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The transition-metal kagome lattice materials host frustrated, correlated, and topological quantum states of matter1–9. Recently, a new family of vanadium-based kagome metals AV3Sb5 (A=K, Rb, and Cs) with topological band structures has been discovered10,11. These layered compounds are nonmagnetic and undergo charge density wave transitions before developing superconductivity at low temperatures11–19. Here we report the observation of unconventional superconductivity and pair density wave (PDW) in CsV3Sb5 using scanning tunneling microscope/spectroscopy (STM/STS) and Josephson STS. We find that CsV3Sb5 exhibits a V-shaped pairing gap Δ~0.5 meV and is a strong-coupling superconductor (2∆/kBTc~5) that coexists with 4a0 unidirectional and 2a0×2a0 charge order. Remarkably, we discover a 3Q PDW accompanied by bidirectional 4a0/3 spatial modulations of the superconducting gap, coherence peak and gap-depth in the tunneling conductance. We term this novel quantum state a roton-PDW associated with an underlying vortex-antivortex lattice that can account for the observed conductance modulations. Probing the electronic states in the vortex halo in an applied magnetic field, in strong-field that suppresses superconductivity, and in zero-field above Tc reveals that the PDW is a primary state responsible for an emergent pseudogap and intertwined electronic order. Our findings show striking analogies and distinctions to the phenomenology of high-Tc cuprate superconductors, and provide groundwork for understanding the microscopic origin of correlated electronic states and superconductivity in vanadium-based kagome metals.

We have calculated the energy spectra for almost all the filling fractions in the Jain series low energy fundamental mode as well as higher energy modes using composite fermion theory. The nature of the low energy mode and higher energy mode is nearly identical to the roton mode. We have observed that, these series of filling fractions $\nu=\frac{n}{2pn+1}$ possess $n$ number of roton minima in their higher energy mode., Comment: 9 pages, 3 figures

Roton dispersion relations, displaying a pronounced “roton” minimum at finite momentum, were firstly predicted by Landau and have been extensively explored in correlated quantum systems at low temperatures. Recently, the roton-like dispersion relations were theoretically extended to classical acoustics, which, however, have remained elusive in reality. Here, we report the experimental observation of roton-like dispersions in acoustic metamaterials with beyond-nearest-neighbour interactions at ambient temperatures. The resulting metamaterial supports multiple coexisting modes with different wavevectors and group velocities at the same frequency and broadband backward waves, analogous to the “return flow” termed by Feynman in the context of rotons. Moreover, by increasing the order of long-range interaction, we observe multiple rotons on a single dispersion band, which have never appeared in Landau’s prediction or any other condensed matter study. The realization of roton-like dispersions in metamaterials could pave the way to explore novel physics and applications on quantum-inspired phenomena in classical systems.

A density functional theory-based method is developed to describe the static and dynamic response of superfluid helium at finite temperatures. The model relies on the well-established 0 K Orsay–Trento functional, which has been extensively used to study the response of bulk superfluid helium as well as superfluid helium droplets. By including a phenomenological stochastic term in this model, it is possible to obtain thermodynamic equilibrium corresponding to a given temperature by propagating the system in imaginary time. The temperature dependence of thermodynamic quantities, such as the internal energy and entropy, is computed and is compared well with experimental reference data for the bulk liquid up to about 2 K, but begins to gradually deviate above that temperature. Inspection of pseudovorticity during real-time evolution of the system near 2 K reveals the presence of roton quasiparticles, which are suggested to be precursors for quantized vortex rings (Onsager’s ghosts), as well as weaker analogs of extended vortex loops.

Detection of moving charge in free space is presented in the framework of single electron CMOS devices. It opens the perspective for construction of new type detectors for beam diagnostic in accelerators. General phenomenological model of noise acting on position based qubit implemented in semiconductor quantum dots is given in the framework of simplistic tight-binding model. At first linear position-based qubit also known as Wannier qubit is considered with the situation of being excited by external movement of charged particle in its proximity. Analytical formulas describing the change of qubit state are derived analytically. In the next steps the roton semiconductor qubit representing closed loop of coupled quantum dots is given and the effect of external moving charge on the change of qubit state is determined by analytical formulas. Roton qubit physical state can be controlled by external magnetic and electric field what is the extension of controlling mechanism in comparison with linear Wannier qubit, when we have N quantum dots aligned.

