2 If a fluid of bosons is cooled to sufficiently low temperature, a significant fraction will condense into the lowest quantum state, forming a Bose condensate. Bose condensation is a consequence of the even symmetry of the many-body wave function of bosons under particle interchange, and allows for the manifestation of macroscopic quantum phenomena, the most striking being superfluidity.Traditionally, Bose condensates are said to come in two types. Bose-Einstein condensates (BECs) occur in systems of stable bosons, such as dilute atomic gases or liquid Excitons are bosons that are bound states between an electron and hole in a solid, and were predicted long ago to Bose condense (2,3,4). Because of their light mass and high binding energy, exciton condensates should be stable at higher temperature than traditional BEC or BCS phases (5,6).Different theories predict that a Bose condensate of excitons could be a superfluid (5) or innately insulating (7), so there is tremendous need for experimental input. Identifying an exciton condensate in nature could have a profound impact on future understanding of macroscopic quantum phenomena, as well the classic problem of the metal-insulator transition in band solids, in which exciton condensation has long been believed to play a fundamental role (2,3,4).Condensed phases of photogenerated excitons have been realized in semiconductor quantum wells in resonance with a Fabry-Perot cavity which, although not fully thermally equilibrated, have exhibited evidence for transient superfluidity (8). Excitonic phases have also been realized in quantumHall bilayers in a perpendicular magnetic field (9). Although the order in these two-dimensional structures is not strictly long-ranged, and the order parameter cannot be measured directly, compelling evidence for excitonic correlations has been observed in Coulomb drag experiments (9). Despite these 3 achievements, there is a great need to identify an exciton condensate in a fully thermalized, threedimensional system in which the order is long-ranged.An ideal approach would be to identify a material in which an exciton condensate forms "naturally." Long ago, a BCS condensate of excitons was predicted to arise spontaneously in semimetals in which an indirect band gap is tuned close to zero ( Fig. 1) (2,3,4). This condensate is expected to break a spatial symmetry, rather than the U(1) symmetry broken by a superconductor, and in the absence of pinning should exhibit perfect conductivity without a Meissner effect (10). This phase can be thought of as a solid crystal of excitons, which early authors dubbed "excitonium" (4), and is the two-band analogue of the Wigner crystal instability of an interacting electron gas (10). This condensate is closely related to that in bilayer quantum wells (9), the coherence developing between electrons and holes in different bands ( Fig. 1) rather than in different layers. If found, this exciton condensate would be threedimensional, guaranteed to reside in thermodynamic equilibrium, and could potentially...

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this wavevector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) kBTC . This is in good agreement with the canonical value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c-axis, indicating that the magnetic fluctuations are two-dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.PACS numbers: 78.70.Nx, 74.20.Mn Understanding the physics of superconductivity in high-T c cuprates and other unconventional superconductors remains a central unresolved problem at the forefront of condensed matter physics. One widespread school of thought maintains that magnetic fluctuations are intimately involved in the pairing mechanism. This view is supported by a growing number of neutron scattering investigations showing the appearance of a magnetic excitation coincident with the onset of superconductivity [1,2,3,4,5,6,7,8]. The spectrum shows a resonance at a wavevector related to the antiferromagnetic order in the non-superconducting parent compounds. The apparent resonance energy scales with T C for different cuprate materials exhibiting a wide range of superconducting transition temperatures [9], providing tantalizing evidence for a common mechanism related to magnetic fluctuations.The discovery of a new family of Fe-based high temperature superconductors with T C as high as 55 K [10,11,12,13,14,15,16] presents an exciting opportunity to examine the relationship of spin excitations to the superconducting condensate in unconventional superconductors. The new materials are composed of Fe containing planes (FeAs or FeSe). Both theory and experiment indicate that simple electron-phonon coupling cannot describe superconductivity in these materials [17,18]. Furthermore, the superconducting state exists in close proximity to magnetism as the parent compounds exhibit spin-density wave order [19,20]. These observations have been put forth as evidence that the superconductivity in the Fe-based materials is unconventional. The presence of the Fe planes suggests quasi-two-dimensionality, as observed in the cuprates. However, neutron scattering investigations of the spin waves in the undoped parent compounds SrFe 2 As 2 [21], BaFe 2 As 2 [22], and CaFe 2 As 2 [23], indicate anisotropic exchange that cannot be classified as two dimensional. Band structure calculations [24,25] indicate that doping should enhance the twodimensionality of the Fermi surface, favoring superconductivity [25]. Directly probing the magnetic fluctuations in superconducting Fe-based systems is crucial for further progress.Recent measurements on a polycrystalline sample of Ba 0.6 K 0.4 Fe 2 As 2 found a spin excitation that appears at the onset...

