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...
Low-dimensional electron systems, as realized in layered materials, often tend to spontaneously break the symmetry of the underlying nuclear lattice by forming so-called density waves 1 ; a state of matter that at present attracts enormous attention 2-6 . Here we reveal a remarkable and surprising feature of charge density waves, namely their intimate relation to orbital order. For the prototypical material 1T-TaS 2 we not only show that the charge density wave within the two-dimensional TaS 2 layers involves previously unidentified orbital textures of great complexity. We also demonstrate that two metastable stackings of the orbitally ordered layers allow manipulation of salient features of the electronic structure. Indeed, these orbital e ects provide a route to switch 1T-TaS 2 nanostructures from metallic to semiconducting with technologically pertinent gaps of the order of 200 meV. This new type of orbitronics is especially relevant for the ongoing development of novel, miniaturized and ultrafast devices based on layered transition metal dichalcogenides 7,8 .Among the various transition metal dichalcogenides (TMDs), 1T-TaS 2 stands out because of its particularly rich electronic phase diagram as a function of pressure and temperature 9 . This phase diagram not only features incommensurate, nearly commensurate and commensurate charge density waves (CDWs), but also pressure-induced superconductivity below 5 K. In addition to this, it was proposed early on that the low-temperature commensurate CDW (C-CDW), which is illustrated in Fig. 1a,c, also features manybody Mott physics 10 . Experimental evidence for the presence of Mott physics in 1T-TaS 2 has indeed been obtained recently by timeresolved spectroscopies, which observed the ultrafast collapse of a charge excitation gap, which has been interpreted as a fingerprint of significant electron-electron interactions [11][12][13] . Even though the above scenario for the C-CDW is widely accepted, important experimental facts remain to be understood: the very strong suppression of the C-CDW with external pressure is puzzling. Already above 0.6 GPa, the C-CDW is no longer stable, although nesting conditions, band widths and lattice structure remain essentially unchanged. It is also not clear how ordered defects within the C-CDW, which emerge in the nearly commensurate phase (NC-CDW) on heating 14,15 and do not cause significant changes in the bandwidths, can render the electronelectron on-site interaction U completely ineffective 16 . In the following we will show that all these issues are explained consistently in terms of orbital textures that are intertwined with the CDW. Furthermore, we demonstrate that this new twist to the physics of
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...
Electrons in graphene behave like Dirac fermions, permitting phenomena from high energy physics to be studied in a solid state setting. A key question is whether or not these Fermions are critically influenced by Coulomb correlations. We performed inelastic x-ray scattering experiments on crystals of graphite, and applied reconstruction algorithms to image the dynamical screening of charge in a freestanding, graphene sheet. We found that the polarizability of the Dirac fermions is amplified by excitonic effects, improving screening of interactions between quasiparticles. The strength of interactions is characterized by a scale-dependent, effective fine structure constant,, whose value approaches 0.14 0.092~1 / 7 at low energy and large distances. This value is substantially smaller than the nominal 2.2 g , suggesting that, on the whole, graphene is more weakly interacting than previously believed.
Time-division multiplexing (TDM) is a mature scheme for the readout of arrays of transition-edge sensors (TESs). TDM is based on superconducting-quantum-interference-device (SQUID) current amplifiers. Multiple spectrometers based on gamma-ray and X-ray microcalorimeters have been operated with TDM readout, each at the scale of 200 sensors per spectrometer, as have several astronomical cameras with thousands of sub-mm or microwave bolometers. Here we present the details of two different versions of our TDM system designed to read out X-ray TESs. The first has been field-deployed in two 160-sensor (8 columns × 20 rows) spectrometers and four 240-sensor (8 columns × 30 rows) spectrometers. It has a three-SQUID-stage architecture, switches rows every 320 ns, and has total readout noise of 0.41 μΦ0/√Hz. The second, which is presently under development, has a two-SQUID-stage architecture, switches rows every 160 ns, and has total readout noise of 0.19 μΦ0/√Hz. Both quoted noise values are non-multiplexed and referred to the first-stage SQUID. In a demonstration of this new architecture, a multiplexed 1-column × 32-row array of NIST TESs achieved average energy resolution of 2.55±0.01 eV at 6 keV.
We present temperature-dependent x-ray diffraction and temperature- and field-dependent Raman scattering studies of single-crystal Mn3O4, which reveal the magnetostructural phases that evolve in the spinels due to the interplay between strong spin-orbital coupling, geometric frustration, and applied magnetic field. We present evidence that the magnetoelastic and magnetodielectric behavior in this material is governed by magnetic-field-controlled tetragonal-to-monoclinic phase changes. Most interestingly, for an applied field transverse to the ferrimagnetic ordering direction, H parallel [110], we find evidence for a field-tuned quantum phase transition to a tetragonal spin-disordered phase, indicating that a structurally symmetric, spin frustrated phase can be recovered at T approximately 0 for intermediate transverse fields in Mn3O4.
We describe a series of microcalorimeter X-ray spectrometers designed for a broad suite of measurement applications. The chief advantage of this type of spectrometer is that it can be orders of magnitude more efficient at collecting X-rays than more traditional high-resolution spectrometers that rely on wavelength-dispersive techniques. This advantage is most useful in applications that are traditionally photon-starved and/or involve radiation-sensitive samples. Each energy-dispersive spectrometer is built around an array of several hundred transition-edge sensors (TESs). TESs are superconducting thin films that are biased into their superconducting-to-normal-metal transitions. The spectrometers share a common readout architecture and many design elements, such as a compact, 65 mK detector package, 8-column time-division-multiplexed superconducting quantum-interference device readout, and a liquid-cryogen-free cryogenic system that is a two-stage adiabatic-demagnetization refrigerator backed by a pulse-tube cryocooler. We have adapted this flexible architecture to mate to a variety of sample chambers and measurement systems that encompass a range of observing geometries. There are two different types of TES pixels employed. The first, designed for X-ray energies below 10 keV, has a best demonstrated energy resolution of 2.1 eV (full-width-at-half-maximum or FWHM) at 5.9 keV. The second, designed for X-ray energies below 2 keV, has a best demonstrated resolution of 1.0 eV (FWHM) at 500 eV. Our team has now deployed seven of these X-ray spectrometers to a variety of light sources, accelerator facilities, and laboratory-scale experiments; these seven spectrometers have already performed measurements related to their applications. Another five of these spectrometers will come online in the near future. We have applied our TES spectrometers to the following measurement applications: synchrotron-based absorption and emission spectroscopy and energy-resolved scattering; accelerator-based spectroscopy of hadronic atoms and particle-induced-emission spectroscopy; laboratory-based time-resolved absorption and emission spectroscopy with a tabletop, broadband source; and laboratory-based metrology of X-ray-emission lines. Here, we discuss the design, construction, and operation of our TES spectrometers and show first-light measurements from the various systems. Finally, because X-ray-TES technology continues to mature, we discuss improvements to array size, energy resolution, and counting speed that we anticipate in our next generation of TES-X-ray spectrometers and beyond.
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