Experimentally observed with dielectric disc resonator technique the resonant microwave absorptionamplification in superfluid He II below λ-point has been interpreted theoretically as a phenomenon in electrically active dielectric medium with low-energy excitations which exist near the ground state of the He–He interatomic bond due to fine structure of spin subsystem in condensed helium phase. The experimentally registered microwave resonant absorption line with fres = 180.3 GHz at T = 1.4 K and fres = 150.0 GHz at T = 2.1 K is closely related to the standard value of roton gap ∆/kB = 8.64 K (179.36 GHz). The measured temperature dependence of the resonant absorption demonstrates an excellent agreement with the corresponding neutron diffraction data for ∆(T) known from different literature sources. From the common point of view the obtained resonant absorption provides a typical example of the microwave spectroscopy which occurs widely among molecular systems with rotational and vibrational degrees of freedom, but for the first time discovered on as simple atomic matter as a superfluid helium. We explain the phenomenon as an effect of spin-phonon interaction within superfluid He II phase, and our theoretical estimation gives an upper limit ~ 250 GHz for the microwave resonant response of the system at T = 0. We interpret the dielectric 4 He superfluid as an electrically active working substance for low temperature MASER, and clarify the atomic mechanism of microwave absorption– amplification in the condensed helium phases. Резонансні поглинання–підсилення електромагнітної енергії НВЧ діапазону у надплинному He II при температурах нижчих λ -точки, що спостерігаються за допомогою техніки діелектричного дискового резонатора, інтерпретовано теоретично як ефекти у електрично активному діелектричному середовищі, яке вміщує низькоенергетичні збудження, що існують поблизу основного стану міжатомного зв’язку He–He завдяки тонкій структурі спінової підсистеми у конденсованих фазах гелію. НВЧ лінії резонансного поглинання fres = 180,3 ГГц при T = 1,4 К та fres = 150 ГГц при T = 2,1 К, які експериментально спостерігаються, однозначно корельовані зі стандартним значенням величини ротонної щілини ∆/ = 8,64 kB К (179,36 ГГц). Виміряна температурна залежність резонансного поглинання добре узгоджується з різними літературними даними по ∆( ) T . Згідно з загальноприйнятими уявленнями резонансне поглинання, яке спостерігається у НВЧ діапазоні, є ефектом, типовим для молекулярних систем з обертальними та коливальними ступінями свободи, однак про це явище вперше повідомлюється для такого простого атомарного середовища як надплинний гелій. Ми пояснюємо це явище як ефект спін-фононної взаємодії у надплинній фазі He II. Наша теоретична оцінка дає верхню границю ~ 250 ГГц щодо НВЧ резонансного відгуку системи при T = 0. Ми інтерпретуємо надплинний діелектрик 4 He як електрично активне робоче середовище для низькотемпературного MASERа та пояснюємо атомарний механізм НВЧ поглинання–підсилення у конденсованих фазах гелію. Резонансные поглощения–усиления электромагнитной энергии СВЧ диапазона в сверхтекучем He II при температурах ниже λ -точки, наблюдаемые с помощью техники диэлектрического дискового резонатора, интерпретированы теоретически как эффекты в электрически активной диэлектрической среде, содержащей низкоэнергетические возбуждения, существующие вблизи основного состояния межатомной связи He–He благодаря тонкой структуре спиновой подсистемы в конденсированных фазах гелия. Экспериментально регистрируемые СВЧ линии резонансного поглощения fres = 180,3 ГГц при T = 1,4 К и fres = 150 ГГц при T = 2,1 К однозначно коррелированны со стандартным значением величины ротонной щели ∆/ = 8,64 kB К (179,36 ГГц). Измеренная температурная зависимость резонансного поглощения демонстрирует прекрасное согласие с литературными данными по ∆( ) T . Согласно общепринятым представлениям, наблюдаемое резонансное поглощение в диапазоне СВЧ является эффектом, типичным для молекулярных систем с вращательными и колебательными степенями свободы, однако об этом явлении впервые сообщается для простой атомарной среды — сверхтекучего гелия. Мы объясняем это явление как эффект спин-фононного взаимодействия в сверхтекучей фазе He II. Наша теоретическая оценка дает верхнюю границу ~ 250 ГГц для СВЧ резонансного отклика системы при T = 0. Мы интерпретируем сверхтекучий диэлектрик 4 He как электрически активную рабочую среду для низкотемпературного MASERа и объясняем атомарный механизм СВЧ поглощения–усиления в конденсированных фазах гелия.

