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Register a new userHybridization driven quantum critical behavior in weakly-itinerant ferromagnets
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Marcello Silva Neto Dr.
Publication date
2011
fields
Physics
and research's language is
English
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We investigate the unusual magnetic properties of nearly-critical, weakly-itinerant ferromagnets with general formula UTX, where T=Rh,Co and X=Ge,Si. As a unique feature about these systems, we show that changes in the V_{df} hybridization control their proximity to a ferromagnetic instability, and determine the evolution of: the ground state magnetization, M_0, the Curie Temperature, T_C, the density of states at the Fermi level, N(E_F), the T^2 resistivity coefficient, A, and the specific heat coefficient, gamma. The universal aspect of our findings comes from the dependence on only two parameters: the T_d bandwidth, W_d, and the distance between T_d and U_f band centers, C_{T_d}-C_{U_f}.
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An important problem in contemporary physics concerns quantum-critical fluctuations in metals. A scaling function for the momentum, frequency, temperature and magnetic field dependence of the correlation function near a 2D-ferromagnetic quantum-critical point (QCP) is constructed, and its singularities are determined by comparing to the recent calculations of the correlation functions of the dissipative quantum XY model (DQXY). The calculations are motivated by the measured properties of the metallic compound YFe$_2$Al$_{10}$, which is a realization of the DQXY model in 2D. The frequency, temperature and magnetic field dependence of the scaling function as well as the singularities measured in the experiments are given by the theory without adjustable exponents. The same model is applicable to the superconductor-insulator transitions, classes of metallic AFM-QCPs, and as fluctuations of the loop-current ordered state in hole-doped cuprates. The results presented here lend credence to the solution found for the 2D-DQXY model, and its applications in understanding quantum-critical properties of diverse systems.
A quantum critical point (QCP) of the heavy fermion Ce(Ru_{1-x}Rh_x)_2Si_2 (x = 0, 0.03) has been studied by single-crystalline neutron scattering. By accurately measuring the dynamical susceptibility at the antiferromagnetic wave vector k_3 = 0.35 c^*, we have shown that the energy width Gamma(k_3), i.e., inverse correlation time, depends on temperature as Gamma(k_3) = c_1 + c_2 T^{3/2 +- 0.1}, where c_1 and c_2 are x dependent constants, in a low temperature range. This critical exponent 3/2 +- 0.1 proves that the QCP is controlled by that of the itinerant antiferromagnet.
We re-examine the experimental results for the magnetic response function $chi({bf q}, E, T)$, for ${bf q}$ around the anti-ferromagnetic vectors ${bf Q}$, in the quantum-critical region, obtained by inelastic neutron scattering, on an Fe-based superconductor, and on a heavy Fermion compound. The motivation is to compare the results with a recent theory, which shows that the fluctuations in a generic anti-ferromagnetic model for itinerant fermions map to those in the universality class of the dissipative quantum-XY model. The quantum-critical fluctuations in this model, in a range of parameters, are given by the correlations of spatial and of temporal topological defects. The theory predicts a $chi({bf q}, E, T)$ (i) which is a separable function of $({bf q -Q})$ and of ($E$,$T$), (ii) at crticality, the energy dependent part is $propto tanh (E/2T)$ below a cut-off energy, (iii) the correlation time departs from its infinite value at criticality on the disordered side by an essential singularity, and (iv) the correlation length depends logarithmically on the correlation time, so that the dynamical critical exponent $z$ is $infty$ . The limited existing experimental results are found to be consistent with the first two unusual predictions from which the linear dependence of the resistivity on T and the $T ln T$ dependence of the entropy also follow. More experiments are suggested, especially to test the theory of variations on the correlation time and length on the departure from criticality.
We measure the band structure of nickel along various high-symmetry lines of the bulk Brillouin zone with angle-resolved photoelectron spectroscopy. The Gutzwiller theory for a nine-band Hubbard model whose tight-binding parameters are obtained from non-magnetic density-functional theory resolves most of the long-standing discrepancies between experiment and theory on nickel. Thereby we support the view of itinerant ferromagnetism as induced by atomic correlations.
Metallic quantum criticality is among the central theme in the understanding of correlated electronic systems, and converging results between analytical and numerical approaches are still under calling. In this work, we develop state-of-art large scale quantum Monte Carlo simulation technique and systematically investigate the itinerant quantum critical point on a 2D square lattice with antiferromagnetic spin fluctuations at wavevector $mathbf{Q}=(pi,pi)$ -- a problem that resembles the Fermi surface setup and low-energy antiferromagnetic fluctuations in high-Tc cuprates and other critical metals, which might be relevant to their non-Fermi-liquid behaviors. System sizes of $60times 60 times 320$ ($L times L times L_tau$) are comfortably accessed, and the quantum critical scaling behaviors are revealed with unprecedingly high precision. We found that the antiferromagnetic spin fluctuations introduce effective interactions among fermions and the fermions in return render the bare bosonic critical point into a new universality, different from both the bare Ising universality class and the Hertz-Mills-Moriya RPA prediction. At the quantum critical point, a finite anomalous dimension $etasim 0.125$ is observed in the bosonic propagator, and fermions at hot spots evolve into a non-Fermi-liquid. In the antiferromagnetically ordered metallic phase, fermion pockets are observed as energy gap opens up at the hot spots. These results bridge the recent theoretical and numerical developments in metallic quantum criticality and can be served as the stepping stone towards final understanding of the 2D correlated fermions interacting with gapless critical excitations.
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