Unconventional Density Waves Research Articles

Unconventional charge density wave and photoinduced lattice symmetry change in the kagome metal CsV3Sb5 probed by time-resolved spectroscopy

The spin-orbit entangled quantum states in 4d/5d compounds, e.g., the Jeff=1/2 and Jeff=3/2 states, have attracted great interests for their unique physical roles in unconventional superconductivity and topological states. Here, the key role of tetragonal distortion is clarified, which determines the ground states of 4d1/5d1 systems to be the Jeff=3/2 one (e.g. K2NbCl6) or S=1/2 one (e.g. Rb2NbCl6). By tuning the tetragonal distortion via epitaxial strain, the occupation weights of dxy/dyz/dxz orbitals can be subtly modulated, competing with the spin-orbit coupling. Consequently, quantum phase transitions between S=1/2 state and Jeff=3/2 state, as well as between different Jeff=3/2 states, can be achieved, resulting in significant changes of local magnetic moments. Our prediction points out a reliable route to engineer new functionality of Jeff states in these quantum materials.

Unconventional density waves and superconductivities in Fe-based superconductors and other strongly correlated electron systems

To seek high-$T_c$ pairing mechanism, many scientists have focused on the mysterious spontaneous rotational symmetry breaking above Tc, such as nematic order at $q=0$ and smectic order at $q\ne0$. Such exotic correlation-driven symmetry breaking in metals has become a central issue in condensed matter physics. We demonstrate the emergence of the nematic and smectic orders due to orbital polarization ($n_{xz}\ne n_{yz}$) and the symmetry breaking in the correlated intersite hopping (= bond order $\delta t_{i,j}$) in Fe-based and cuprate superconductors. In addition, we discuss exotic spontaneous loop current orders driven by the pure imaginary $\delta t_{i,j}$. These interesting ``unconventional density-waves'' originate from the quantum interference between different spin fluctuations that is described by the vertex correction (VC) in the field theory. In the next stage, we discuss electron-correlation driven superconductivity due to the fluctuations of unconventional density-waves. For this purpose, we suggest the beyond-Migdal-Eliashberg gap equation by including the VCs into the equation. In Fe-based superconductors, high-$T_c$ $s$-wave superconductivity can be mediated by nematic and smectic fluctuations because the pairing interaction is magnified by the VCs. We also discuss the multipolar fluctuation pairing mechanism in heavy fermion systems, owing to the cooperation between the strong spin-orbit interaction and the strong electron correlation. To summarize, we suggest that the quantum interference mechanism described by the VCs is the key ingredients to explain not only various unconventional density-waves, but also exotic superconducting states in many strongly correlated metals. We finally discuss some interesting future issues with respect to the quantum interference mechanism.

Hybridization Dynamics in CeCoIn_{5} Revealed by Ultrafast Optical Spectroscopy.

We investigate the quasiparticle dynamics in the prototypical heavy fermion CeCoIn_{5} using ultrafast optical pump-probe spectroscopy. Our results indicate that this material system undergoes hybridization fluctuations before the establishment of heavy electron coherence, as the temperature decreases from ∼120 K (T^{†}) to ∼55 K (T^{*}). We reveal that the anomalous coherent phonon softening and damping reduction below T^{*} are directly associated with the emergence of collective hybridization. We also discover a distinct collective mode with an energy of ∼8 meV, which may be experimental evidence of the predicted unconventional density wave. Our findings provide important information for understanding the hybridization dynamics in heavy fermion systems.