We consider SU(N) Chern-Simons theory coupled to a scalar field in the fundamental representation at strictly zero temperature and finite chemical potential for the global U(1)_B particle number or flavour symmetry. In the semiclassical regime we identify a Bose condensed ground state with a vacuum expectation value (VEV) for the scalar accompanied by noncommuting background gauge field matrix VEVs. These matrices coincide with the droplet ground state of the Abelian quantum Hall matrix model. The ground state spontaneously breaks U(1)_B and Higgses the gauge group whilst preserving spatial rotations and a colour-flavour locked global U(1) symmetry. We compute the perturbative spectrum of semiclassical fluctuations for the SU(2) theory and show the existence of a single massless state with a linear phonon dispersion relation and a roton minimum (and maximum) determining the Landau critical superfluid velocity. For the massless scalar theory with vanishing self interactions, the semiclassical dispersion relations and location of roton extrema take on universal forms., Comment: Version to appear in JHEP; added discussions on quantum corrections

We have studied the collective excitation of fractional quantum Hall effect (FQHE) in the rotating Bose-Einstein condensate (BEC) using CF theory at the filling fraction $\nu=1/2$. The roton type of excitation in the FQHE of electron system is established over the years for all the filling fraction, whereas the collective excitation at $\nu=1/2$ filling fraction in the rotating BEC shows no roton minimum. We have investigated this using composite fermion theory with the P$\ddot{o}$schl-Teller interaction potential between the particles. We have seen that the long range interaction will give the roton, whereas short range interaction gives no roton minimum.

Using data from equilibrium Molecular Dynamics computer simulations we have built up a catalogue of response functions for the Coulomb one-component plasma (OCP) over a wide range of $\Gamma$ coupling values, including the strongly coupled $\Gamma > 1$ liquid regime. We focus on the domain of negative compressibility ($\Gamma>3$), where causality and the concomitant Kramers-Kronig relations are violated. We give a description of the details of the violation, both in the frequency and time domains. We show that the viscoelastic pole of the density response function morphs into an imaginary pole in the upper $\omega$ half-plane that is responsible for the anomalous behavior of the response in this coupling range. By examining the plasmon dispersion relation through the dielectric response function, rather than via the peaks of the dynamical structure function, we obtain a more reliable representation for the dispersion. We demonstrate that there is an intimate link between the formation of the roton minimum in the dispersion and the negative compressibility of the system. The feasibility of the extension of our analysis to systems with a short range interaction is explored., Comment: 17 pages, 15 figures

We consider the problem of calculating the excitation spectrum of a gas of nonrelativistic anyons. When the anyons have statistics close to fermionic and the statistical angle has the form $\theta=\pi (1-\frac{1}{k})$ where $k$ is a large integer, the problem can be solved by employing the method of bosonization, which maps the problem to that of an infinite number of bosonic excitations coupled to a $U(1)$ Chern-Simons gauge field. The spectrum consists of a Goldstone boson branch and a large number of massive branches, each having roton minima and maxima. The dispersion curves asymptote to the Landau levels at large momentum., Comment: 19 pages, 2 figures; clarifications added, typos fixed

We carry out a theoretical investigation of overpressurized superfluid phases of He-4 by means of quantum Monte Carlo (QMC) simulations. As a function of density, we study structural and superfluid properties, and estimate the energy of the roton excitation by inverting imaginary-time density correlation functions computed by QMC, using Maximum Entropy. We estimate the pressure at which the roton energy vanishes to be about 100 bars, which we identify with the spinodal density, i.e., the upper limit for the existence of a metastable superfluid phase., Comment: 6 pages, 7 figures

We observe signatures of radial and angular roton excitations around a droplet crystallization transition in dipolar Bose-Einstein condensates. In situ measurements are used to characterize the density fluctuations near this transition. The static structure factor is extracted and used to identify the radial and angular roton excitations by their characteristic symmetries. These fluctuations peak as a function of interaction strength indicating the crystallization transition of the system. We compare our observations to a theoretically calculated excitation spectrum allowing us to connect the crystallization mechanism with the softening of the angular roton modes., Comment: 10 pages, 4 + 4 figures

