Showing 1–50 of 87 results for author: Pi, M

$d$-wave altermagnetism revealed by resonant inelastic X-ray scattering

Authors: Guangkai Zhang, Yuehong Li, Xubin Ye, Vincent C. Morano, Sze Tung Li, Jaewon Choi, Rebecca Scatena, Shuai Tang, Maocai Pi, Mengqi Ye, Mirian Garcia-Fernandez, Alessandro Bombardi, Xiaomei Qin, Zhao Pan, Daniel G. Mazzone, Qisi Wang, Yi Lu, Yao Shen, Youwen Long

Abstract: Altermagnetism defines a third fundamental class of collinear magnetic order, featuring compensated magnetic moments with antiparallel spin alignment, yet lifted Kramers degeneracy without the need for relativistic spin-orbit coupling. Its ability to host spin-polarized electronic bands and unconventional chiral magnons makes it a promising platform for functional materials. However, experimental… ▽ More Altermagnetism defines a third fundamental class of collinear magnetic order, featuring compensated magnetic moments with antiparallel spin alignment, yet lifted Kramers degeneracy without the need for relativistic spin-orbit coupling. Its ability to host spin-polarized electronic bands and unconventional chiral magnons makes it a promising platform for functional materials. However, experimental verification has proven challenging; while circular dichroism in resonant inelastic X-ray scattering (RIXS) has been suggested as a signature of chiral magnons, it remains controversial whether this effect is an intrinsic property of altermagnetism or an artifact of experimental geometry. In this work, we resolve this debate and provide unambiguous experimental evidence of $d$-wave altermagnetism in the strongly correlated Lieb-lattice magnet La$_2$O$_3$Mn$_2$Se$_2$. The RIXS spectra exhibit a $d$-wave-symmetry circular dichroism in the magnetic excitations that vanishes in the paramagnetic phase. Through RIXS-operator symmetry analysis and exact-diagonalization calculations, we prove that the observed dichroism is a direct consequence of altermagnetic symmetry constraints, independent of magnon branch splitting. Our results provide definitive evidence for the experimental realization of $d$-wave altermagnetism in La$_2$O$_3$Mn$_2$Se$_2$ and establish circularly polarized RIXS as a highly symmetry-sensitive spectroscopic framework for detecting magnetic phases that evade conventional probes. △ Less

Submitted 17 June, 2026; originally announced June 2026.

Comments: Supplementary Information available upon request

Revisiting Thomson's model with multiply charged superfluid helium nanodroplets

Authors: Ernesto Garcia-Alfonso, Francesco Ancilotto, Manuel Barranco, Fausto Cargnoni, Nadine Halberstadt, Marti Pi

Abstract: We study superfluid helium droplets multiply charged with ions. When stable, the charges are found to reside in equilibrium close to the droplet surface, thus representing a physical realization of Thomson's model. We find the minimum radius of the helium droplet that can host a given number of ions using a model whose physical ingredients are the solvation energy of the cations, calculated within… ▽ More We study superfluid helium droplets multiply charged with ions. When stable, the charges are found to reside in equilibrium close to the droplet surface, thus representing a physical realization of Thomson's model. We find the minimum radius of the helium droplet that can host a given number of ions using a model whose physical ingredients are the solvation energy of the cations, calculated within the He-DFT approach, and their mutual Coulomb repulsion energy. Our model goes beyond the often used liquid drop model, where charges are smeared out either within the droplet or on its surface, and which neglects the solid-like helium shell around the ions. We find that below a threshold droplet radius R_0, the total energy of the system becomes higher than that of the separated system of the pristine helium droplet and the charges embedded in their solvation microcluster ("snowball"). However, the ions are still kept within the droplet by the presence of energy barriers which hinder Coulomb explosion. A further reduction of the droplet radius below a value R_expl eventually results in the disappearance of such barrier, leading to Coulomb explosion. Surprisingly, our results are rather insensitive to the ion atomic species. This makes room to discuss them in the context of intrinsic multicharged helium droplets, where the charges are triatomic He3+ ions. Our calculated values for R_expl display the correct scaling with the number of cations compared to available experimental results, at variance with other estimates for the critical radii. △ Less

Submitted 22 October, 2024; originally announced October 2024.