Dxy-density wave in fermion-fermion cold-atom mixtures

We study density wave instabilities in a doubly-degenerate Fermi-Fermi mixture with $SU(2)\times SU(2)$ symmetry on a square lattice. For sufficiently large on-site inter-species repulsion, when the two species of fermions are both at half-filling, two conventional ($s$-wave) number density waves are formed with a $\pi$-phase difference between them to minimize the inter-species repulsion. Upon moving one species away from half-filling, an unconventional density wave with $d_{xy}$-wave symmetry emerges. When both species are away from the vicinity of half-filling, superconducting instabilities dominate. We present results of a functional renormalization-group calculation that maps out the phase diagram at weak couplings. Also, we provide a simple explanation for the emergence of the $d_{xy}$-density wave phase based on a four-patch model. We find a robust and general mechanism for $d_{xy}$-density-wave formation that is related to the shape and size of the Fermi surfaces. The density imbalance between the two species of fermions in the vicinity of half-filling leads to phase-space discrepancy for different inter-species Umklapp couplings. Using a phase space argument for leading corrections in the one-loop renormalization group approach to fermions, we show that the phase-space discrepancy in our system causes opposite flows for the two leading intra-species Umklapp couplings and that this triggers the $d_{xy}$-density-wave instability.

Theory of Orbital Fluctuation and Electrical Transport in Unconventional Charge Density Wave Phase of Skutterudite PrRu4P12

A theory of charge density wave (CDW) ordering and the metal–insulator transition in skutterudite PrRu 4 P 12 is presented. We introduce a simplified model Hamiltonian which involves an interaction of conduction electrons with localized f electrons occupying singlet–triplet crystal field levels, and calculate the solutions of the coherent potential approximation. It is shown that the fluctuation of the f -electron occupancy plays very important roles in the mechanism of unusual transport properties. On the one hand, the fluctuation leads to a severe suppression of the quasi-particle gap in the CDW phase and resultingly produces a characteristic temperature dependence of the resistivity. On the other hand, the fluctuation induces breaking of the particle–hole symmetry, giving a drastic enhancement of the Hall conductivity in the CDW phase. The mechanism of sign reversal of the Hall conductivity is discussed in connection with the properties of the fluctuation. Comparisons of the theoretical results with th...

Enhanced diamagnetism and Nernst signal from a chiral d-density wave state in the pseudogap regime of the cuprates

In this paper we propose an alternative explanation for the pseudogap regime of the cuprate superconductors. Specifically, a solution to this puzzle can be provided by associating at least a part of the pseudogap regime with a chiral dxy +idx2-y2 density wave. As we demonstrate for the first time, violation of parity and time reversal from this unconventional density wave gives rise to the Topological Meissner effect and the concomitant Anomalous Nernst effect. As a matter of fact, the enhanced diamagnetism and Nerst signal, recently observed in the cuprates, could be simultaneously explained in terms of a chiral d-density wave.

Unconventional Charge Density Wave Arising from Electron–Phonon Interaction

We report of a theoretical study on quasi-one dimensional unconventional charge density wave (UCDW) driven by electron–phonon interaction. Within mean field theory, we find that the wavevector dependence of the coupling leads to a momentum dependent single particle gap on the Fermi surface. The presence of small energy single particle excitations around the gap nodes significantly changes the optical conductivity compared to the conventional CDW result. In addition to that, the collective phase excitation arising from fluctuation of the order parameter leads to further qualitative changes of the conductivity and results in an effective mass that is nonmonotonic in temperature.

Impurity scattering in unconventional density waves: non-crossing approximation for arbitrary scattering rate

We present a detailed theoretical study on the thermodynamic properties of impure quasi-one-dimensional unconventional charge and spin density waves in the framework of mean-field theory. The impurities are of the ordinary non-magnetic type. Making use of the full self-energy that takes into account all ladder- and rainbow-type diagrams, we are able to calculate the relevant low temperature quantities for arbitrary scattering rates. These are the density of states, specific heat and the shift in the chemical potential. Our results therefore cover the whole parameter space: they include both the self-consistent Born and the resonant unitary limits, and most importantly give exact results in between.