We present the collective excitation spectrum analysis of binary Bose-Einstein condensates (BECs) with spin-orbit (SO) and Rabi couplings in a quasi-two-dimensional system. In particular, we investigate the role of SO and Rabi coupling strengths in determining the dynamical stability of the coupled BECs using Bogoliubov-de Gennes (BdG) theory. Using the eigenergy of BdG spectrum, we confirm the existence of phonon, roton, and maxon modes with weak repulsive intra- and inter-species contact interactions. The depth of the minimum corresponding to the roton mode depends strongly on the coupling strength. We find that the increase of the SO coupling leads to instability, while the increase in the Rabi coupling stabilizes the system. Also the eigenvectors of BdG spectrum indicates the presence of density like mode in the stable regime and spin like modes in unstable regimes. A phase diagram demonstrating the stability regime in the plane of SO and Rabi coupling strengths is obtained. Finally, we complement the observation of the excitation spectrum with the direct numerical simulation results of coupled Gross-Pitaevskii equations., Comment: 14 pages, 15 figures

We study the superfluid dynamics of the outer core of neutron stars by means of a hydrodynamic model made of a neutronic superfluid and a protonic superconductor, coupled by both the dynamic entrainment and the Skyrme SLy4 nucleon-nucleon interactions. The resulting nonlinear equations of motion are probed in the search for dynamical instabilities triggered by the relative motion of the superfluids that could be related to observed timing anomalies in pulsars. Through linear analysis, the origin and expected growth of the instabilities is explored for varying nuclear-matter density. Differently from previous findings, the dispersion of linear excitations in our model shows rotonic structures below the pair-breaking energy threshold, which lies at the origin of the dynamical instabilities, and could eventually lead to emergent vorticity along with modulations of the superfluid density., 10 pages, 5 figures. v2: minor changes

The paper describes the millimeter-wave electromagnetic field generator tuned to resonance with the roton line in the superfluid helium (He II) spectrum. It is shown that periodic oscillations of the normal component in the wave of the second sound are accompanied by a flux of electromagnetic radiation in the opposite direction.

In the presence of dipolar interactions the excitation spectrum of a Bose gas can acquire a local minimum. The corresponding quasiparticles are known as rotons. They are gaped and do not decay at zero temperature. Here we study the decay of rotons in one-dimensional Bose gases at low temperatures. It predominantly occurs due to the backscattering of thermal phonons on rotons. The resulting rate scales with the third power of temperature and is inversely proportional to the sixth power of the roton gap near the solidification phase transition. The hydrodynamic approach used here enables us to find the decay rate for quasiparticles at practically any momenta, with minimal assumptions on the exact form of the interparticle interactions. Our results are an essential prerequisite for the description of all the dissipative phenomena in dipolar gases and have direct experimental relevance., 6 pages, 3 figures

We study the properties of a magnetic dipolar Bose-Einstein condensate (BEC) in an elongated (cigar shaped) confining potential in the beyond quasi-one-dimensional (quasi-1D) regime. In this system the dipole-dipole interactions (DDIs) develop a momentum-dependence related to the transverse confinement and the polarization direction of the dipoles. This leads to density fluctuations being enhanced or suppressed at a length scale related to the transverse confinement length, with local atom number measurements being a practical method to observe these effects in experiments. We use meanfield theory to describe the ground state, excitations and the local number fluctuations. Quantitative predictions are presented based on full numerical solutions and a simplified variational approach that we develop. In addition to the well-known roton excitation, occurring when the dipoles are polarized along a tightly confined direction, we find an "anti-roton" effect for the case of dipoles polarized along the long axis: a nearly non-interacting ground state that experiences strongly repulsive interactions with excitations of sufficiently short wavelength., 12 pages, 8 figures

Features of the phonon spectrum of a chiral crystal are examined within the micropolar elasticity theory. This formalism accounts for not only translational micromotions of a medium but also rotational ones. It is found that there appears the phonon band splitting depending on the left/right-circular polarization in a purely phonon sector without invoking any outside subsystem. The phonon spectrum reveals parity breaking while preserving time-reversal symmetry, i.e. it possesses true chirality. We find that hybridization of the micro-rotational and translational modes gives rise to the acoustic phonon branch with a "roton" minimum reminiscent of the elementary excitations in the superfluid helium-4. We argue that a mechanism of this phenomena is in line with Nozi\`{e}res' reinterpretation [J. Low Temp. Phys. \textbf{137}, 45 (2004)] of the rotons as a manifistation of an incipient crystallization instability. We discuss a close analogy between the translational and rotational micromotions in the micropolar elastic medium and the Bogoliubov quasiparticles and gapful density fluctuations in ${}^{4}$He., Comment: Main text (6 pages, 5 figures) + supplementary material (5 pages, 1 figure); accepted for publication in Physical Review Letters