Comments: 12 pages, 10 figures

Large low-field magnetocaloric response in a ferromagnetic gadolinium orthophosphate

Authors: Ziyu W. Yang, Jie Zhang, Maocai Pi, Xubin Ye, Chenxu Kang, Xiaoliang Weng, Wei Tang, Hongzhi Cui, Yu-Jia Zeng, Youwen Long

Abstract: Bulk magnetic and thermodynamic measurements, along with mean-field calculations, were conducted on the ferromagnetic K3Gd5(PO4)6 powders. No magnetic ordering was observed until 2 K, while the application of an external field B > 1 T resulted in the splitting of the Gd3+ ground state multiplet and induced a non-cooperative Schottky effect. The average nearest-neighbor exchange strength |J1/kB| is… ▽ More Bulk magnetic and thermodynamic measurements, along with mean-field calculations, were conducted on the ferromagnetic K3Gd5(PO4)6 powders. No magnetic ordering was observed until 2 K, while the application of an external field B > 1 T resulted in the splitting of the Gd3+ ground state multiplet and induced a non-cooperative Schottky effect. The average nearest-neighbor exchange strength |J1/kB| is determined to be 0.017 K, which leads to a remarkably large low field magnetic entropy change ΔSm = 36.2 J kg-1 K-1 under applied field change B = 2 T at temperature T = 2 K, as well as a maximum adiabatic temperature change Tad = 10.9 K. We contend that ferromagnetic gadolinium orthophosphates serve as a promising reservoir for exploring advanced magnetic refrigerants applicable under low magnetic fields. △ Less

Submitted 31 May, 2024; originally announced May 2024.

Comments: 7 pages, 5 figures

Tracking Intrinsic Non-Hermitian Skin Effect in Lossy Lattices

Authors: Liwei Xiong, Qicheng Zhang, Xiling Feng, Yufei Leng, Min Pi, Shuaishuai Tong, Chunyin Qiu

Abstract: Non-Hermitian skin effect (NHSE), characterized by a majority of eigenstates localized at open boundaries, is one of the most iconic phenomena in non-Hermitian lattices. Despite notable experimental studies implemented, most of them witness only certain signs of the NHSE rather than the intrinsic exponential localization inherent in eigenstates, owing to the ubiquitous and inevitable background lo… ▽ More Non-Hermitian skin effect (NHSE), characterized by a majority of eigenstates localized at open boundaries, is one of the most iconic phenomena in non-Hermitian lattices. Despite notable experimental studies implemented, most of them witness only certain signs of the NHSE rather than the intrinsic exponential localization inherent in eigenstates, owing to the ubiquitous and inevitable background loss. Even worse, the experimental observation of the NHSE would be completely obscured in highly lossy cases. Here, we theoretically propose a dual test approach to eliminate the destructive loss effect and track the intrinsic NHSE that is essentially irrelevant to background loss. Experimentally, the effectiveness of this approach is precisely validated by one- and two-dimensional non-Hermitian acoustic lattices. Our study sheds new light on the previously untapped intrinsic aspect of the NHSE, which is of particular significance in non-Hermitian topological physics. △ Less

Submitted 1 December, 2023; originally announced December 2023.

Breakup of quantum liquid filaments into droplets

Authors: Francesco Ancilotto, Manuel Barranco, Marti Pi

Abstract: We have investigated how the Rayleigh-Plateau instability of a filament made of a 41K-87Rb self-bound mixture may lead to an array of identical quantum droplets, with typical breaking times which are shorter than the lifetime of the mixture. If the filament is laterally confined -- as it happens in a toroidal trap -- and atoms of one species are in excess with respect to the optimal, equilibrium r… ▽ More We have investigated how the Rayleigh-Plateau instability of a filament made of a 41K-87Rb self-bound mixture may lead to an array of identical quantum droplets, with typical breaking times which are shorter than the lifetime of the mixture. If the filament is laterally confined -- as it happens in a toroidal trap -- and atoms of one species are in excess with respect to the optimal, equilibrium ratio, the droplets are immersed into a superfluid background made by the excess species which provides global phase coherence to the system, suggesting that the droplets array in the unbalanced system may display supersolid character. This possibility has been investigated by computing the non-classical translational inertia coefficient. The filament may be a reasonable representation of a self-bound mixture subject to toroidal confinement when the bigger circle radius of the torus is much larger than the filament radius. △ Less

Submitted 22 June, 2023; v1 submitted 27 April, 2023; originally announced April 2023.

