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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

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 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

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.

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.

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.

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

Motivated by a recent experiment [L.Chomaz et al., Nature Physics 14, 442 (2018)], we perform numerical simulations of a dipolar Bose-Einstein Condensate (BEC) in a tubular confinement at T=0 within Density Functional Theory, where the beyond-mean-field correction to the ground state energy is included in the Local Density Approximation. We study the excitation spectrum of the system by solving the corresponding Bogoliubov-de Gennes equations. The calculated spectrum shows a roton minimum, and the roton gap decreases by reducing the effective scattering length. As the roton gap disappears, the system spontaneously develops in its ground-state a periodic, linear structure formed by denser clusters of atomic dipoles immersed in a dilute superfluid background. This structure shows the hallmarks of a supersolid system, i.e. (i) a finite non-classical translational inertia along the tube axis and (ii) the appearance, besides the phonon mode, of the Nambu-Goldstone gapless mode corresponding to phase fluctuations, and related to the spontaneous breaking of the gauge symmetry. A further decrease in the scattering length eventually leads to the formation of a periodic linear array of self-bound droplets., Comment: 5 pages, 4 figures (version accepted for publication in PRA Rapid Communications)

We study by means of first principle Quantum Monte Carlo simulations the ground state phase diagram of a system of dipolar bosons with aligned dipole moments, and with the inclusion of a two-body repulsive potential of varying range. The system is shown to display a supersolid phase in a relatively broad region of the phase diagram, featuring different crystalline patterns depending on the density and on the range of the repulsive part of the interaction (scattering length). The supersolid phase is sandwiched between a classical crystal of parallel filaments and a homogeneous superfluid phase. We show that a "roton" minimum appears in the elementary excitation spectrum of the superfluid as the system approaches crystallization., Comment: 7 pages, 8 figures

Important information for the roton-maxon spectrum of a flattened dipolar Bose-Einstein condensate is extracted by applying a static perturbation exhibiting a periodic in-plane modulation. By solving the Gross-Pitaevskii equation in the presence of the weak perturbation we evaluate the linear density response of the system and use it, together with sum rules, to provide a Feynman-like upper-bound prediction for the excitation spectrum, finding excellent agreement with the predictions of full Bogoliubov calculations. By suddenly removing the static perturbation, while still maintaining the trap, we find that the density modulations -- as well as the weights of the perturbation-induced side peaks of the momentum distribution -- undergo an oscillatory behavior with double the characteristic frequency of the excitation spectrum. The measurement of the oscillation periods could provide an easy determination of dispersion relations., 6 pages, 3 figures

The competition of dipole-dipole and contact interactions leads to exciting new physics in dipolar gases, well-illustrated by the recent observation of quantum droplets and rotons in dipolar condensates. We show that the combination of the roton instability and quantum stabilization leads under proper conditions to a novel regime that presents supersolid properties, due to the coexistence of stripe modulation and phase coherence. In a combined experimental and theoretical analysis, we determine the parameter regime for the formation of coherent stripes, whose lifetime of a few tens of milliseconds is limited by the eventual destruction of the stripe pattern due to three-body losses. Our results open intriguing prospects for the development of long-lived dipolar supersolids., Comment: Revised version with a theoretical analysis not present in the previous version

We present the results of computer simulations of the motion of an electron bubble through superfluid helium-4 when acted upon by an electric field. The simulations are based on an extended version of the Gross–Pitaevskii equation. The temperature is assumed to be sufficiently low for the drag exerted on the bubble by thermal excitations to be negligible, and the calculations are made for velocities below the critical velocitie for nucleation of vortices and roton production. We calculate the effective mass $$m*$$ of the bubble and obtain results in excellent agreement with the measurements of Poitrenaud and Williams, and Ellis, McClintock, and Bowley.

Based on numerous experimental data on inelastic neutron and X-ray scattering in liquids, we assert that the phonon–roton spectrum of collective excitations, predicted by Landau for superfluid helium, is a universal property of the liquid state. We show that the existence of the roton minimum in the spectrum of collective excitations is caused by the short-range order in liquids. Using the virial theorem, we assume that one more branch of excitations should exist in He II, whose energy spectrum differs from the phonon–roton spectrum. Such excitations are associated with the pole of single-particle Green function, which can have a gap at small values of momenta.

