Anomalous femtosecond quasiparticle dynamics of hidden order state in URu2Si2

Abstract

At T0 = 17.5 K an exotic phase emerges from a heavy fermion state in URu2Si2. The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at HO transition, however, experiments and ab-initio calculations have been unable to reveal the essential character of the QP. Here we use femtosecond pump-probe time-and angle-resolved photoemission spectroscopy (tr-ARPES) to elucidate the ultrafast dynamics of the QP. We show how the Fermi surface is renormalized by shifting states away from the Fermi level at specific locations, characterized by vector q<110> = 0.56 ± 0.08Å −1 . Measurements of the temperature-time response reveal that upon entering the HO the QP lifetime in those locations increases from 42 fs to few hundred fs. The formation of the long-lived QPs is identified here as a principal actor of the HO.Over the last 25 years, the nature of the hidden order transition in a heavy fermion system URu 2 Si 2 has remained a mystery. The sharp second-order transition at T 0 = 17.5 K [1-13], marked by a large jump in specific heat, corresponds to removal of more than 10 percent of total entropy [14]. The phase transition might have been consistent with magnetism, but several years of intensive search showed the absence of magnetism in HO phase. Antiferromagnetism is observed only under pressure [15]. Partial gapping of the Fermi surface was proposed in the context of itinerant models [8,9,11,12] and the HO gap was shown to behave similarly to a BCS order parameter [16]. Entropy removal at HO transition also suggested that the Fermi surface instability induces reconstruction of the density of states. It was proposed that a commensurate [6,9,10,17,18] or incommensurate [4,11,12] renormalization of the Fermi surface is the driver of HO transition, with magnetic fluctuations[9] or hybridization wave [11,12] as mechanisms for the gap formation. However, the underlying physics behind the key QPs, the gap formation, symmetry, and momentumdependence remain elusive, in spite of several models of HO proposed over the years [4, 7-10, 12, 17, 18].The picture of HO is also obscured by the formation of a hybridization gap, a typical feature of heavy fermion materials [19]. Such a gap formation is due to hybridization of the flat f-band with a strongly dispersive d-band, as evidenced by numerous experiments [1-3, 14, 20, 21].The hybridization gap onset is related to the coherence temperature T*, usually from 60 K to 100 K for uraniumbased heavy fermion materials [22] and estimated at 70 K in URu 2 Si 2 . At T 0 the stage is already set by a well established f-d hybridization gap structure evolving in a mean-field behavior [23,24]. To reveal the nature of the HO transition within the hybridization gap, one needs to track the QPs responsible for the Fermi surface renormalization. Several recent investigations using ARPES, INS and Scanning Tunneling Microscopy (STM) demonstrate that the HO transition is mark...

Photoemission investigations of URu2Si2

“…A tr-ARPES gas-cell harmonic generator system at Los Alamos provided temporal resolution of roughly 30 fs and probe photon energy of 29.5 eV, suitable for 5f elements in terms of photoionization cross-sections [44]. As shown in Figures 6 and 7, experiments revealed the dramatic increase of quasi-particle lifetime at specific, strongly energy-and momentum-dependent locations in the BZ [45]. An almost order-of-magnitude increase in lifetime from 44 fs at 19 K to over 300 fs at 12 K was interpreted as evidence for formation of very long lived, narrow quasi-particle states at HO transition.…”

“…The results underscore the three-dimensional nature of the URu 2 Si 2 , and a comparison of critical nesting vectors obtained with different Downloaded by [New York University] at 08:32 19 June 2015 Figure 7. (colour online) Time-resolved ARPES study of URu 2 Si 2 in the hidden-order state, from [45], measured at 12K along the (1 1 0) direction. (a) The case of a very long, "negative" 1-ps delay between probe and subsequent pump pulses, subtracted from all other spectra, collected for the probe pulse arriving after the pump pulse with delays between 0 and 533 fs, with (b) the 0 fs case corresponding to the overlap of pump and probe pulses.…”

Colloquium: Hidden order, superconductivity, and magnetism: The unsolved case ofURu2Si2

“…A new technique, time and angular resolved photoemission spectroscopy (tr-ARPES) was very recently applied to study the dynamics of the HO quasiparticles on the femtosecond time scale (Dakovski et al, 2011). Within reciprocal space the probed position is first tuned to one of the Fermi surface hot spots (see below), and subsequently the femtosecond time-resolved quasiparticle dynamics at this location is probed after stimulation with a pump laser.…”

“…Moreover, these bulk sensitive ARPES measurements do not detect a narrow heavy band in the vicinity of E F , in contrast to the He I study of Santander-Syro et al (2009). This narrow heavy band is consequently attributed to a surface contribution.A new technique, time and angular resolved photoemission spectroscopy (tr-ARPES) was very recently applied to study the dynamics of the HO quasiparticles on the femtosecond time scale (Dakovski et al, 2011). Within reciprocal space the probed position is first tuned to one of the Fermi surface hot spots (see below), and subsequently the femtosecond time-resolved quasiparticle dynamics at this location is probed after stimulation with a pump laser.…”

Can there be “Hidden Order” in transition metals?

“…Thermodynamic and transport measurements [2,6] indicate that approximately 40% of the Fermi-surface is gapped in the ''Hidden Order'' phase. There is evidence that the ''Hidden Order'' state also has a modified spatial periodicity [7] and involves Fermi-surface nesting [8]. Electronic structure calculations give support for the involvement of Fermi-surface nesting [9,10].…”