Comments: 11 pages, 8 figures

Journal ref: Published in Phys. Rev. A 107, 063312 (2023)

Self-sustained deformable rotating liquid He cylinders: The pure normal fluid $^3$He and superfluid $^4$He cases

Authors: Martí Pi, Francesco Ancilotto, Manuel Barranco, Samuel L. Butler, José María Escartín

Abstract: We have studied self-sustained, deformable, rotating liquid He cylinders of infinite length. In the normal fluid $^3$He case, we have employed a classical model where only surface tension and centrifugal forces are taken into account, as well as the Density Functional Theory (DFT) approach in conjunction with a semi-classical Thomas-Fermi approximation for the kinetic energy. In both approaches, i… ▽ More We have studied self-sustained, deformable, rotating liquid He cylinders of infinite length. In the normal fluid $^3$He case, we have employed a classical model where only surface tension and centrifugal forces are taken into account, as well as the Density Functional Theory (DFT) approach in conjunction with a semi-classical Thomas-Fermi approximation for the kinetic energy. In both approaches, if the angular velocity is sufficiently large, it is energetically favorable for the $^3$He cylinder to undergo a shape transition, acquiring an elliptic-like cross section which eventually becomes two-lobed. In the $^4$He case, we have employed a DFT approach that takes into account its superfluid character, limiting the description to vortex-free configurations where angular momentum is exclusively stored in capillary waves on a deformed cross section cylinder. The calculations allow us to carry out a comparison between the rotational behavior of a normal, rotational fluid ($^3$He) and a superfluid, irrotational fluid ($^4$He). △ Less

Submitted 22 March, 2023; originally announced March 2023.

Comments: 13 pages, 10 figures

Nanoscopic jets and filaments of superfluid He-4 at zero temperature: a DFT study

Authors: Francesco Ancilotto, Manuel Barranco, Marti Pi

Abstract: Helium droplets produced by the instability of a cryogenic helium jet exiting a source chamber leads to the formation of He drops which are considered as ideal matrices for spectroscopic studies of embedded atoms and molecules. Here, we present a He-DFT description of droplet formation resulting from jet breaking and contraction of superfluid He-4 filaments. Whereas the fragmentation of long jets… ▽ More Helium droplets produced by the instability of a cryogenic helium jet exiting a source chamber leads to the formation of He drops which are considered as ideal matrices for spectroscopic studies of embedded atoms and molecules. Here, we present a He-DFT description of droplet formation resulting from jet breaking and contraction of superfluid He-4 filaments. Whereas the fragmentation of long jets closely follows the predictions of linear theory for inviscid fluids, leading to droplet trains interspersed with smaller satellite droplets, the contraction of filaments with an aspect ratio larger than a threshold value leads to the nucleation of vortex rings which hinder their breakup into droplets. △ Less

Submitted 13 April, 2023; v1 submitted 31 January, 2023; originally announced January 2023.

Comments: 14 pages, 12 figures, published version

Journal ref: J. Chem. Phys. 158, 144306 (2023)

Physical realization of topological Roman surface by spin-induced ferroelectric polarization in cubic lattice

Authors: Guangxiu Liu, Maocai Pi, Long Zhou, Zhehong Liu, Xudong Shen, Xubin Ye, Shijun Qin, Xinrun Mi, Xue Chen, Lin Zhao, Bowen Zhou, Jia Guo, Xiaohui Yu, Yisheng Chai, Hongming Weng, Youwen Long

