Charge-Density Wave and One-dimensional Electronic Spectra in Blue Bronze: Incoherent Solitons and Spin-Charge Separation Daixiang Mou, 1 R. M. Konik, 2 A. M. Tsvelik ∗ , 2 I. Zaliznyak, 2 and Xingjiang Zhou ∗1 1 National Laboratory for Superconductivity, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2 CMPMS Dept., Brookhaven National Laboratory, Upton, NY 11973-5000, USA 2 (Dated: November 23, 2018) We present new high resolution angle resolved photoemission (ARPES) data for K0.3MoO3 (blue bronze) and propose a novel theoretical description of these results. The observed Fermi surface, with two quasi-one-dimensional sheets, is consistent with a ladder material with a weak inter-ladder coupling. Hence, we base our description on spectral properties of one-dimensional ladders. The marked broadening of the ARPES lineshape, a significant fraction of an eV, is interpreted in terms of spin-charge separation. A high energy feature, which is revealed for the first time in the spectra near the Fermi momentum thanks to improved energy resolution, is seen as a signature of a higher energy bound state of soliton excitations on a ladder. PACS numbers: 74.81.Fa, 74.90.+n Systems of two coupled chains (called ladders) can be viewed as a first step in crossing over from one dimen- sion (1D), with its exotic physics of Luttinger liquid and spin-charge separation, to higher dimensions. In spite of being unusual and seemingly enigmatic, this 1D physics is now well understood, thanks to remarkable progress in applying field theoretic methods in condensed matter [1]. How this physics transforms in the course of crossover to higher dimension is why ladders have been intensely studied. While theoretical progress on this problem has been considerable, with work in predicting rich excitation spectra, dynamical generation of spectral gaps, existence of preformed pairs [2–16], the progress on the experimen- tal side has been slower both due to limited numbers of materials with a suitable structure and limited experi- mental accuracy. The molybdenum blue bronzes, A 0.3 MoO 3 are among the most interesting and intensely studied examples of quasi-1D conductors [17, 18]. They feature MoO 6 octa- hedra forming an array of weakly coupled pairs of con- ducting ladders [19]. According to band structure calcu- lations the chemical potential is crossed by two slightly warped bands with 3/4-filling [20, 21]. These are bonding (B) and anti-bonding (AB) bands arising from the elec- tron hopping between the two legs of a given ladder, with Fermi wave vectors K FB and K F AB , respectively. The resulting Fermi surface nesting suggests a charge density wave (CDW) formation with a wave vector K FB + K F AB along the chains. In K 0.3 MoO 3 a 3D phase transition into a charge- ordered insulating phase takes place at a temperature T CDW ≈ 180K [22, 23] even though the magnetic suscep- tibility starts to decrease well above the transition, ex- periencing a one-third drop in the interval between 700K and 180K [22]. The low-energy lattice responses mea- sured in neutron and X-ray experiments also show pre- cursor effects up to at least ∼ 2T CDW [18, 24–26]. Pre- vious angle-resolved photoemission (ARPES) measure- ments, while confirming the band structure predictions, also found significant breadth of the electronic spectra, extending to photoelectron energies ∼ eV [27–29]. By virtue of the lattice involvement in the CDW forma- tion, this surprising incoherence was assigned to electron- phonon interactions and small polarons, in spite of the marked mismatch in the energy scales. Here we report the results of new high resolution ARPES measurements of K 0.3 MoO 3 , which we analyze based on the electronic spectral properties of ladders. With much improved instrumental resolution, we focus upon examining the fine structure of the measured pho- toemission spectra. We resolve for the first time two broad peaks in regions of the Brillouin zone near the Fermi vector, dispersing through energies a significant fraction of an eV. We interpret these features as ladder excitations originating from the same interaction that un- derpins the 3D charge order existing in this material at low temperatures. The breadth of these peaks arises from the presence of gapless charge excitations (holons) and gapful spin solitons (spinons) with markedly different ve- locities. We obtain a good description of the measured spectra with two SU(2) Thirring models describing the spectral properties of a ladder, thus assuming that the physics is electron-driven and the electron-lattice inter- action plays only a secondary role. The K 0.3 MoO 3 single crystals were grown by an elec- trolytic reduction method [30]. High resolution angle- resolved photoemission measurements were carried out with a Scienta R4000 electron energy analyzer [31]. For the band structure measurements and Fermi surface mapping (Fig. 1), we used a helium discharge lamp as the light source with a photon energy of hν =21.218 eV. The overall energy resolution was set at 10 meV and the an- gular resolution was 0.3 ◦ , corresponding to a momentum resolution of 0.0091 ˚ A −1 at the photon energy of 21.218 eV. For the high-precision ARPES measurements (Fig. 2 and Fig. 3), a vacuum ultra-violet (VUV) laser with a photon energy hν =6.994 eV was used as a light source. The energy resolution in this case was 1 meV. The an- arXiv:1207.7066v2 [cond-mat.str-el] 25 Mar 2014