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Register a new userLadder-like optical conductivity in the spin-fermion model
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In the nested limit of the spin-fermion model for the cuprates, one-dimensional physics in the form of half-filled two-leg ladders emerges. We show that the renormalization group flow of the corresponding ladder is towards the d-Mott phase, a gapped spin-liquid with short-ranged d-wave pairing correlations, and reveals an intermediate SO(5)$times$SO(3) symmetry. We use the results of the renormalization group in combination with a memory-function approach to calculate the optical conductivity of the spin-fermion model in the high-frequency regime, where processes within the hot spot region dominate the transport. We argue that umklapp processes play a major role. For finite temperatures, we determine the resistivity in the zero-frequency (dc) limit. Our results show an approximate linear temperature dependence of the resistivity and a conductivity that follows a non-universal power law. A comparison to experimental data supports our assumption that the conductivity is dominated by the antinodal contribution above the pseudogap.
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A link is established between the spin-fermion (SF) model of the cuprates and the approach based on the analogy between the physics of doped Mott insulators in two dimensions and the physics of fermionic ladders. This enables one to use nonperturbative results derived for fermionic ladders to move beyond the large-N approximation in the SF model. It is shown that the paramagnon exchange postulated in the SF model has exactly the right form to facilitate the emergence of the fully gapped d-Mott state in the region of the Brillouin zone at the hot spots of the Fermi surface. Hence the SF model provides an adequate description of the pseudogap.
Inelastic-neutron-scattering measurements were performed on a single crystal of the heavy-fermion paramagnet UTe$_2$ above its superconducting temperature. We confirm the presence of antiferromagnetic fluctuations with the incommensurate wavevector $mathbf{k}_1=(0,0.57,0)$. A quasielastic signal is found, whose momentum-transfer dependence is compatible with fluctuations of magnetic moments $muparallelmathbf{a}$, with a sine-wave modulation of wavevector $mathbf{k}_1$ and in-phase moments on the nearest U atoms. Low dimensionality of the magnetic fluctuations, consequence of the ladder structure, is indicated by weak correlations along the direction $mathbf{c}$. These fluctuations saturate below the temperature $T_1^*simeq15$~K, in possible relation with anomalies observed in thermodynamic, electrical-transport and nuclear-magnetic-resonance measurements. The absence or weakness of ferromagnetic fluctuations, in our data collected at temperatures down to 2.1 K and energy transfers from 0.6 to 7.5 meV, is emphasized. These results constitute constraints for models of magnetically-mediated superconductivity in UTe$_2$.
Experiments carried over the last years on the underdoped cuprates have revealed a variety of symmetry-breaking phenomena in the pseudogap state. Charge-density waves, breaking of $C_{4}$ rotational symmetry as well as time-reversal symmetry breaking have all been observed in several cuprate families. In this regard, theoretical models where multiple non-superconducting orders emerge are of particular interest. We consider the recently introduced (Phys. Rev. B 93, 085131 (2016)) spin-fermion model with overlapping hot spots on the Fermi surface. Focusing on the particle-hole instabilities we obtain a rich phase diagram with the chemical potential relative to the dispersion at $(0,pi);;(pi,0)$ and the Fermi surface curvature in the antinodal regions being the control parameters. We find evidence for d-wave Pomeranchuk instability, d-form factor charge density waves as well as commensurate and incommensurate staggered bond current phases similar to the d-density wave state. The current orders are found to be promoted by the curvature. Considering the appropriate parameter range for the hole-doped cuprates, we discuss the relation of our results to the pseudogap state and incommensurate magnetic phases of the cuprates.
We have measured the resistivity, optical conductivity, and magnetic susceptibility of LaSb$_2$ to search for clues as to the cause of the extraordinarily large linear magnetoresistance and to explore the properties of the superconducting state. We find no evidence in the optical conductivity for the formation of a charge density wave state above 20 K despite the highly layered crystal structure. In addition, only small changes to the optical reflectivity with magnetic field are observed indicating that the MR is due to scattering rate, not charge density, variations with field. Although a superconducting ground state was previously reported below a critical temperature of 0.4 K, we observe, at ambient pressure, a fragile superconducting transition with an onset at 2.5 K. In crystalline samples, we find a high degree of variability with a minority of samples displaying a full Meissner fraction below 0.2 K and fluctuations apparent up to 2.5 K. The application of pressure stabilizes the superconducting transition and reduces the anisotropy of the superconducting phase.
With angle-resolved photoemission spectroscopy, we studied the electronic structure of TaFe$_{1.23}$Te$_3$, which is a two-leg spin ladder compound with a novel antiferromagnetic ground state. Quasi-two-dimensional Fermi surface is observed, indicating sizable inter-ladder hopping, which would facilitate the in-plane ferromagnetic ordering through double exchange interactions. Moreover, an energy gap is not observed at the Fermi surface in the antiferromagnetic state. Instead, the shifts of various bands have been observed. Combining these observations with density-functional-theory calculations, we propose that the large scale reconstruction of the electronic structure, caused by the interactions between the coexisting itinerant electrons and local moments, is most likely the driving force behind the magnetic transition. TaFe$_{1.23}$Te$_3$ thus provides a simpler system that contains similar ingredients as the parent compounds of iron-based superconductors, which yet could be readily modeled and understood.
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