Theoretical study of the electrical activity of the saturated superfluid helium (He II) film upon the relative motion of the normal and superfluid components in the film was performed. The polarization vector due to the dipole moments of the quantized vortex rings in He II in the field of van der Waals forces was calculated taking into account the relative motion of the normal and superfluid components. An explicit analytical expression for the electric potential difference arising upon the relative motion of the normal and superfluid components in a torsional oscillator was derived. The obtained time, temperature and relative velocity dependences of the potential difference were in agreement with the experimental data.

We report on the first observation of 3He diffusion anisotropy in 3He–4He liquid mixture confined in ordered aerogels at 1.5–4.2 K temperatures. The used aerogels are arrays of long Al2O3 parallel 8 nm strands. The possible origins of diffusion anisotropy are considered and the changes of roton properties introduced by parallel aerogel strands are discussed. Among the responsible mechanisms we account for Knudsen diffusion, potential anisotropy of layer mode excitations or of bulklike excitations, and helium vortices. The observed reduced 3He diffusion in aerogels is discussed and suggested to appear due to helium excitations at strong confinement conditions. These observations pave the way for future experiments to gain insight into the crossover regime expected at lower temperatures (below 1 K) for which roton density is lower and 3He collisions with strands play significant role.

We study a quasi-two dimensional gas of bosonic dipoles, calculating the beyond mean field corrections to the ground state energy and chemical potential neglecting the transverse mode structure. We show that the corrections are sensitive to the high momentum part of the interaction and cannot be expressed solely in terms of the scattering length and the dipole strength. While nonuniversal, the correction is found to be negative, which provides an additional attractive term in the extended Gross-Pitaevskii equation, enhancing the roton instability.

Formation of vortex rings around moving spherical objects in superfluid He-4 at 0 K is modeled by time-dependent density functional theory. The simulations provide detailed information of the microscopic events that lead to vortex ring emission through characteristic observables such as liquid current circulation, drag force, and hydrodynamic mass. A series of simulations were performed to determine velocity thresholds for the onset of dissipation as a function of the sphere radius up to 1.8 nm and at external pressures of zero and 1 bar. The threshold was observed to decrease with the sphere radius and increase with pressure thus showing that the onset of dissipation does not involve roton emission events (Landau critical velocity), but rather vortex emission (Feynman critical velocity), which is also confirmed by the observed periodic response of the hydrodynamic observables as well as visualization of the liquid current circulation. An empirical model, which considers the ratio between the boundary layer kinetic and vortex ring formation energies, is presented for extrapolating the current results to larger length scales. The calculated critical velocity value at zero pressure for a sphere that mimics an electron bubble is in good agreement with the previous experimental observations at low temperatures. The stability of the system against symmetry breaking was linked to its ability to excite quantized Kelvin waves around the vortex rings during the vortex shedding process. At high vortex ring emission rates, the downstream dynamics showed complex vortex ring fission and reconnection events that appear similar to those seen in previous Gross-Pitaevskii theory-based calculations, and which mark the onset of turbulent behavior., Comment: 23 pages, 7 figures

${\mathrm{RbFe}}^{2+}{\mathrm{Fe}}^{3+}{\mathrm{F}}_{6}$ is an example of a charge ordered antiferromagnet where iron sites, with differing valences, are structurally separated into two interpenetrating sublattices. The low temperature magnetically ordered ${\mathrm{Fe}}^{2+}$ ($S=2$) and ${\mathrm{Fe}}^{3+}$ ($S=5/2$) moments form a noncollinear orthogonal structure with the ${\mathrm{Fe}}^{3+}$ site displaying a reduced static ordered moment. Neutron spectroscopy on single crystals finds two distinct spin wave branches with a dominant coupling along the ${\mathrm{Fe}}^{3+}$ chain axis ($b$ axis). High resolution spectroscopic measurements find an intense energy and momentum broadened magnetic band of scattering bracketing a momentum-energy region where two magnon processes are kinematically allowed. These anharmonic excitations are enhanced in this noncollinear magnet owing to the orthogonal spin arrangement.