Local density of states and Friedel oscillations around a nonmagnetic impurity in unconventional density waves

We present a mean-field theoretical study on the effect of a single nonmagnetic impurity in quasi-one-dimensional unconventional density wave. The scattering potential is treated within the self-consistent $T$-matrix approximation. The local density of states around the impurity exhibits resonant states in the vicinity of the Fermi level, much the same way as in $d$ density waves or unconventional superconductors. The assumption for different forward and backward scattering, characteristic to quasi-one-dimensional systems in general, leads to a resonance state that is double peaked in the pseudogap. The Friedel oscillations around the impurity are also explored, both within and beyond the density wave coherence length ${\ensuremath{\xi}}_{0}$. Beyond ${\ensuremath{\xi}}_{0}$, we find power-law behavior as opposed to the exponential decay of conventional density wave.

D-Wave density waves in high Tc cuprates and CeCoIn5

Unconventional density waves (UDW) have a long history starting with the speculation of Halperin and Rice in 1968. However, a more realistic approach started around 1999 in order to clarify the nature of the pseudogap in the underdoped region of hole-doped high Tc cuprates. Also d-wave density waves (dDW) evolved from early unrealistic 2D model with Z2 symmetry to more realistic 3D mean-field condensate with U(1) gauge symmetry. More recently, the giant Nernst effect and the angle dependent magnetoresistance in LSCO, YBCO, Bi2212 and CeCoIn5 are successfully described in terms of dDW, where the Landau quantization of the quasiparticle spectrum in dDW in a magnetic field (the Nersesyan effect) plays the crucial role.

Resonant Nernst Effect in the Metallic and Field-Induced Spin Density Wave States of(TMTSF)2ClO4

We examine an unusual phenomenon where, in tilted magnetic fields near magic angles parallel to crystallographic planes, a "giant" resonant Nernst signal has been observed by Wu et al. [Phys. Rev. Lett. 91, 056601 (2003)] in the metallic state of an organic conducting Bechgaard salt. We show that this effect appears to be a general feature of these materials and is also present in the field-induced spin density wave phase with even larger amplitude. Our results place conditions on any model that treats the metallic state as a state with finite Cooper pairing.

Electronic Raman scattering in unconventional density waves

We investigate the electronic Raman scattering in pure, quasi-one-dimensional conductors with a density wave ground state. In particular, we develop the theory of light scattering on spin and charge density waves, both conventional and unconventional. We calculate the electronic Raman response of the interacting-electron system with a single, highly anisotropic conduction band. The calculation is carried out in the mean-field approximation. In addition to the quasiparticle contribution, the electron-electron interaction is also included on the level of the random phase approximation. The contribution of collective modes and the effect of Coulomb screening are investigated. In analogy with unconventional superconductivity, the obtained Raman spectra---which are finite in the low-temperature phase possessing a gap, and vanish identically in the normal state---show unique and strong dependence on the polarization of the incoming and scattered light. We have found distinct, characteristic line shapes, especially in the unconventional situation, depending on the various scattering geometries and the particular momentum dependence of the density wave order parameter.

Gapped optical excitations from gapless phases: Imperfect nesting in unconventional density waves

We consider the effect of imperfect nesting in quasi-one-dimensional unconventional density waves in the case, when the imperfect nesting and the gap depends on the same wavevector component. The phase diagram is very similar to that in a conventional density wave. The density of states is highly asymmetric with respect to the Fermi energy. The optical conductivity at T=0 remains unchanged for small deviations from perfect nesting. For higher imperfect nesting parameter, an optical gap opens, and considerable amount of spectral weight is transferred to higher frequencies. This makes the optical response of our system very similar to that of a conventional density wave. Qualitatively similar results are expected in d-density waves.

Unconventional density wave inCeCoIn5

Very recently large Nernst effects and Seebeck effects were observed [R. Bel et al., Phys. Rev. Lett. 92, 217002 (2004)] above the superconducting transition temperature 2.3 K in a heavy fermion superconductor ${\mathrm{CeCoIn}}_{5}$. We shall interpret this large Nernst effect in terms of an unconventional density wave (UDW), which appears around $T=18\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Also the temperature dependence of the Seebeck coefficient below $T=18\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is described in terms of an UDW.

