Session E06: Electronic States in Se Chalcogenides: Including Excitonic Behavior

Response of FeSe to in-plane anisotropic strain

By affixing thin single crystals of FeSe to rigid sample carriers and then applying uniaxial stress to the carrier, we apply in-plane uniaxial strains of up to ~0.7% to FeSe. Above the structural transition temperature T_s, anisotropic strain drives partial polarization of the nematic order, and a corresponding strong resistive anisotropy [1, 2]. However the resistive anisotropy saturates rapidly as strain is applied, and the resistivity then varies nonmonotonically for compressions above ~0.3%. Below T_s, the extrinsic contribution to the resistance from twin boundaries can be identified. The twin boundaries are weakly pinned and can be partially annealed, allowing the intrinsic elastoresistivity below T_s to be resolved.

[1] H.-H. Kuo, J.-H. Chu, J. C. Palmstrom, S. A. Kivelson, and I. R. Fisher, Science 352, 958 (2016).
[2] S. Hosoi et al, Proc. Nat. Acad. Sciences 113, 8139 (2016).   

Nonequilibrium dynamics of local lattice distortions in FeSe

The role of the lattice in unique properties of iron-based superconductors, such as electronic nematicity, is still unclear. This rises an interest in investigating the local structure modifications during the phase transitions in these materials. We report the study of the local crystal structure of FeSe and its connection to electronic nematicity using ultrafast electron diffraction, x-ray diffraction, and transmission electron microscopy. The study reveals the local distortions in form of iron dimers, ordered within nanometrer -sized domains. The observed nonequilibrium lattice dynamics implies sensitivity of the lattice distortions to the nematic order parameter.   

Character of excitonic insulator phase in a transition metal dichalcogenide 1T-TiSe2.

We revisit the question of the transition metal dichalcogenide 1T-TiSe₂ charge density wave (CDW) state being an excitonic insulator. The BCS-like CDW gap equation with statically screened Coulomb interaction was employed to calculate the transition temperature (T_c) as a function of the energy gap. Realistic dispersions for one hole band centered at Γ and three electron bands at M points in two-dimensional periodic hexagonal lattice were considered to model the TiSe₂. The obtained T_c (~240K) is comparable to measured one for monolayer (~232K), and T_c suppression as a function of doping concentration and pressure are reproduced by controlling chemical potential and energy gap. We discuss possibility of the 1T-TiSe₂ CDW state originating from exciton condensation through the calculated T_c, ARPES intensity and density of states.   

Intrinsic Insulating Ground State in Transition Metal Dichalcogenide TiSe2

TiSe₂ has received significant research attention over the past four decades, in large part due to the uniqueness of its charge-ordered state. Different techniques can suppress the charge density wave transition, vary low temperature resistivity by orders of magnitude, and stabilize magnetic or superconducting states. This talk will present the results of a new synthesis method whereby samples were grown in an argon gas environment at elevated pressures up to 180 bar. Above 100 K, properties (including those of the 200 K CDW) are unchanged from prior reports. However, a hysteretic resistance region beginning around 80 K, accompanied by insulating low temperature behavior, is distinct from anything previously observed. This new feature suggests that pressure growth may allow access to a nonmetallic ground state in a material long speculated to be an excitonic insulator.   

Folded superstructure and degeneracy-enhanced band gap in the weak-coupling charge density wave system 2H−TaSe2

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we have mapped out the reconstructed electronic structure in the commensurate charge-density-wave (CDW) state of quasi-two-dimensional transition metal dichalcogenide 2H−TaSe₂. The observation of the fine structure near Brillouin zone (BZ) center supplements the picture of Fermi surface folding in the 3×3 CDW state. In addition to the anisotropic CDW band gaps that energetically stabilize the system at the Fermi level in the first-order lock-in transition, we found band reconstruction at high binding energy, which can be well explained by the hybridization between main bands (MBs) and folded bands (FBs). Furthermore, in contrast to the perfectly nested quasi-one-dimensional system, triple-nesting-vector-induced CDW FBs increase the degeneracy of the band crossing and thus further enlarge the magnitude of band gap at certain momentum-energy positions. The visualization and modeling of CDW gaps in momentum-energy space reconcile the long-lasting controversy on the gap magnitude and suggests a weak-coupling Peierls physics in this system.   