The concept of a roton, a special kind of elementary excitation, forming a minimum of energy at finite momentum, has been essential to understand the properties of superfluid 4He 1. In quantum liquids, rotons arise from the strong interparticle interactions, whose microscopic description remains debated 2. In the realm of highly-controllable quantum gases, a roton mode has been predicted to emerge due to magnetic dipole-dipole interactions despite of their weakly-interacting character 3. This prospect has raised considerable interest 4–12; yet roton modes in dipolar quantum gases have remained elusive to observations. Here we report experimental and theoretical studies of the momentum distribution in Bose-Einstein condensates of highly-magnetic erbium atoms, revealing the existence of the long-sought roton mode. Following an interaction quench, the roton mode manifests itself with the appearance of symmetric peaks at well-defined finite momentum. The roton momentum follows the predicted geometrical scaling with the inverse of the confinement length along the magnetisation axis. From the growth of the roton population, we probe the roton softening of the excitation spectrum in time and extract the corresponding imaginary roton gap. Our results provide a further step in the quest towards supersolidity in dipolar quantum gases 13.

We use a slave-rotor approach within a mean-field theory to study the competition of metallic, Mott-insulating, and superconducting phases of spin-3/2 fermions subjected to a periodic optical lattice potential. In addition to the metallic, the Mott-insulating, and the superconducting phases that are associated with the gauge symmetry breaking of the spinon field, we identify a novel emerging superconducting phase that breaks both roton and spinon field gauge symmetries. This novel superconducting phase emerges as a result of the competition between spin-0 singlet and spin-2 quintet interaction channels naturally available for spin-3/2 systems. The two superconducting phases can be distinguished from each other by quasiparticle weight. We further discuss the properties of these phases for both two-dimensional square and three-dimensional cubic lattices at zero and finite temperatures., Comment: 10 pages and 6 figures. Published version

Atoms excited into high-lying Rydberg states and under strong dipole-dipole interactions exhibit phenomena associated with highly correlated and complex systems. We perform first principles numerical simulations on the dynamics of such systems. The emergence of a roton minimum in the excitation spectrum, as expected in strongly correlated gases and accurately described by Feynman's theory of liquid helium, is shown to significantly inhibit particle transport, with a strong suppression of the diffusion coefficient, due to the emerging spatial order. We also demonstrate how the ability to temporally tune the interaction strength among Rydberg atoms can be used in order to overcome the effects of disorder induced heating, allowing the study of unprecedented highly coupled regimes.

Frustrated magnetic compounds, in particular low-dimensional, are topical research due to persistent uncover of novel nontrivial quantum states and potential applications. The problem of this field is that many important results are scattered over the localized parameter ranges, while areas in between still contain hidden interesting effects. We consider $J_1-J_2-J_3$ Heisenberg model on the square lattice and use the spherically symmetric self-consistent approach for spin-spin Green's functions in "quasielastic" approximation. We have found a new local order in spin liquids: antiferromagnetic isotropical helices. On the structure factor we see circular concentric dispersionless structures, while on any radial direction the excitation spectrum has "roton" minima. That implies nontrivial magnetic excitations and consequences in magnetic susceptibility and thermodynamics. On the $J_1-J_2-J_3$ exchange parameters globe we discover a crossover between antiferromagnetic-like local order and ferromagnetic-like; we find stripe-like order in the middle. In fact, our "quasielastic" approach allows investigation of the whole $J_1-J_2-J_3$ globe., Comment: 9 pages, 12 figures

The anomalous thermodynamic properties of the paradigmatic frustrated spin-1/2 triangular lattice Heisenberg antiferromagnet (TLH) has remained an open topic of research over decades, both experimentally and theoretically. Here we further the theoretical understanding based on the recently developed, powerful exponential tensor renormalization group (XTRG) method on cylinders and stripes in a quasi one-dimensional (1D) setup, as well as a tensor product operator approach directly in 2D. The observed thermal properties of the TLH are in excellent agreement with two recent experimental measurements on the virtually ideal TLH material Ba$_8$CoNb$_6$O$_{24}$. Remarkably, our numerical simulations reveal two crossover temperature scales, at $T_l/J \sim 0.20$ and $T_h/J\sim 0.55$, with $J$ the Heisenberg exchange coupling, which are also confirmed by a more careful inspection of the experimental data. We propose that in the intermediate regime between the low-temperature scale $T_l$ and the higher one $T_h$, the gapped "roton-like" excitations are activated with a strong chiral component and a large contribution to thermal entropies, which suppress the incipient 120$^\circ$ order that emerges for temperatures below $T_l$., Comment: 4+15 pages, 4 + 16 figures