We performed numerical simulations of decaying quantum turbulence by using a generalized Gross-Pitaevskii equation, that includes a beyond mean field correction and a nonlocal interaction potential. The nonlocal potential is chosen in order to mimic He II by introducing a roton minimum in the excitation spectrum. We observe that at large scales the statistical behavior of the flow is independent of the interaction potential, but at scales smaller than the intervortex distance a Kelvin wave cascade is enhanced in the generalized model. In this range, the incompressible kinetic energy spectrum obeys the weak wave turbulence prediction for Kelvin wave cascade not only for the scaling with wave numbers but also for the energy fluxes and the intervortex distance., 14 pages, 9 figures

Rotons can exist in ultracold atomic gases either with long-range interactions or with spin-orbitcoupled dispersions. We find that two different kinds of rotons coexist in a joint system combining long-range interactions and spin-orbit coupling. One roton originates from spin-orbit coupling and two others come from long-range interactions. Their softening can be controlled separately. The interesting new phenomenon which we find is that spin-orbit-coupled roton can push down the energy of one long-range-interaction roton. The spin-orbit coupling accelerates the softening of this roton. The post phase of spin-orbit-coupling-assisted roton softening and instability is identified as a superstripe., Published Version

We investigate the dynamics of a three-dimensional Bose-Einstein condensate of ultracold atomic gases with a soft-core shape long-range interaction, which is induced by laser dressing the atoms to a highly excited Rydberg state. For a homogeneous condensate, the long-range interaction drastically alters the dispersion relation of the excitation, supporting both roton and maxon modes. Rotons are typically responsible for the creation of supersolids, while maxons are normally dynamically unstable in BECs with dipolar interactions. We show that maxon modes in the Rydberg-dressed condensate, on the contrary, are dynamically stable. We find that the maxon modes can be excited through an interaction quench, i.e. turning on the soft-core interaction instantaneously. The emergence of the maxon modes is accompanied by oscillations at high frequencies in the quantum depletion, while rotons lead to much slower oscillations. The dynamically stable excitation of the roton and maxon modes leads to persistent oscillations in the quantum depletion. Through a self-consistent Bogoliubov approach, we identify the dependence of the maxon mode on the soft-core interaction. Our study shows that maxon and roton modes can be excited dynamically and simultaneously by quenching Rydberg-dressed long-range interactions. This is relevant to current studies in creating and probing exotic states of matter with ultracold atomic gases., Comment: 10 pages, 8 figures

Using the logarithmic superfluid model of physical vacuum, one can formulate a quantum theory, which successfully recovers Einstein's theory of relativity in low-momenta limit, but otherwise has different foundations and predictions. We present an analytical example of the dispersion relation and argue that it should have a Landau "roton" form which ensures the suppression of dissipative fluctuations. We show that at small momenta, a dispersion relation becomes relativistic with small deformations, such that a photon acquires effective mass, but a much more complex picture arises at large momenta., Comment: 3 pages, 1 figure, based on the talk given at the Tenth Alexander Friedmann International Seminar on Gravitation and Cosmology, Saint Petersburg, 24-28 June 2019

Ultracold dipolar droplets have been realized in a series of ground-breaking experiments, where the stability of the droplet state is attributed to beyond-mean-field effects in the form of the celebrated Lee-Huang-Yang (LHY) correction. We scrutinize the dipolar droplet states in a one-dimensional context using a combination of analytical and numerical approaches, and identify experimentally viable parameters for accessing our findings for future experiments. In particular we identify regimes of stability in the restricted geometry, finding multiple roton instabilities as well as regions supporting quasi-one-dimensional droplet states. By applying an interaction quench to the droplet, a modulational instability is induced and multiple droplets are produced, along with bright solitons and atomic radiation. We also assess the droplets robustness to collisions, revealing population transfer and droplet fission., Comment: Expanded and revised version, J. Phys. Commun. in press

