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1. Nanofluidic Platform for Studying the First-Order Phase Transitions in Superfluid Helium-3

   https://doi.org/10.1007/s10909-024-03146-6  

   Heikkinen, Petri_J; Eng, Nathan; Levitin, Lev_V; Rojas, Xavier; Singh, Angadjit; Autti, Samuli; Haley, Richard_P; Hindmarsh, Mark; Zmeev, Dmitry_E; Parpia, Jeevak_M; et al (May 2024, Journal of Low Temperature Physics)

   Abstract The symmetry-breaking first-order phase transition between superfluid phases$$^3$$ ${}^3$He-A and$$^3$$ ${}^3$He-B can be triggered extrinsically by ionising radiation or heterogeneous nucleation arising from the details of the sample cell construction. However, the role of potential homogeneous intrinsic nucleation mechanisms remains elusive. Discovering and resolving the intrinsic processes may have cosmological consequences, since an analogous first-order phase transition, and the production of gravitational waves, has been predicted for the very early stages of the expanding Universe in many extensions of the Standard Model of particle physics. Here we introduce a new approach for probing the phase transition in superfluid$$^3$$ ${}^3$He. The setup consists of a novel stepped-height nanofluidic sample container with close to atomically smooth walls. The$$^3$$ ${}^3$He is confined in five tiny nanofabricated volumes and assayed non-invasively by NMR. Tuning of the state of$$^3$$ ${}^3$He by confinement is used to isolate each of these five volumes so that the phase transitions in them can occur independently and free from any obvious sources of heterogeneous nucleation. The small volumes also ensure that the transitions triggered by ionising radiation are strongly suppressed. Here we present the preliminary measurements using this setup, showing both strong supercooling of$$^3$$ ${}^3$He-A and superheating of$$^3$$ ${}^3$He-B, with stochastic processes dominating the phase transitions between the two. The objective is to study the nucleation as a function of temperature and pressure over the full phase diagram, to both better test the proposed extrinsic mechanisms and seek potential parallel intrinsic mechanisms. 

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2. Chiral Superfluid Helium-3 in the Quasi-Two-Dimensional Limit

   https://doi.org/10.1103/PhysRevLett.134.136001  

   Heikkinen, Petri J; Levitin, Lev V; Rojas, Xavier; Singh, Angadjit; Eng, Nathan; Casey, Andrew; Saunders, John; Vorontsov, Anton; Zhelev, Nikolay; Sebastian, Abhilash Thanniyil; et al (March 2025, Physical Review Letters)

   Anisotropic pair breaking close to surfaces favors the chiral $A$phase of the superfluid ${}^3\mathrm{He}$over the time-reversal invariant $B$phase. Confining the superfluid ${}^3\mathrm{He}$into a cavity of height $D$of the order of the Cooper pair size characterized by the coherence length ${\xi }_0$—ranging between 16 nm (34 bar) and 77 nm (0 bar)—extends the surface effects over the whole sample volume, thus allowing stabilization of the $A$phase at pressures $P$and temperatures $T$where otherwise the $B$phase would be stable. In this Letter, the surfaces of such a confined sample are covered with a superfluid ${}^4\mathrm{He}$film to create specular quasiparticle scattering boundary conditions, preventing the suppression of the superfluid order parameter. We show that the chiral $A$phase is the stable superfluid phase under strong confinement over the full $P\text{−}T$phase diagram down to a quasi-two-dimensional limit $D/{\xi }_0=1$, where $D=80\mathrm{nm}$. The planar phase, which is degenerate with the chiral $A$phase in the weak-coupling limit, is not observed. The gap inferred from measurements over the wide pressure range from 0.2 to 21.0 bar leads to an empirical ansatz for temperature-dependent strong-coupling effects. We discuss how these results pave the way for the realization of the fully gapped two-dimensional $p_x+i{p}_y$superfluid under more extreme confinement. Published by the American Physical Society2025 