We consider a two-component Bose-condensed mixture characterized by positive s-wave scattering lengths. We assume equal densities and intra-species interactions. By doing the Bogoliubov transformation of an effective Hamiltonian we obtain the lower energy magnon dispersion incorporating the superfluid entrainment between the components. We argue that p-wave pairing of distinct bosons should be accompanied by self-localization of magnons and formation of a magnetoroton. We demonstrate the effect on a model system of particles interacting via step potentials., Comment: 5 pages, 1 figure

We investigate the behavior of the collective excitations of adsorbed $^4$He in an ordered hexagonal mesopore, examining the crossover from a thin film to a confined fluid. Here we present the inelastic scattering results as a function of filling at constant temperature. We find a monotonic transition of the maxon excitation as a function of filling. This has been interpreted as corresponding to an increasing density of the adsorbed helium, which approaches the bulk value as filling increases. The roton minimum exhibits a more complicated behavior that does not monotonically approach bulk values as filling increases. The full pore scattering resembles the bulk liquid accompanied by a layer mode. The maxon and roton scattering, taken together, at intermediate fillings does not correspond to a single bulk liquid dispersion at negative, low, or high pressure., Comment: 9 pages, 10 figures

Liquid helium and spin-1/2 cold-atom Fermi gases both exhibit in their superfluid phase two distinct types of excitations, gapless phonons and gapped rotons or fermionic pair-breaking excitations. In the long wavelength limit, revising and extending Landau and Khalatnikov's theory initially developed for helium [ZhETF 19, 637 (1949)], we obtain universal expressions for three- and four-body couplings among these two types of excitations. We calculate the corresponding phonon damping rates at low temperature and compare them to those of a pure phononic origin in high-pressure liquid helium and in strongly interacting Fermi gases, paving the way to experimental observations.

An analysis of the existing theoretical ideas as to the relationship between the spectrum of collective density excitations and the spectrum of single-particle excitations in liquids with a Bose-Einstein condensate. Using available experimental results, we examine the problem of the relationship between the collective phonon-roton excitations in different liquids, and the Bose-Einstein condensate.

We show a possibility for a thermodynamically equilibrium nanocrystalline structure consisting of nanosized solid inclusions to appear in a melt just beyond the melting curve. Thermodynamic stability of the nanocrystalline structure in the melt results from the free energy lowering due to rotational motion of nanoparticles. The main contribution to the reduction of the free energy of the system is due to an increase in the rotational entropy and change in formation energy of nanocrystals, i.e. the nanocrystalline structure in the melt, like vacancies in a crystal, is an equilibrium defect structure of the melt. It is demonstrated that similar nanocrystalline structures can also appear in the vapour phase in the form of liquid nanodrops and in liquid solutions, e.g. in He II.

A necessary condition for the damping of the long-wavelength excitations of the superfluid phase (referred to as superfluid phonons) due to the three-particle process is to have an anomalous phonon dispersion. The existence of anomalous phonon dispersion has been confirmed in superfluid $$^4\mathrm {He}$$ . There are no experimental data suggesting that this phenomenon exists in superfluid Fermi gases. To the best of our knowledge, the existence of anomalous dispersion has been theoretically predicted only in atomic spin balanced Fermi gas close to the unitarity limit. The numerical results reported here suggest that the anomalous long-wavelength dispersion can be realized in mass and spin imbalanced atomic Fermi gases away from the unitary limit. In particular, the numerical solution of the Bethe–Salpeter equation in a weak-coupling regime shows that the long-wavelength part of the collective-mode dispersion of the superfluid Fulde–Ferrell phase of a mixture of population-imbalanced Lithium-6 and Potassium-40 atoms in a square lattice at some values of polarization, interacting strength and temperature initially bends upward before bending over.