Large-Nlimit of a magnetic impurity in unconventional density waves

We investigate the effect of unconventional density wave (UDW) condensate on an Anderson impurity using large-$N$ technique at $T=0$. In accordance with previous treatments of a Kondo impurity in pseudogap phases, we find that Kondo effect occurs only in a certain range of parameters. The $f$-electron density of states reflects the influence of UDW at low energies and around the maximum of the density wave gap. The static spin susceptibility diverges at the critical coupling, indicating the transition from strong to weak coupling. In the dynamic spin susceptibility an additional peak appears showing the presence of the of UDW gap. Predictions concerning nonlinear density of states are made. Our results apply to other unconventional condensates such as $d$-wave superconductors and $d$-density waves as well.

Unconventional density wave as possible explanation of the Nernst signal in CeCoIn5

AbstractRecently large Nernst effect and Seebeck effect were observed above the superconducting transition temperature 2.3 K in a heavy fermion superconductor CeCoIn5. We interpret this large Nernst effect in terms of unconventional density wave (UDW), which appears around T = 18 K. In the presence of magnetic field, Landau levels are formed in UDW, and the transport is dominated by thermally excited quasiparticles across the levels. Our theory describes successfully the available experimental data, and the deduced fitting parameters are in accordance with what is expected in the vicinity of a quantum critical point. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Giant Nernst effect in the pseudogap phase of high Tc superconductors

Abstract We have investigated theoretically the Nernst effect in unconventional (d-wave) charge and spin density waves (UDW). In the presence of magnetic field, Landau levels are formed, and the gapless behaviour of the low energy excitations change into gapped behaviour. When additional electric field is applied, the quasiparticles drift with a velocity of E × B/B2, and carry entropy. From this, the Nernst coefficient can be calculated using the Kelvin relation. The present results account very nicely for the measured Nernst signal in the pseudogap phase of high Tc superconductor La2−xSrxCuO4 and Bi2Sr2−yLayCuO6. This indicates that the large Nernst effect is a clear signiture of UDW.

Non-Linear Conduction in the Density Wave State of Quasi-Two Dimensional Organic Conductor α-(BEDT-TTF)2KHg(SCN)4

Current–voltage characteristics are measured in the quasi-two dimensional organic conductor α-(BEDT-TTF) 2 KHg(SCN) 4 at temperatures down to 0.5 K and in the magnetic field up to 25 T. The non-linear conduction with a threshold electric field is found in the density wave state. The features of threshold electric field obtained in the low magnetic field region are explained by the unconventional charge density wave model. In the high magnetic field region, where the Shubnikov–de Haas oscillations appear, the current–voltage characteristics reveal that the density wave state synchronizes with the filling of the electron on the Landau level and continues even above a kink field 23 T.

RECENT ADVANCES IN UNCONVENTIONAL DENSITY WAVES

The unconventional density wave (UDW) was speculated on as a possible electronic ground state in the excitonic insulator in 1968. The recent surge of interest in UDW's is partly due to the proposal that the pseudogap phase in high Tc cuprate superconductors is a d-wave density wave (d-DW). Here we review our recent works on UDW's within the framework of mean field theory. In particular, we have shown that many properties of the low temperature phase (LTP) in α-( BEDT-TTF )2 MHg ( SCN )4, with M = K , Rb and Tl , are well characterized in terms of the unconventional charge density wave (UCDW). In this identification the Landau quantization of the quasiparticle motion in a magnetic field (the Nersesyan effect) plays the crucial role. Indeed, the angle-dependent magnetoresistance and the negative giant Nernst effect are two hallmarks of UDW's.

Magnetothermopower in unconventional charge density wave ground state of α-(BEDT-TTF)2KHg(SCN)4

We have investigated theoretically the thermoelectric power and the Nernst effect in unconventional density waves (UDW). Due to the presence of magnetic field, Landau levels are formed, and the low energy excitations change from gapless to gapped. The present results account consistently for the recent data of magnetothermopower in α-(BEDT-TTF) 2 KHg(SCN) 4 obtained by Choi et al. (Phys. Rev. B, 65, 205119 (2002)). This confirms further our identification of low temperature phase (LTP) in this salt as UCDW.