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Excitons, electron-hole pairs bound by the Coulomb potential, are fundamental quasiparticles of coherent light-matter interaction energizing processes from photosynthesis to optoelectronics 1−5 . Excitons are observed in semiconductors, and their existence is implicit in the quantum theory of metals, yet their appearance is tenuous due to the screening of the Coulomb interaction on few femtosecond timescale 6−8 .Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full screening of the Coulomb interaction, in the course of a three-photon photoemission process with <15 femtosecond laser pulses. Electron-hole pair interaction through the excitonic response introduces coherent quasiparticle correlations beyond the single-particle description of the optics of metals, which dominate the multi-photon photoemission process.Reflection of light has made metal mirrors valued optical instruments since the bronze age 9 . At the macroscopic level the coherent optical response of a metallic surface is well described by the classical Maxwell's equations. At the quantum level, a photon interacting with a metal surface polarizes an electron-hole (e-h) pair to create an exciton-polariton, the arXiv:1405.0977v2 [cond-mat.mes-hall]

The Mott insulator κ-(BEDT-TTF)2Ag2(CN)3 forms a highly-frustrated triangular lattice of S = 1/2 dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasi-degenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound κ-(BEDT-TTF)2Cu2(CN)3.PACS numbers: 75.10. Kt, 77.22.Gm, Electronic ferroelectricity and multiferroicity attracts great attention of condensed matter physicists due to their fundamental and technological importance. 1-3 They are identified in systems with strong electronic correlations such as transition-metal oxides and low-dimensional charge-transfer molecular solids. In the latter category, electric polarization arises from valence instability and charge ordering. In both cases, breaking the inversionsymmetry results in the concurrence of non-equivalent charge-sites and bonds. 4 There is no doubt that electron correlations are fundamental for stabilizing the ferroelectric ground state, nevertheless, experimental evidence indicates that the delicate interplay of Coulomb forces and structural changes within the coupled molecular-anion system have to be taken into account. Along these lines a solid understanding of electronic ferroelectricity was achieved for the families of quasi-one-dimensional organic charge-transfer salts: (TMTTF) 2 X and TTF-X, but also some layered (BEDT-TTF) 2 X systems. [5][6][7] However, no consensus has been reached yet on the origin of the ferroelectric signatures detected in the strongly dimerized κ-(BEDT-TTF) 2 X salts. [8][9][10][11][12] In these compounds, the BEDT-TTF dimers are arranged in a triangular lattice with a relatively high geometrical frustration. In some of them, indications of charge-ordering phenomena have been reported, but in-depth studies are missing 13,14 . On the other hand, the Mott dimer insulators κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl and κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , called κ-CuCN, have been thoroughly studied because they are discussed as prototypes of a molecular multiferroic and quantum spin liquid (QSL) systems. 9,15 It turns out to be extremely challenging to reconcile the idea of quantum electric dipoles on molecular dimers interacting via dipolar-spin coupling [16][17][18][19] with the experimentally evidenced absence of any considerable charge imbalance. So far no global structural changes and no charge disproportionation between molecular dimer sites larger than 2δ ρ ≈ ±0.01e that could break the symmetry have been found. 20,21 In the case of the QSL κ-...

Using inelastic scattering of thermal energy He atoms we have determined the phonon dispersion curves in commensurate ͑ p 3 3p 3 ͒R30 ± Xe monolayers adsorbed on Cu(111). In addition to the well known transverse acoustic mode the dispersion curve of the longitudinally polarized phonon branch could be measured. Since this is the first case where a finite zone-center phonon gap (here 0.4 6 0.15 meV) has been seen for a Q-resolved phonon branch, a normal mode analysis and theoretical calculations on the phonon excitation probabilities have been carried out to confirm the assignment. From the gap magnitude important information on the corrugation of the Xe-atom-substrate potential is derived. [S0031-9007(97)04932-6] PACS numbers: 68.35.Ja, 63.22. + mRecently the properties of monolayers of noble gas atoms, in particular Xe, adsorbed on solid substrates have attracted interest in connection with fundamental experiments on the microscopic origin of friction carried out by Krim and co-workers [1]. Up to now, however, no consensus has been achieved concerning the proper theoretical modeling of these layers. Two recent attempts to theoretically analyze the results of the friction experiments [1] using molecular dynamics have obtained rather different results [2,3]. These discrepancies may in part be due to uncertainties concerning the proper model potentials describing the corrugation of the metal substrate as seen by the Xe atoms. These uncertainties result from the lack of direct experimental information on the potential energy surface governing the lateral motion of Xe atoms on the substrate. So far it has only been possible to extract indirect information from, e.g., detailed analyses of thermodynamical properties and thermal desorption spectroscopy [4].In the present Letter we will describe the outcome of experiments where high resolution scattering of thermal energy He atoms (HAS) has been successfully used to determine the dispersion of the longitudinal phonon in the commensurate ͑ p 3 3p 3 ͒R30 ± Xe overlayer on Cu(111). A detailed analysis of this mode provides two important pieces of information: (1) At the zone center of the surface Brillouin zone [cf. Fig. 1(a)] this mode corresponds to a lateral displacement of the whole Xe layer, and the corresponding frequency thus directly measures the curvature of the potential energy surface describing the interaction of the individual Xe atoms with the substrate.(2) From the slope of the longitudinal phonon dispersion curve (or the velocity of sound) the force-constant coupling between adjacent Xe atoms can be obtained. As to (1) a Q-resolved phonon branch with a finite zone-center phonon gap has not yet been seen experimentally, although in previous neutron scattering studies this quantity has been extracted from the total density of states [5]. With regard to (2) one may expect the force-constant coupling of the adsorbed Xe atoms to be very similar to that predicted from accurate gas-phase potentials [6] or from the analysis of bulk phonon data [7], but recent work ha...