We develop a variational theory for a dipolar condensate in an elongated (cigar shaped) confinement potential. Our formulation provides an effective one-dimensional extended meanfield theory for the ground state and its collective excitations. We apply our theory to investigate the properties of rotons in the system comparing the variational treatment to a full numerical solution. We consider the effect of quantum fluctuations on the scattering length at which the roton excitation softens to zero energy., Comment: 9 pages, 5 figures

Here, it has been shown that the homogeneous state of the medium can be unstable in melts near the melting curve, in multicomponent liquids, in dense gases, as well as in superfluid helium. Nanoclusters may be spontaneously formed in the medium; their metastability is associated with the increase in orientational entropy during spontaneous rotation of the nanoparticles. With a positive energy of formation of nanoparticles, their concentration is determined by the increase in entropy of the system configuration. Like vacancies in crystals, roton clusters are structural defects of the melt.

We study the emergence of density waves in dipolar Bose-Einstein condensates (BEC) when the strength of dipole-dipole atomic interactions is periodically varied in time. The proposed theoretical model, based on the evolution of small perturbations of the background density, allows to compute the growth rate of instability (gain factor) for arbitrary set of input parameters, thus to identify the regions of instability against density waves. We find that among other modes of the system the roton mode is most effectively excited due to the contribution of sub-harmonics of the excitation frequency. The frequency of temporal oscillations of emerging density waves coincides with the half of the driving frequency, this being the hallmark of the parametric resonance, is characteristic to Faraday waves. The possibility to create density waves in dipolar BECs, which can persist after the emergence, has been demonstrated. The existence of a stationary spatially periodic solution of the nonlocal Gross-Pitaevskii equation has been discussed. The effect of three-body atomic interactions, which is relevant to condensates with increased density, upon the properties of emerging waves has been analyzed too. Significant modification of the condensate's excitation spectrum owing to three-body effects is shown., 12 pages, 11 eps figures

Author(s): Hertel, SA; Biekert, A; Lin, J; Velan, V; McKinsey, DN | Abstract: A promising technology concept for sub-GeV dark matter detection is described, in which low-temperature microcalorimeters serve as the sensors and superfluid He4 serves as the target material. We name this concept "HeRALD," helium roton apparatus for light dark matter. A superfluid helium target has several advantageous properties, including a light nuclear mass for better kinematic matching with light dark matter particles, copious production of scintillation light, extreme intrinsic radiopurity, high impedance to external vibration noise, and a unique "quantum evaporation" signal channel enabling the detection of phononlike modes via liberation of He4 atoms into a vacuum. In this concept, both scintillation photons and triplet excimers are detected using calorimeters, including calorimeters immersed in the superfluid. Kinetic excitations of the superfluid medium (rotons and phonons) are detected using quantum evaporation and subsequent atomic adsorption onto a calorimeter suspended in vacuum above the target helium. The energy of adsorption amplifies the phonon/roton signal before calorimetric sensing, producing a gain mechanism that can reduce the technology's recoil energy threshold below the calorimeter energy threshold. We describe signal production and signal sensing probabilities, and estimate the resulting electron recoil discrimination. We simulate radioactive backgrounds from gamma rays and construct an overall background spectrum expectation also including neutrons and solar neutrinos. Finally, we calculate projected sensitivities to dark matter-nucleon elastic scattering, demonstrating that even very small (sub-kg) target masses can probe wide regions of as-yet untested dark matter parameter space.

The entanglement spectrum of superfluid phases of 3He, the 3D B-phase and the planar phase in two dimensions, has been analyzed. The wavefunctions of the low-lying eigenstates, including Majorana zero modes, as well as the corresponding part of the spectrum of the entanglement Hamiltonian, have been found explicitly.

We analyze the quantum phase transition-like behavior in the lowest energy state of a two-site coupled atom-cavity system, where each cavity contains one atom but the total excitation number is not limited to two. Utilizing the variance of the total excitation number to distinguish the insulator and superfluid states, and the variance of the atomic excitation number to identify the polaritonic characteristics of these states, we find that the total excitation number plays a significant role in the lowest-energy-state phase transitions. In both the small hopping regime and the small atom-field interaction regime, we identify an interesting coexisting phase involving characteristics of both photonic superfluid and atomic insulator. For small hopping, we find that the signature of the photonic superfluid state becomes more pronounced with the increase in total excitation number, and that the boundaries of the various phases shift with respect to the case of $N=2$. In the limit of small atom-field interaction, the polaritonic superfluid region becomes broader as the total excitation number increases. We demonstrate that the variance of the total excitation number in a single site has a linear dependence on the total excitation number in the large-detuning limit., 7 pages, 9 figures, comments are welcome!