Observation of anomalous electron relaxation in optically excited 1T-TaSe2

Ultrafast light pulses can drive materials far from their equilibrium states and is a powerful approach for manipulating their states. The resultant multi-step energy and momentum relaxation processes can give us insight into the dominant interactions in these materials. In this work, we present a series of anomalous behaviors observed in the electron relaxation in the charge density wave (CDW) material 1T-TaSe₂. After exciting the material with a femtosecond laser pulse, we measure the temporal evolution of the band structure and electron temperature. We observe a band oscillation that is coherently coupled to the CDW amplitude mode. Meanwhile, the hot electron temperature relaxes anomalously fast, and very different from that predicted by the widely-used N-temperature model. Moreover, this anomaly shows a critical change at the laser fluence corresponding to the ultrafast phase transformation to a new long-lived metastable state. These results offer a rare opportunity for better understanding of the coherent electron-phonon coupling.   

Spinon Fermi Surface in a Cluster Mott Insulator Model on a Triangular Lattice and Possible Application to 1T-TaS2

1T-TaS₂ is a cluster Mott insulator on the triangular lattice with 13 Ta atoms forming a star of David cluster as the unit cell. We derive a two-dimensional XXZ spin-1=2 model with a four-spin ring exchange term to describe the effective low energy physics of a monolayer 1T-TaS₂, where the effective spin-1/2 degrees of freedom arises from the Kramers degenerate spin-orbital states on each star of David. A large scale density matrix renormalization group simulation is further performed on this effective model and we find a gapless spin liquid phase with a spinon Fermi surface at a moderate to large strength region of the four-spin ring exchange term. All peaks in the static spin structure factor are found to be located on the“2kF” surface of a half-filled spinon on the triangular lattice. Experiments to detect the spinon Fermi surface phase in 1T-TaS2 are discussed.   

Ultrafast electron calorimetry to uncover new transient states in charge density wave and magnetic materials

Understanding and harnessing phase transitions in a wide range of quantum materials remains a great challenge. The strongly coupled interactions between the charges, spins and lattice often make it challenging to unambiguously uncover the underlying mechanisms. Here we present a new ultrafast electron calorimetry technique to map the laser-driven phase space of materials. It relies on the small mass and heat capacity of the electrons, which means that they can react very quickly to any phase changes in a material. Specifically, we use time- and angle-resolved photoemission spectroscopy to systematically measure the band structure, electron temperature and heat capacity as a function of both time delay and laser fluence. First, we find a new metastable state in the charge density wave material 1T-TaSe₂, which is characterized by a significantly reduced effective heat capacity. Our results also reveal new manifestations of the electron-phonon coupling in the dynamics of both the electron and phonon bathes. Second, we uncover a highly-excited spin state that launches the ultrafast magnetic phase transition in Ni. Finally, we note that our approach is general, and can be used to uncover the presence of hidden phases in other materials.   

Terahertz spectroscopy of excitonic insulator candidate Ta2NiSe5

An excitonic insulator (EI) is a correlated-electron state in which excitons condense into an insulating ground state in analogy with the condensation of Cooper pairs in a superconductor. A handful of EI candidates have been proposed, but to date experimental verification of such a state has not been conclusively found. Recent transport and specific heat measurements have placed Ta₂NiSe₅ as the leading EI candidate with T_c = 326 K. Here we present measurements of the terahertz and far-infrared optical conductivity of Ta₂NiSe₅, which show that at temperatures far below T_c a thermally-activated Drude peak gradually develops inside the optical band gap E_g^op = 160 meV. At the same time, the gap gradually fills from the high-frequency side with significant spectral weight from above-gap states. This transfer of spectral weight can be interpreted as predominant incoherent hopping of charge carriers along the Ta-Ni chains in the orthorhombic phase and as evidence for near-zero-gap behavior at high temperature. A phonon centered at 4.71 meV in both the a- and c-axis response suggests that the monoclinic distortion associated with the formation of the EI state allows bidirectional ac-plane activity of the B_1u/B_3u modes.   