The dimer Mott insulator κ-(BEDT-TTF)2Cu2(CN)3 exhibits unusual electrodynamic properties. Numerical investigations of the electronic ground state and the molecular and lattice vibrations reveal the importance of the Cu2(CN) − 3 anion network coupled to the BEDT-TTF molecules: The threefold cyanide coordination of copper and linkage isomerism in the anion structure cause a loss of symmetry, frustration, disorder, and domain formation. Our findings consistently explain the temperature and polarization-dependent THz and infrared measurements, reinforce the understanding of dielectric properties and have important implications for the quantum spin-liquid state, which should be treated beyond two-dimensional, purely electronic models.In the field of quasi-two-dimensional strongly correlated electron systems, the κ-salts of the BEDT-TTF family have attracted considerable interest [1] because their triangular arrangement of the dimers is close to geometrical frustration leading to the first realization of spinliquid behavior in κ-(BEDT-TTF) 2 Cu 2 (CN) 3 [2]. The antiferromagnetic exchange coupling among the dimers is rather strong (J ≈ 250 K), however, neither the magnetic order nor structural distortions have been detected down to lowest temperatures. Despite enormous theoretical and experimental efforts, the nature of this phase is still inconclusive and the question of a possible gap in the spin excitations remains open [3][4][5][6]. From the residual spin susceptibility and the power-law temperature dependence of the NMR spin-lattice relaxation rate, low-lying spin excitations are concluded, while their contribution to the optical conductivity is still under discussion [7][8][9].Interacting spins on a triangular lattice cannot order in a simple antiferromagnetic ground state, quantum fluctuations prevent a stable spin-liquid phase in the presence of finite hopping t and electronic repulsion U . More advanced microscopic models have been put forward to explain the properties of κ-BEDT-TTF salts [10-13] based on the presence of charge dipoles on the dimers which couple to spin degrees of freedom and thereby prevent ordering. While the electronic order has been proven in related compounds [14] in which the intra-dimer Coulomb repulsion U is reduced with respect to inter-site interaction V , the ratio V /t is of minor relevance in the case of κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , called κ-CN (Fig. 1). The anomalous dielectric response observed in the kHz and MHz range below 60 K [20, 21] evidences some unexpected charge excitations in the spin-liquid state which must be explained satisfactorily. Recently, time-domain optical measurements rendered a rather broad absorption band around 1 THz that develops below T ≈ 80 K. Itoh et al. [22] attributed it to collective excitation of intra-dimer electric dipoles. However, NMR, Raman and infrared spectroscopies rule out any sizable charge imbalance all the way down to low temperatures [24][25][26].In order to shed light on the excitation properties in the quantum spin-liquid state in...

Photons can excite the collective and single-particle excitations in metals; the collective plasmonic excitations are of keen interest in physics, chemistry, optics, and nanotechnology because they enhance coupling of the electromagnetic energy and can drive nonlinear processes in electronic materials, particularly where their dielectric function ε(ω) approaches zero. We investigate the nonlinear angle-resolved two-photon photoemission (2PP) spectroscopy of Ag(111) surface through the ε(ω) near-zero region. In addition to the Einsteinian single-particle photoemission, the 2PP spectra report unequivocal signatures of nonlocal dielectric, plasmonically enhanced, excitation processes.

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