We report neutron scattering measurements on single crystals of BaFe1.92Co0.08As2. The magnetic Bragg peak intensity is reduced by 6 % upon cooling through TC . The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional (3d) interactions. Below TC additional intensity appears at an energy of ∼4.5(0.5) meV similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below TC for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance demonstrating the direct competition between static magnetic order and superconductivity.PACS numbers: 78.70.Nx, 74.20.Mn Despite intense experimental and theoretical efforts directed towards understanding the recently discovered Febased superconductors [1,2,3,4,5,6,7], the mechanism behind superconductivity remains unclear (e.g. [8,9,10,11,12,13]). However, one common feature is the presence of magnetism in large regions of the phase diagrams for these materials (e.g. [15,16,17,18]). As such, there have been many suggestions that the pairing mechanism originates from the spin fluctuations. The observation of a resonance, strongly coupled to T C , in the spin dynamics of BaFe 2 As 2 doped with potassium [19], cobalt [20], and nickel [21] supports this contention and emphasizes the need for a comprehensive understanding of the fundamental magnetic behavior of these materials.Consequently, numerous studies have been initiated to probe the magnetic behavior of the Fe-based superconductors. In AFe 2 As 2 (A=Ba,Sr,Ca) studies of the static magnetic properties have revealed spin density wave order with magnetic moments aligned along the a-axis of the low temperature orthorhombic structure [18]. The spin excitations of these compounds are the subject of intense scrutiny as means of establishing the degree to which the magnetic behavior originates with localized or itinerant Fe 3d -electrons [18]. As noted above the spin dynamics in superconducting samples have been studied in several members of optimally doped BaFe 2 As 2 [19,20,21] where both the tetragonal-orthorhombic structural distortion and long range magnetic order have been suppressed. In particular, for doping levels where static magnetism coexists with superconductivity (either microscopically or as separate phases) the nature of the magnetic excitations has yet to be established in detail.BaFe 2−x Co x As 2 [23] has emerged as one of the most important systems in large part due to the availability of large homogenous single crystals. The phase diagram [22,24,25] for these materials shows that in the underdoped regime, there is both a structural phase transition and a magnetic ordering transition, as in the parent compound. For the range of concentrations, 0.06 < x < 0.12, samples exhibit both magnetic order and superconductivity. The topic of this letter is a neutron scattering study of single crystal samples of underdoped BaFe 1.92 Co 0.08 As ...

