Strong-coupling electron-phonon superconductivity in noncentrosymmetric quasi-one-dimensional K2_2Cr3_3As3_3

I study the lattice dynamics and electron-phonon coupling in non-centrosymmetric quasi-one-dimensional K$_2$Cr$_3$As$_3$ using density functional theory based first principles calculations. The phonon dispersions show stable phonons without any soft-mode behavior. They also exhibit features that point to a strong interaction of K atoms with the lattice. I find that the calculated Eliashberg spectral function shows a large enhancement around 50 cm$^{-1}$. The phonon modes that show large coupling involve in-plane motions of all three species of atoms. The $\mathbf{q}$ dependent electron-phonon coupling decreases strongly away from the $q_z = 0$ plane. The total electron-phonon coupling is large with a value of $\lambda_{\textrm{ep}} = 3.0$, which readily explains the experimentally observed large mass enhancement

Proposal for ultrafast switching of ferroelectrics using mid-infrared pulses

I propose a method for ultrafast switching of ferroelectric polarization using mid-infrared pulses. This involves selectively exciting the highest frequency $A_1$ phonon mode of a ferroelectric material with an intense mid-infrared pulse. Large amplitude oscillations of this mode provides a unidirectional force to the lattice such that it displaces along the lowest frequency $A_1$ phonon mode coordinate because of a nonlinear coupling of the type $g Q_{\textrm{P}} Q_{\textrm{IR}}^2$ between the two modes. First principles calculations show that this coupling is large in transition-metal oxide ferroelectrics, and the sign of the coupling is such that the lattice displaces in the switching direction. Furthermore, I find that the lowest frequency $A_1$ mode has a large $Q_{\textrm{P}}^3$ order anharmonicity, which causes a discontinuous switch of electric polarization as the pump amplitude is continuously increased

Orthorhombic-to-monoclinic transition in Ta2_2NiSe5_5 due to a zone-center optical phonon instability

I study dynamical instabilities in Ta$_2$NiSe$_5$ using density functional theory based calculations. The calculated phonon dispersions show two unstable optical branches. All the acoustic branches are stable, which shows that an elastic instability is not the primary cause of the experimentally observed orthorhombic-to-monoclinic structural transition in this material. The largest instability of the optical branches occurs at the zone center, consistent with the experimental observation that the size of the unit cell does not multiply across the phase transition. The unstable modes have the irreps $B_{1g}$ and $B_{2g}$. Full structural relaxations minimizing both the forces and stresses find that the monoclinic $C2/c$ structure corresponding to the $B_{2g}$ instability has the lowest energy. Electronic structure calculations show that this low-symmetry structure has a sizable band gap. This suggest that a $B_{2g}$ zone-center optical phonon instability is the primary cause of the phase transition. An observation of a softening of a $B_{2g}$ zone-center phonon mode as the transition is approached from above would confirm the mechanism proposed here. If none of the $B_{2g}$ modes present in the material soften, this would imply that the transition is caused by electronic or elastic instability

Interplay between structure and chemistry of materials and their physical properties

First principles calculations provide a powerful tool for sorting out the interplay of chemical composition and structure with the physical properties of materials. In this dissertation, I discuss the physical properties and their microscopic basis within this framework for following illustrative examples. (i) The Zintl phase hydrides, where I find H is anionic and the formation of covalent sp2 bonds in the Al/Ga/Al-Si planes, which is a highly unusual bonding configuration for these elements. (ii) PbTe, which shows strong coupling between the longitudinal acoustic and transverse optic modes that may explain its low thermal conductivity. (iii) The double perovskites BiPbZnNbO6 and BiSrZnNbO6, where introducing size disorder at A-site prevents the BO6 octahedra from tiling and enhances the polar behavior. (iv) FeSe, which shares the salient electronic and magnetic features of other Fe superconductors and cannot be described as a conventional electron phonon superconductor. (v) NbFe2, which is near a magnetic quantum critical point and shows strong competition between various magnetic orderings that may explain its unusual non-Fermi liquid behavior at very low temperatures. (vi) The nickel analogues of Fe superconductors LaNiPO and BaNi2As2, where I show that superconductivity is of conventional electron-phonon type in contrast to the Fe-based superconductors. (vii) Noncentrosymmetric LaNiC2, which I find is a conventional electron-phonon superconductor with intermediate coupling

Electron-phonon superconductivity in AAPt3_3P compounds: from weak to strong coupling

We study the newly discovered Pt phosphides $A$Pt$_3$P ($A$=Sr, Ca, La) [ T. Takayama et al. Phys. Rev. Lett. 108, 237001 (2012)] using first-principles calculations and Migdal-Eliashberg theory. Given the remarkable agreement with the experiment, we exclude the charge-density wave scenario proposed by previous first-principles calculations, and give conclusive answers concerning the superconducting state in these materials. The pairing increases from La to Ca and Sr due to changes in the electron-phonon matrix elements and low-frequency phonons. Although we find that all three compounds are well described by conventional s-wave superconductivity and spin-orbit coupling of Pt plays a marginal role, we show that it could be possible to tune the structure from centrosymmetric to noncentrosymmetric opening new perspectives towards the understanding of unconventional superconductivity.Comment: updated Journal referenc