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3. Imaging interlayer exciton superfluidity in a 2D semiconductor heterostructure

   https://doi.org/10.1126/sciadv.adr1772  

   Cutshall, Jacob; Mahdikhany, Fateme; Roche, Anna; Shanks, Daniel N; Koehler, Michael R; Mandrus, David G; Taniguchi, Takashi; Watanabe, Kenji; Zhu, Qizhong; LeRoy, Brian J; et al (January 2025, Science Advances)

   Excitons, which are Coulomb bound electron-hole pairs, are composite bosons and thus at low temperature can form a superfluid state with a single well-defined amplitude and phase. We directly image this macroscopic exciton superfluid state in an hBN-separated MoSe₂-WSe₂heterostructure. At high density, we identify quasi-long-range order over the entire active area of our sample, through spatially resolved coherence measurements. By varying the exciton density and sample temperature, we map out the phase diagram of the superfluid. We observe the superfluid phase persisting to a temperature of 15 K, which is in excellent agreement with theoretical predictions. This works paves the way to realizing on chip superfluid structures capable of studying fundamental physical behaviors and quantum devices that use superfluidity. 

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4. Two types of charge order with distinct interplay with superconductivity in the kagome material CsV3Sb5

   https://doi.org/10.1038/s42005-022-01011-0  

   Gupta, Ritu; Das, Debarchan; Mielke, Charles; Ritz, Ethan T.; Hotz, Fabian; Yin, Qiangwei; Tu, Zhijun; Gong, Chunsheng; Lei, Hechang; Birol, Turan; et al (December 2022, Communications Physics)

   Abstract The kagome metals of the family A V 3 Sb 5 , featuring a unique structural motif, harbor an array of intriguing phenomena such as chiral charge order and superconductivity. CsV 3 Sb 5 is of particular interest because it displays a double superconducting dome in the region of the temperature-pressure phase diagram where charge order is still present. However, the microscopic origin of such an unusual behavior remains an unsolved issue. Here, to address it, we combine high-pressure, low-temperature muon spin relaxation/rotation with first-principles calculations. We observe a pressure-induced threefold enhancement of the superfluid density, which also displays a double-peak feature, similar to the superconducting critical temperature. This leads to three distinct regions in the phase diagram, each of which features distinct slopes of the linear relation between superfluid density and the critical temperature. These results are attributed to a possible evolution of the charge order pattern from the superimposed tri-hexagonal Star-of-David phase at low pressures (within the first dome) to the staggered tri-hexagonal phase at intermediate pressures (between the first and second domes). Our findings suggest a change in the nature of the charge-ordered state across the phase diagram of CsV 3 Sb 5 , with varying degrees of competition with superconductivity. 

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5. The neutron electric dipole moment experiment at the Spallation Neutron Source

   https://doi.org/10.1051/epjconf/201921902005  

   Leung, K.K.H.; Ahmed, M.; Alarcon, R.; Aleksandrova, A.; Baeßler, S.; Barrón-Palos, L.; Bartoszek, L.; Beck, D.H.; Behzadipour, M.; Bessuille, J.; et al (January 2019, EPJ Web of Conferences)

   Novel experimental techniques are required to make the next big leap in neutron electric dipole moment experimental sensitivity, both in terms of statistics and systematic error control. The nEDM experiment at the Spallation Neutron Source (nEDM@SNS) will implement the scheme of Golub & Lamoreaux [Phys. Rep., 237, 1 (1994)]. The unique properties of combining polarized ultracold neutrons, polarized 3 He, and superfluid 4 He will be exploited to provide a sensitivity to ∼ 10 −28   e  · cm. Our cryogenic apparatus will deploy two small (3 L) measurement cells with a high density of ultracold neutrons produced and spin analyzed in situ. The electric field strength, precession time, magnetic shielding, and detected UCN number will all be enhanced compared to previous room temperature Ramsey measurements. Our 3 He co-magnetometer offers unique control of systematic effects, in particular the Bloch-Siegert induced false EDM. Furthermore, there will be two distinct measurement modes: free precession and dressed spin. This will provide an important self-check of our results. Following five years of “critical component demonstration,” our collaboration transitioned to a “large scale integration” phase in 2018. An overview of our measurement techniques, experimental design, and brief updates are described in these proceedings. 

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