We consider excitons in a two-dimensional periodic potential and study the linear response of the excitonic superfluid to an electromagnetic wave at low and high densities. It turns out that the static structure factor for small wavevectors is very sensitive to a change of density and temperature. It is a consequence of the fact that thermal fluctuations play a crucial role at small wavevectors, since exchanging the order of the two limits, zero temperature and vanishing wavevector, leads to different results for the structure factor. This effect could be used for high accuracy measurements in the superfluid exciton phase, which might be realized by a gated electron-hole gas. The transition of the exciton system from the superfluid state to a non-superfluid state and its manifestation by light scattering are discussed., 9 pages, 5 figures

We have studied collective excitation almost all of the filling fraction, in the Jain series of positive flux attachment states. In this article we have presented excitation spectrum of all positive flux attachment states for a real two dimensional electron system to get the complete scenario. The fundamental roton energy becomes zero at the filling fractions associated with four pair of magnetic flux quanta attachment composite fermion, which suggest that there is a limitation of flux attachment to the electron to form a composite fermion in fractional quantum Hall effect. The roton energy and energy gap become zero at filling fraction ν = 1 / 2 , which shows that the gapless state ν = 1 / 2 can be naturally obtained as an extrapolation of the collective excitation of the incompressible gaped states.

We have carried out a study of the momentum distribution and of the spectrum of elementary excitations of liquid $$^4$$ He across the normal–superfluid transition temperature, using the path integral Monte Carlo method. Our results for the momentum distribution in the superfluid regime show that a kink is present in the range of momenta corresponding to the roton excitation. This effect disappears when crossing the transition temperature to the normal fluid, in a behavior currently unexplained by theory.

We theoretically investigate the spectrum of elementary excitations of a trapped dipolar quantum gas across the BEC-supersolid phase transition. Our calculations reveal the existence of distinct Higgs amplitude and Nambu-Goldstone modes that emerge from the softening roton modes of the dipolar BEC at the phase transition point. On the supersolid side of the transition, the energy of the Higgs amplitude mode increases rapidly, leading to a strong coupling to higher-lying modes. Our Letter highlights how the symmetry-breaking nature of the supersolid state translates to finite-size systems.

Experiments and theories describing the metastable phases of liquid and solid $$^4$$ He are presented and discussed. For the case of metastable liquid $$^4$$ He with respect to its gaseous phase, it is shown that different measurements of its destabilization threshold (cavitation threshold) and their comparisons to available theories reveal that the nucleation mechanism is not totally understood. Then experiments measuring typical lifetime of cavitation bubbles in He I and He II are shortly considered showing the important role of heat transport mechanism during their collapse. Finally for liquid $$^4$$ He, its metastability with respect to its solid phase and the possibility of the liquid destabilization due to the vanishing of the roton gap is presented. The last part of the review is devoted to metastable solid $$^4$$ He with respect to its liquid phase. The first experimental production of such a state is described and its destabilization limit possibly invoking the creation and proliferation of crystalline defects is discussed.

We measure the excitation spectrum of a stable dipolar Bose--Einstein condensate over a wide momentum-range via Bragg spectroscopy. We precisely control the relative strength, $\epsilon_{\rm dd}$, of the dipolar to the contact interactions and observe that the spectrum increasingly deviates from the linear phononic behavior for increasing $\epsilon_{\rm dd}$. Reaching the dipolar dominated regime $\epsilon_{\rm dd}>1$, we observe the emergence of a roton minimum in the spectrum and its softening towards instability. We characterize how the excitation energy and the strength of the density-density correlations at the roton momentum vary with $\epsilon_{\rm dd}$. Our findings are in excellent agreement with numerical calculations based on mean-field Bogoliubov theory. When including beyond-mean-field corrections, in the form of a Lee-Huang-Yang potential, we observe a quantitative deviation from the experiment, questioning the validity of such a description in the roton regime., Comment: 11 pages, 8 figures