Theory of nonlinear phononics for coherent light-control of solids

We present a microscopic theory for ultrafast control of solids with high-intensity terahertz frequency optical pulses. When resonant with selected infrared-active vibrations, these pulses transiently modify the crystal structure and lead to new collective electronic properties. The theory predicts the dynamical path taken by the crystal lattice using first-principles calculations of the energy surface and classical equations of motion, as well as symmetry considerations. Two classes of dynamics are identified. In the perturbative regime, displacements along the normal mode coordinate of symmetry-preserving Raman active modes can be achieved by cubic anharmonicities. This explains the light-induced insulator-to-metal transition reported experimentally in manganites. We predict a regime in which ultrafast instabilities that break crystal symmetry can be induced. This nonperturbative effect involves a quartic anharmonic coupling and occurs above a critical threshold, below which the nonlinear dynamics of the driven mode displays softening and dynamical stabilization.Comment: updated to reflect the published versio

Low-energy description of the metal-insulator transition in the rare-earth nickelates

We propose a simple theoretical description of the metal-insulator transition of rare-earth nickelates. The theory involves only two orbitals per nickel site, corresponding to the low-energy anti-bonding $e_g$ states. In the monoclinic insulating state, bond-length disproportionation splits the manifold of $e_g$ bands, corresponding to a modulation of the effective on-site energy. We show that, when subject to a local Coulomb repulsion $U$ and Hund's coupling $J$, the resulting bond-disproportionated state is a paramagnetic insulator for a wide range of interaction parameters. Furthermore, we find that when $U-3J$ is small or negative, a spontaneous instability to bond disproportionation takes place for large enough $J$. This minimal theory emphasizes that a small or negative charge-transfer energy, a large Hund's coupling, and a strong coupling to bond-disproportionation are the key factors underlying the transition. Experimental consequences of this theoretical picture are discussed.Comment: 17 pages, 10 figures; published version in the updat

Thermoelectric transport properties of electron doped pyrite FeS2

Pyrite FeS$_2$ has been investigated for a wide range of applications, including thermoelectrics due to previous observation of large thermopower at room-temperature. However, the values of thermopower reported in the literature is extremely sensitive to the nature of sample -- whether they are natural or lab grown, bulk crystals or thin films -- and an ambiguity in the magnitude and sign of thermopower of pure FeS$_2$ exists. Variation in the magnitude of room-temperature thermopower has also been observed in Co-doped samples. Therefore, it is of interest to clarify the intrinsic thermopower of this system that could be measured in more pure samples. In this paper, we investigate the thermoelectric properties of Co-doped FeS$_2$ using first principles calculations. We apply three different doping schemes to understand the effect of electron doping in FeS$_2$, namely explicit Co-substitution, jellium doping and electron addition within rigid band approximation (RBA) picture. The calculated thermopower is less than $-50$ $\mu$V/K for all values of Co doping that we studied, suggesting that this system may not be useful in thermoelectric applications. Interestingly, we find that RBA substantially overestimates the magnitude of calculated thermopower compared to the explicit Co-substitution and jellium doping schemes. The overestimation occurs because the changes in the electronic structure due to doping-induced structural modification and charge screening is not taken into account by the rigid shift of the Fermi level within RBA. RBA is frequently used in first principles investigations of the thermopower of doped semiconductors, and Co-substituted FeS$_2$ illustrates a case where it fails.Comment: 9 pages, 6 figures, 2 table

Minority-spin conducting states in Fe substituted pyrite CoS2_2

There has been a longstanding debate whether the pyrite CoS$_2$ or its alloys with FeS$_2$ are half metallic. We argue using first principles calculations that there is a finite occupation of minority-spin states at the Fermi level throughout the series Co$_{1-x}$Fe$_x$S$_2$. Although the exchange-correlation functional influences the specifics of the electronic structure, we observe a similar trend with increasing Fe concentration in both LDA and GGA calculations. Specifically, even as band filling is decreased through Fe substitution, the lowest-lying conduction band in the minority-spin channel broadens such that these states keep getting lowered relative to the Fermi level, which is contrary to the expectations from a rigid band picture. Furthermore, the exchange splitting decreases as more Co atoms are replaced by Fe, and this again brings the minority-spin states closer to the Fermi level. These two mechanisms, in conjunction with the experimental observation that minority-spin bands cross the Fermi level in stoichiometric CoS$_2$, indicate that minority-spin charge carriers will always be present in Co$_{1-x}$Fe$_x$S$_2$.Comment: 8 pages, 4 figure, 2 table