skip to main content

Title: Magnon Bose–Einstein condensation and superconductivity in a frustrated Kondo lattice

Motivated by recent experiments on magnetically frustrated heavy fermion metals, we theoretically study the phase diagram of the Kondo lattice model with a nonmagnetic valence bond solid ground state on a ladder. A similar physical setting may be naturally occurring inYbAl3C3,CeAgBi2, andTmB4compounds. In the insulating limit, the application of a magnetic field drives a quantum phase transition to an easy-plane antiferromagnet, which is described by a Bose–Einstein condensation of magnons. Using a combination of field theoretical techniques and density matrix renormalization group calculations we demonstrate that in one dimension this transition is stable in the presence of a metallic Fermi sea, and its universality class in the local magnetic response is unaffected by the itinerant gapless fermions. Moreover, we find that fluctuations about the valence bond solid ground state can mediate an attractive interaction that drives unconventional superconducting correlations. We discuss the extensions of our findings to higher dimensions and argue that depending on the filling of conduction electrons, the magnon Bose–Einstein condensation transition can remain stable in a metal also in dimensions two and three.

; ;
Publication Date:
Journal Name:
Proceedings of the National Academy of Sciences
Page Range or eLocation-ID:
p. 20462-20467
Proceedings of the National Academy of Sciences
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We report the temperature dependence of the Yb valence in the geometrically frustrated compoundYbB4from 12 to 300 K using resonant x-ray emission spectroscopy at the YbLα1transition. We find that the Yb valence,v, is hybridized between thev = 2 andv = 3 valence states, increasing fromv=2.61±0.01at 12 K tov=2.67±0.01at 300 K, confirming thatYbB4is a Kondo system in the intermediate valence regime. This result indicates that the Kondo interaction inYbB4is substantial, and is likely to be the reason whyYbB4does not order magnetically at low temperature, rather than this being an effect of geometric frustration. Furthermore, the zero-point valence of the system is extracted from our data and compared with other Kondo lattice systems. The zero-point valence seems to be weakly dependent on the Kondo temperature scale, but not on the valence change temperature scaleTv.

  2. Abstract

    The best upper limit for the electron electric dipole moment was recently set by the ACME collaboration. This experiment measures an electron spin-precession in a cold beam of ThO molecules in their metastableH(3Δ1)state. Improvement in the statistical and systematic uncertainties is possible with more efficient use of molecules from the source and better magnetometry in the experiment, respectively. Here, we report measurements of several relevant properties of the long-livedQ(3Δ2)state of ThO, and show that this state is a very useful resource for both these purposes. TheQstate lifetime is long enough that its decay during the time of flight in the ACME beam experiment is negligible. The large electric dipole moment measured for theQstate, giving rise to a large linear Stark shift, is ideal for an electrostatic lens that increases the fraction of molecules detected downstream. The measured magnetic moment of theQstate is also large enough to be used as a sensitive co-magnetometer in ACME. Finally, we show that theQstate has a large transition dipole moment to theC(1Π1)state, which allows for efficient population transfer between the ground stateXmore »width='0.50em'/>(1Σ+)and theQstate viaXCQStimulated Raman Adiabatic Passage (STIRAP). We demonstrate 90 % STIRAP transfer efficiency. In the course of these measurements, we also determine the magnetic moment ofCstate, theXCtransition dipole moment, and branching ratios of decays from theCstate.

    « less
  3. Ferroquadrupole order associated with local4fatomic orbitals of rare-earth ions is a realization of electronic nematic order. However, there are relatively few examples of intermetallic materials which exhibit continuous ferroquadrupole phase transitions, motivating the search for additional materials that fall into this category. Furthermore, it is not clear a priori whether experimental approaches based on transport measurements which have been successfully used to probe the nematic susceptibility in materials such as the Fe-based superconductors will be as effective in the case of4fintermetallic materials, for which the important electronic degrees of freedom are local rather than itinerant and are consequently less strongly coupled to the charge-carrying quasiparticles near the Fermi energy. In the present work, we demonstrate that the intermetallic compoundYbRu2Ge2exhibits a tetragonal-to-orthorhombic phase transition consistent with ferroquadrupole order of the Yb ions and go on to show that elastoresistivity measurements can indeed provide a clear window on the diverging nematic susceptibility in this system. This material provides an arena in which to study the causes and consequences of electronic nematicity.

  4. Abstract

    Cosmic reionization was the last major phase transition of hydrogen from neutral to highly ionized in the intergalactic medium (IGM). Current observations show that the IGM is significantly neutral atz> 7 and largely ionized byz∼ 5.5. However, most methods to measure the IGM neutral fraction are highly model dependent and are limited to when the volume-averaged neutral fraction of the IGM is either relatively low (x¯HI103) or close to unity (x¯HI1). In particular, the neutral fraction evolution of the IGM at the critical redshift range ofz= 6–7 is poorly constrained. We present new constraints onx¯HIatz∼ 5.1–6.8 by analyzing deep optical spectra of 53 quasars at 5.73 <z< 7.09. We derive model-independent upper limits on the neutral hydrogen fraction based on the fraction of “dark” pixels identified in the Lyαand Lyβforests, without any assumptions on the IGM model or the intrinsic shape of the quasar continuum. They are the first model-independent constraints on the IGM neutral hydrogen fraction atz∼ 6.2–6.8 using quasar absorption measurements. Our results give upper limits ofx¯more »mathvariant='normal'>HI(z=6.3)<0.79±0.04(1σ),x¯HI(z=6.5)<0.87±0.03(1σ), andx¯HI(z=6.7)<0.940.09+0.06(1σ). The dark pixel fractions atz> 6.1 are consistent with the redshift evolution of the neutral fraction of the IGM derived from Planck 2018.

    « less
  5. The negatively charged silicon monovacancyVSiin 4H silicon carbide (SiC) is a spin-active point defect that has the potential to act as a qubit in solid-state quantum information applications. Photonic crystal cavities (PCCs) can augment the optical emission of theVSi, yet fine-tuning the defect–cavity interaction remains challenging. We report on two postfabrication processes that result in enhancement of theV1optical emission from our PCCs, an indication of improved coupling between the cavity and ensemble of silicon vacancies. Below-bandgap irradiation at 785-nm and 532-nm wavelengths carried out at times ranging from a few minutes to several hours results in stable enhancement of emission, believed to result from changing the relative ratio ofVSi0(“dark state”) toVSi(“bright state”). The much faster change effected by 532-nm irradiation may result from cooperative charge-state conversion due to proximal defects. Thermal annealing at 100 °C, carried out over 20 min, also results in emission enhancements and may be explained by the relatively low-activation energy diffusion of carbon interstitialsCi, subsequently recombining with other defects to create additionalVSis. These PCC-enabled experiments reveal insights into defect modifications and interactions within a controlled, designated volume and indicate pathways tomore »improved defect–cavity interactions.

    « less