Many correlated electron materials, such as high-temperature superconductors 1 , geometrically frustrated oxides 2 and lowdimensional magnets 3,4 , are still objects of fruitful study because of the unique properties that arise owing to poorly understood many-body effects. Heavy-fermion metals 5 -materials that have high effective electron masses due to those effects-represent a class of materials with exotic properties, ranging from unusual magnetism, unconventional superconductivity and 'hidden' order parameters 6 . The heavy-fermion superconductor URu 2 Si 2 has held the attention of physicists for the past two decades owing to the presence of a 'hidden-order' phase below 17.5 K. Neutron scattering measurements indicate that the ordered moment is 0.03μ B , much too small to account for the large heat-capacity anomaly at 17.5 K. We present recent neutron scattering experiments that unveil a new piece of this puzzle-the spin-excitation spectrum above 17.5 K exhibits well-correlated, itinerant-like spin excitations up to at least 10 meV, emanating from incommensurate wavevectors. The large entropy change associated with the presence of an energy gap in the excitations explains the reduction in the electronic specific heat through the transition.The central issue in URu 2 Si 2 concerns the identification of the order parameter that explains the reduction in the specific heat coefficient, γ = C/T, and thus the change in entropy, through the transition at 17.5 K (ref. 6). Numerous speculations about the ground state have been advanced, from quadrupolar ordering 7 , to spin-density wave formation 8 , to 'orbital currents' 9 to account for the missing entropy. Here, we present cold-neutron time-offlight spectroscopy results that shed some light on the 'hiddenorder' (HO) in URu 2 Si 2 . We have carried out experiments above and below the ordering temperature to measure how the spin excitations evolve. It is clear from our data that above T 0 the spectrum is dominated by fast, itinerant-like spin excitations emanating from incommensurate wavevectors at positions located 0.4a* from the antiferromagnetic (AF) points. From the group velocity and temperature dependence of these modes, we surmise that these are heavy-quasiparticle excitations that form below the 'coherence temperature' and play a crucial role in the formation of the heavy-fermion and HO states. The gapping of these excitations, which corresponds to a loss of accessible states, accounts for the reduction in γ through the transition at 17.5 K. Figure 1 shows the excitation spectrum of URu 2 Si 2 at 1.5 K in the H00 plane. The characteristic gaps at ∼2 meV at the AF zone centre (1, 0, 0) and ∼4 meV at the incommensurate wavevectors (0.6, 0, 0) and (1.4, 0, 0) have been known for some time 10 . The incommensurate wavevector corresponds to a displacement of ∼0.4a * from the AF zone centres (that is, where h + k + l = an odd integer, and is thus forbidden in the body-centred-cubic chemical structure). A scenario for this modesoftening at the incommensurate positi...

Superconductivity (SC) in so-called "unconventional superconductors" is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between SC and fluctuations in some other order parameter. To better understand this connection, we used high-pressure x-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe 2 , which was previously shown to exhibit SC when the CDW is suppressed by pressure [1] or intercalation of Cu atoms [2]. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at P c = 5.1 ± 0.2 GPa, which is more than 1 GPa beyond the end of the SC region. Unexpectedly, at P = 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that SC in TiSe 2 may not be connected to the QCP itself, but to the formation of CDW domain walls. *The term "unconventional superconductor" once referred to materials whose phenomenology does not conform to the Bardeen-Cooper-Schrieffer (BCS) paradigm for superconductivity. It is now evident that, by this definition, the vast majority of known superconductors are unconventional, notable examples being the copper-oxide, iron-arsenide, and iron-selenide high temperature superconductors, heavy Fermion materials such as CeIn 3 and CeCoIn 5 , ruthenium oxides, organic superconductors such as ϰ-(BEDT-TTF)2X, filled skutterudites, etc.Despite their diversity in structure and phenomenology, the phase diagrams of these materials all exhibit the common trait that superconductivity (SC) resides near the boundary of an ordered phase with broken translational or spin rotation symmetry. For example, SC resides near antiferromagnetism in CeIn 3 [3], near a spin density wave in iron arsenides [4], orbital order in ruthenates [5], and stripe and nematic order in the copper-oxides [6]. The pervasiveness of this "universal phase diagram" suggests that there exists a unifying framework -more general than BCS -in which superconductivity can be understood as coexisting with some ordered phase, and potentially emerging from its fluctuations.A classic example is the transition metal dichalcogenide family, MX 2 , where M=Nb, Ti, Ta, and X=Se, S, which exhibit a rich competition between superconductivity and Peierls-like charge density wave (CDW) order [7]. A recent, prominent case is 1T-TiSe 2 , which under ambient pressure exhibits CDW order below a transition temperature T CDW = 202 K [8]. This CDW phase can be suppressed either with intercalation of Cu atoms [2,9], or through the application of hydrostatic pressure [1,10], causing SC to emerge. These studies suggest that the emergence of SC coincides with a quantum critical point (QCP) at which T CDW goes to zero, suggesting that TiSe 2 exemplifies the universal phenomenon of superconductivity em...