Cooperative Exciton-Phonon Bose-Einstein Condensation in an Excitonic Insulator

The excitonic insulator is an exotic phase of matter in which excitons spontaneously form and collectively undergo Bose-Einstein condensation. Recently, increasing evidence has shown that this ground state is stabilized in the layered transition metal chalcogenide Ta₂NiSe₅ (TNS). Distinctive signature of exciton condensation is the pronounced flattening of the valence band top with decreasing temperature, signaling the opening of an additional many-body gap, as well as a coherent amplitude-like response observed in optical pump-probe data. Due to its direct bandgap, TNS is believed to realize the pure excitonic insulator state, free from the complications of coexisting density-wave orders or strong coupling to other degrees of freedom. Here, we reveal that a cooperative exciton-phonon mechanism lies instead at the origin of the condensate in TNS. Specifically, we use time- and angle-resolved photoemission spectroscopy to show that the vibrational degrees of freedom play a crucial role in the photoinduced melting of the exciton Bose-Einstein condensate. Our results open new routes towards the selective manipulation of the excitonic insulating state via specific modes of the crystal lattice.   

Time- and Angle-Resolved Photoemission Study on the Excitonic Insulator Candidate Ta2NiSe5

Ultra-high resolution laser-based time- and angle-resolved photoemission is a unique technique in probing the momentum resolved ultrafast electronic dynamics in quantum solid material. In this talk, we will present the electronic evidence of quenching the excitonic insulating state and a hidden semimetallic state upon excited by a strong infrared laser pulse in the excitionic insulator candidate Ta2NiSe5.   

Exciton Mott transition revisited

The dissociation of excitons into holes and electrons in photoexcited semiconductors, despite being one of the first recognized examples of a Mott transition, still defies a complete understanding, especially regarding the character of the transition, which is first order in some cases and second order in others. We tackle this issue by a recently proposed and very powerful variational technique, which extends the conventional Gutzwiller variational wavefunction and has been named ghost Gutzwiller wavefunction (g-GA). The results that we present [1] are in accordance with experiments and allow identifying the key parameter that controls the nature of the transition: the magnitude of the exciton binding energy.
[1] D. Guerci, M. Capone, M. Fabrizio (2018,arXiv:1810.01843).   

A Description of Phases with Induced Hybridisation at Finite Temperatures

In an extended Falicov-Kimball model, an excitonic insulator phase can be stabilised at zero temperature. With increasing temperature, the excitonic order parameter (interaction-induced hybridisation on-site, characterised by the absolute value and phase) eventually becomes disordered, which involves fluctuations of both its phase and (at higher T) its absolute value. In order to build an adequate mean field description, it is important to clarify the nature of degrees of freedom associated with the phase and absolute value of the induced hybridisation, and the corresponding phase space volume. We show that a possible description (including the phase space integration measure) is provided by the on-site density matrix parametrisation. In principle, this allows to describe both the lower-temperature regime where phase fluctuations destroy the long-range order, and the higher temperature crossover corresponding to a decrease of absolute value of the hybridisation relative to the fluctuations level. This picture is also expected to be relevant in other contexts, including the Kondo lattice model.   

Gutzwiller Molecular Dynamics Simulation using Second-Moment Approximation

Molecular dynamics (MD) simulations are crucial to modern computational physics, chemistry, and material science, especially when combined with potentials derived from density-functional theory. However, even in state of the art MD codes, the on-site Coulomb repulsion is only treated at the self-consistent Hartree-Fock level. This standard approximation may miss important effects due to electron correlations. The recently developed Gutzwiller molecular dynamics (GMD) method provides a feasible approach for realistic correlated materials [1]. The Gutzwiller variational method captures the essential physics of correlated electron systems, and is much faster than, for example, the dynamical-mean field theory approach. In its current implementation, however, the GMD method is limited to small system sizes mainly due to the heavy computational cost of solving the renormalized electron Hamiltonian at every time-step. Here we demonstrate significant improvement of the GMD efficiency using the second-moment approximation. Importantly, we show that this approximation still captures the main features of the correlation-induced metal-insulator transition.

[1] G.-W. Chern, K. Barros, C. D. Batista, J. Kress, G. Kotliar, Phys. Rev. Lett. 118, 226401 (2017).