated by a variety of methods, from small to large U. Superconductivity with d-wave symmetry is consistently found close to half filling. After a brief review of the various methods a variational many-electron state is discussed in more detail. This trial state is a natural extension of the Gutzwiller ansatz and provides a substantial improvement thereof.Comment: Proc. Int. Conference on Quantum Phenomena in Complex Matter, Erice, July 19-25, 2010, to appear in Journal of Superconductivity and Novel Magnetism, Springer Verla

Central Bank's Action and Communication

This paper contributes to the ongoing debate about the welfare effect of public information. In an environment characterized by imperfect common knowledge and strategic complementarities, Morris and Shin (2002)argue that noisy public information may be detrimental to welfare because public information is attributed too large a weight relative to its face value since it serves as a focal point. While this argument has received a great deal of attention in central banks and in the financial press, it considers communication as the sole task of a central bank and ignores that communication usually goes with a policy action. This paper accounts for the action task of a central bank and analyzes whether public disclosure is beneficial in the conduct of monetary policy when the central bank primarily tries to stabilize the economy with an instrument that is optimal with respect to its perhaps mistaken view. In this context, it turns out that transparency is particularly beneficial when central bank’s information is poorly accurate because it helps reducing the distortion associated with badly suited policies

Adiabatic continuity and broken symmetry in many-electron systems: a variational perspective

Variational wave functions are very useful for describing the panoply of ground states found in interacting many-electron systems. Some particular trial states are "adiabatically" linked to a reference state, from which they borrow the essential properties. A prominent example is the Gutzwiller ansatz, where one starts with the filled Fermi sea. A simple soluble example, the anisotropic XY chain, illustrates the adiabatic continuity of this class of wave functions. To describe symmetry breaking, one has to modify the reference state accordingly. Alternatively, a quantum phase transition can be described by a pair of variational wave functions, starting at weak and strong coupling, respectively

Central Bank's Action and Communication

This paper contributes to the ongoing debate about the welfare effect of public information. In an environment characterized by imperfect common knowledge and strategic complementarities, Morris and Shin (2002)argue that noisy public information may be detrimental to welfare because public information is attributed too large a weight relative to its face value since it serves as a focal point. While this argument has received a great deal of attention in central banks and in the financial press, it considers communication as the sole task of a central bank and ignores that communication usually goes with a policy action. This paper accounts for the action task of a central bank and analyzes whether public disclosure is beneficial in the conduct of monetary policy when the central bank primarily tries to stabilize the economy with an instrument that is optimal with respect to its perhaps mistaken view. In this context, it turns out that transparency is particularly beneficial when central bank’s information is poorly accurate because it helps reducing the distortion associated with badly suited policies.differential information; monetary policy; transparency

Can Opacity of a Credible Central Bank Explain Excessive Inflation?

Excessive inflation is usually attributed to the lack of central bank’s credibility. In this context, most of the literature considers transparency a means to establish central bank’s credibility. The contribution of this paper is twofold. First, it shows that, even in the absence of inflationary bias, a credible central bank may find it optimal to implement an accommodating monetary policy in response to cost-push shocks whenever the uncertainty surrounding its monetary instrument is high. Indeed, the degree of central bank’s transparency influences the effectiveness of its policy to stabilize inflation in terms of output gap, and thereby whether it will implement an expansionary or contractionary policy in response to cost-push shocks. Second, it stresses that transparency is not just a means to achieve credibility but is essential per se for the optimality of monetary policy of a fully credible central bank

Fate of the Wigner crystal on the square lattice

The phase diagram of a system of electrons hopping on a square lattice and interacting through long-range Coulomb forces is studied as a function of density and interaction strength. The presence of a lattice strongly enhances the stability of the Wigner crystal phase as compared to the case of the two-dimensional electron gas.Comment: ECRYS-2005 proceeding

The signaling role of policy action.

This paper analyzes the conduct of the optimal monetary policy with imperfect information on the shocks hitting the economy where firms’ prices are strategic complements. Monetary policy entails a dual stabilizing role, as a policy response that influences directly the economy and as a vehicle for information that shapes firms’ beliefs. In the case where more information is welfare detrimental, the central bank faces a dilemma, for its monetary instrument aimed at stabilizing the economy may harmfully shape firms’ beliefs. Recognizing the signaling role of its instrument, the central bank finds it optimal to distort its policy response in order to mitigate the detrimental information that it may convey.differential information, monetary policy, transparency.

Superconductivity in the two-dimensional Hubbard model?

A refined variational wave function for the two-dimensional repulsive Hubbard model is studied numerically, with the aim of approaching the difficult crossover regime of intermediate values of U. The issue of a superconducting ground state with d-wave symmetry is investigated for an average electron density n=0.8125 and for U=8t. Due to finite-size effects a clear-cut answer to this fundamental question has not yet been reached.Comment: 5 pages, 1 figure, Proc. 30th Int. Conf. of Theoretical Physics, Ustron, Poland, 2006, to be published in phys. stat. so

Personality effects on cardiovascular reactivity: need for closure moderates the impact of task difficulty on engagement-related myocardial beta-adrenergic activity

An experiment assessed the joint effect of dispositional need for closure (NFC) and task difficulty on engagement-related myocardial beta-adrenergic activity. Participants who scored either low or high on the NFC scale performed an ambiguous categorization task with either low or high difficulty. Confirming the theory-derived predictions, task difficulty effects on pre-ejection period (PEP) reactivity were moderated by NFC. If difficulty was low, PEP reactivity was low and independent of the participants NFC level. If difficulty was high, participants with high NFC showed increased PEP reactivity compared to participants with low NFC. These results extend previous research on Wright's model of engagement-related cardiovascular reactivity and suggest that the model may provide a useful framework for assessing the impact of personality on cardiovascular response

Can Opacity of a Credible Central Bank Explain Excessive Inflation?

Excessive inflation is usually attributed to the lack of central bank’s credibility. In this context, most of the literature considers transparency a means to establish central bank’s credibility. The contribution of this paper is twofold. First, it shows that, even in the absence of inflationary bias, a credible central bank may find it optimal to implement an accommodating monetary policy in response to cost-push shocks whenever the uncertainty surrounding its monetary instrument is high. Indeed, the degree of central bank’s transparency influences the effectiveness of its policy to stabilize inflation in terms of output gap, and thereby whether it will implement an expansionary or contractionary policy in response to cost-push shocks. Second, it stresses that transparency is not just a means to achieve credibility but is essential per se for the optimality of monetary policy of a fully credible central bank.monetary policy; differential information; transparency; cost-push shocks