NMR investigation of the Knight shift anomaly in CeIrIn5 at high magnetic fields

We report nuclear magnetic resonance Knight shift data in the heavy fermion material CeIrIn5 at fields up to 30 T. The Knight shift of the In displays a strong anomaly, and we analyze the results using two different interpretations. We find that the Kondo lattice coherence temperature and the effective mass of the heavy electrons remains largely unaffected by the magnetic field, despite the fact that the Zeeman energy is on the order of the coherence temperature.Comment: 5 pages, 5 figures; to appear in Phys. Rev.

A predictive standard model for heavy electron systems

We propose a predictive standard model for heavy electron systems based on a detailed phenomenological two-fluid description of existing experimental data. It leads to a new phase diagram that replaces the Doniach picture, describes the emergent anomalous scaling behavior of the heavy electron (Kondo) liquid measured below the lattice coherence temperature, T*, seen by many different experimental probes, that marks the onset of collective hybridization, and enables one to obtain important information on quantum criticality and the superconducting/antiferromagnetic states at low temperatures. Because T* is ~J^2\rho/2, the nearest neighbor RKKY interaction, a knowledge of the single-ion Kondo coupling, J, to the background conduction electron density of states, \rho, makes it possible to predict Kondo liquid behavior, and to estimate its maximum superconducting transition temperature in both existing and newly discovered heavy electron families.Comment: 4 pages, 2 figures, submitted to J. Phys.: Conf. Ser. for SCES 201

Local Magnetic Inhomogeneities in Lightly Doped BaFe2_2As2_2

We report $^{75}$As NMR measurements in BaFe$_2$As$_2$ doped with Ni. Like Co, Ni doping suppresses the antiferromagnetic and structural phase transitions and gives rise to superconductivity for sufficiently large Ni doping. The spin lattice relaxation rate diverges at $T_N$, with a critical exponent consistent with 3D ordering of local moments. In the ordered state the spectra quickly broaden inhomogeneously with doping. We extract the average size of the ordered moment as a function of doping, and show that a model in which the order remains commensurate but with local amplitude variations in the vicinity of the dopant fully explains our observations.Comment: 4 pages, 4 figure

Nuclear magnetic resonance studies of pseudospin fluctuations in URu2_2Si2_2

We report $^{29}$Si NMR measurements in single crystals and aligned powders of URu$_2$Si$_2$ in the hidden order and paramagnetic phases. The spin-lattice-relaxation data reveal evidence of pseudospin fluctuations of U moments in the paramagnetic phase. We find evidence for partial suppression of the density of states below 30 K, and analyze the data in terms of a two component spin-fermion model. We propose that this behavior is a realization of a pseudogap between the hidden order transition $T_{HO}$ and 30 K. This behavior is then compared to other materials that demonstrate precursor fluctuations in a pseudogap regime above a ground state with long-range order.Comment: 5 pages, 3 figure

Lending Standards, Bank Risk-Taking, and Monetary Policy

This paper contributes to the burgeoning literature on the so-called “risk-taking channel” of monetary policy by constructing a partial equilibrium model of the commercial loan market, where conventional monetary policy influences bank lending standards and, by extension, the risk of bank default. In the model, borrower heterogeneity in terms of entrepreneurial abilities and project revenue results in optimal commercial bank lending standards. These standards take the form of a minimum ability requirement: only borrowers with abilities greater than or equal to the minimum threshold receive a loan; the rest are completely excluded from the loan market. After loans are made, a negative aggregate shock that lowers all borrowers’ project revenue results in unexpected loan losses for the bank that may push it into default. In the baseline model, lending standards loosen and the probability of bank default rises when the central bank’s policy rate falls, a result that suggests that asset quality and lending standards may play an important role in shaping the risk-taking channel of monetary policy. However, the relationship between bank default and the policy rate is sensitive to the extent of bank capitalization. While default risk and the policy rate are negatively correlated at relatively strongly capitalized banks, the risk of default at more weakly capitalized banks increases with the policy rate. Additionally, default risk at strongly capitalized banks is more responsive to changes in the policy rate than at weakly capitalized banks. This paper contributes to the literature on the risk-taking channel of monetary policy. First, the model enables analysis of the impact of monetary policy on risk-taking in a setting where both bank asset quality and leverage play a role in shaping the risk of bank default. Second, the model’s results on the role of bank capitalization imply that the tradeoff between financial stability and monetary policy depends crucially on bank capital ratios. This suggests that recent changes to bank capital requirements embodied in new post-2008 financial crisis regulations may impact the nature and size of the risk-taking channel

Long range order and two-fluid behavior in heavy electron materials

The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu$_2$Si$_2$, CeIrIn$_5$ and CeRhIn$_5$. In CeRhIn$_5$ we find that the antiferromagnetic order is preceded by a relocalization of the Kondo liquid, providing independent evidence for a local moment origin of antiferromagnetism. In URu$_2$Si$_2$ the hidden order is shown to emerge directly from the Kondo liquid and so is not associated with local moment physics. Our results imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice in agreement with phase diagram proposed by Yang and Pines.Comment: 9 pages, 13 figure