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Register a new userItinerant spin excitations near the hidden order transition in URu2Si2
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By means of neutron scattering we show that the high-temperature precursor to the hidden order state of the heavy fermion superconductor URu$_{2}$Si$_{2}$ exhibits heavily damped incommensurate paramagnons whose strong energy dispersion is very similar to that of the long-lived longitudinal f-spin excitations that appear below T$_{0}$. Since the underlying local f-exchange is preserved we expect only the f-d interactions to change across the phase transition and to cause the paramagnetic damping. The damping exhibits single-ion behavior independent of wave vector and vanishes below the hidden order transition. We suggest that this arises from a transition from valence fluctuations to a hybridized f-d state below T$_{0}$. Here we present evidence that the itinerant excitations, like those in chromium, are due to Fermi surface nesting of hole and electron pockets so that the hidden order phase likely originates from a Fermi-surface instability. We identify wave vectors that span nested regions of a band calculation and that match the neutron spin crossover from incommensurate to commensurate on approach to the hidden order phase.
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The term hidden order refers to an as yet unidentified form of broken-symmetry order parameter that is presumed to exist in the strongly correlated electron system URu2Si2 on the basis of the reported similarity of the heat capacity at its phase transition at To~17 K to that produced by Bardeen-Cooper-Schrieffer (BCS) mean field theory. Here we show that the phase boundary in URu2Si2 has the elliptical form expected for an entropy-driven phase transition, as has been shown to accompany a change in valence. We show one characteristic feature of such a transition is that the ratio of the critical magnetic field to the critical temperature is defined solely in terms of the effective quasiparticle g-factor, which we find to be in quantitative agreement with prior g-factor measurements. We further find the anomaly in the heat capacity at To to be significantly sharper than a BCS phase transition, and, once quasiparticle excitations across the hybridization gap are taken into consideration, loses its resemblance to a second order phase transition. Our findings imply that a change in valence dominates the thermodynamics of the phase boundary in URu2Si2, and eclipses any significant contribution to the thermodynamics from a hidden order parameter.
Solids with strong electron correlations generally develop exotic phases of electron matter at low temperatures. Among such systems, the heavy-fermion semi-metal URu2Si2 presents an enigmatic transition at To = 17.5 K to a `hidden order state whose order parameter remains unknown after 23 years of intense research. Various experiments point to the reconstruction and partial gapping of the Fermi surface when the hidden-order establishes. However, up to now, the question of how this transition affects the electronic spectrum at the Fermi surface has not been directly addressed by a spectroscopic probe. Here we show, using angle-resolved photoemission spectroscopy, that a band of heavy quasi-particles drops below the Fermi level upon the transition to the hidden-order state. Our data provide the first direct evidence of a large reorganization of the electronic structure across the Fermi surface of URu2Si2 occurring during this transition, and unveil a new kind of Fermi-surface instability in correlated electron systems
URu2Si2 is one of the most enigmatic strongly-correlated-electron systems and offers a fertile testing ground for new concepts in condensed matter science. In spite of >30 years of intense research, no consensus on the order parameter of its low-temperature hidden-order phase exists. A strong magnetic field transforms the hidden order into magnetically-ordered phases, whose order parameter has also been defying experimental observation. Here, thanks to an instrumentation breakthrough in high-field neutron scattering, we identify the field-induced phases of URu2Si2 as a spin-density-wave state with wavevector k1 = (0.6 0 0). The transition to the spin-density wave represents a unique touchstone for understanding the hidden-order phase. An intimate relationship between this magnetic structure, the magnetic fluctuations, and the Fermi surface is emphasized, calling for dedicated band structure calculations.
Since the 1985 discovery of the phase transition at $T_{rm HO}=17.5$ K in the heavy-fermion metal URu$_2$Si$_2$, neither symmetry change in the crystal structure nor magnetic ordering have been observed, which makes this hidden order enigmatic. Some high-field experiments have suggested electronic nematicity which breaks fourfold rotational symmetry, but direct evidence has been lacking for its ground state at zero magnetic field. Here we report on the observation of lattice symmetry breaking from the fourfold tetragonal to twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction measurements at zero field, which pins down the space symmetry of the order. Small orthorhombic symmetry-breaking distortion sets in at $T_{rm HO}$ with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultrapure sample, implying a highly unusual coupling nature between the electronic nematicity and underlying lattice.
To elucidate the underlying nature of the hidden order (HO) state in heavy-fermion compound URu2Si2, we measure electrical transport properties of ultraclean crystals in a high field/low temperature regime. Unlike previous studies, the present system with much less impurity scattering resolves a distinct anomaly of the Hall resistivity at H*=22.5 T well below the destruction field of the HO phase ~36 T. In addition, a novel quantum oscillation appears above a magnetic field slightly below H*. These results indicate an abrupt reconstruction of the Fermi surface, which implies a possible phase transition well within the HO phase caused by a band-dependent destruction of the HO parameter. The present results definitely indicate that the HO transition should be described by an itinerant electron picture.
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