Abstract: Topology, a mathematical concept in geometry, has become an ideal theoretical tool for describing topological states and phase transitions. Many topological concepts have found their physical entities in real or reciprocal spaces identified by topological/geometrical invariants, which are usually defined on orientable surfaces such as torus and sphere. It is natural to quest whether it is possible… ▽ More Topology, a mathematical concept in geometry, has become an ideal theoretical tool for describing topological states and phase transitions. Many topological concepts have found their physical entities in real or reciprocal spaces identified by topological/geometrical invariants, which are usually defined on orientable surfaces such as torus and sphere. It is natural to quest whether it is possible to find the physical realization of more intriguing non-orientable surfaces. Herein, we show that the set of spin-induced ferroelectric polarizations in cubic perovskite oxides AMn3Cr4O12 (A = La and Tb) resides on the topological Roman surface, a non-orientable two-dimensional manifold formed by sewing a Mobius strip edge to that of a disc. The induced polarization may travel in a loop along the non-orientable Mobius strip or orientable disc depending on how the spin evolves as controlled by external magnetic field. Experimentally, the periodicity of polarization can be the same or the twice of the rotating magnetic field, being well consistent with the orientability of disc and Mobius strip, respectively. This path dependent topological magnetoelectric effect presents a way to detect the global geometry of the surface and deepens our understanding of topology in both mathematics and physics △ Less

Submitted 16 January, 2022; originally announced January 2022.

Comments: 46 pages, 13 figures

Journal ref: Nature Communications 13, 2373 (2022)

Merging of superfluid Helium nanodroplets with vortices

Authors: J. M. Escartin, F. Ancilotto, M. Barranco, M. Pi

Abstract: Within Density Functional Theory, we have investigated the coalescence dynamics of two superfluid helium nanodroplets hosting vortex lines in different relative orientations, which are drawn towards each other by the Van der Waals mutual attraction. We have found a rich phenomenology depending on how the vortex lines are oriented. In particular, when a vortex and anti-vortex lines are present in t… ▽ More Within Density Functional Theory, we have investigated the coalescence dynamics of two superfluid helium nanodroplets hosting vortex lines in different relative orientations, which are drawn towards each other by the Van der Waals mutual attraction. We have found a rich phenomenology depending on how the vortex lines are oriented. In particular, when a vortex and anti-vortex lines are present in the merging droplets, a dark soliton develops at the droplet contact region, which eventually decays into vortex rings. Reconnection events are observed between the vortex lines or rings, leading to the creation of more vortices. Our simulations show the interplay between vortex creation and reconnections, as well as the effect of the droplet surface which pins the vortex ends and, by reflecting short-wavelength excitations produced by the interactions between vortices, strongly affects the droplet final state. Additional vorticity is nucleated in the proximity of surface indentations produced in the course of the dynamics, which in turn interact with other vortices present in the droplets. These effects, obviously absent in the case of bulk liquid helium, show that the droplet surface may act as a multiplier of vortex reconnections. The analysis of the energy spectrum shows that vortex-antivortex ring annihilation, as well as vortex-antivortex reconnections, yields roton bursts of different intensity. △ Less

Submitted 20 October, 2021; originally announced October 2021.

Comments: 11 pages, 11 figures + Supplemental Material

The microscopic origin of the spin-induced linear and quadratic magnetoelectric effects

Authors: Maocai Pi, Xifan Xu, Mingquan He, Yisheng Chai

Abstract: Understanding of the intimate cross-coupling between electric and magnetic degrees of freedom in solids usually requires sophisticated models and time-consuming calculation methods. Instead of macroscopic symmetry analysis, we present a simple but general approach to explore the microscopic mechanism of magnetoelectric (ME) effects in magnetic ordered materials based on local spin and lattice symm… ▽ More Understanding of the intimate cross-coupling between electric and magnetic degrees of freedom in solids usually requires sophisticated models and time-consuming calculation methods. Instead of macroscopic symmetry analysis, we present a simple but general approach to explore the microscopic mechanism of magnetoelectric (ME) effects in magnetic ordered materials based on local spin and lattice symmetry analysis. Our methods are successfully applied to Cr$_2$O$_3$ and orthorhombic RMnO$_3$ with linear and quadratic ME effects, respectively. We revealed all the possible microscopic origins of every non-zero ME coefficients that cannot be easily fulfilled by other theoretical methods. Moreover, the contribution from each mechanism can be located down to specific spins or spin pairs. Our theoretical approach is capable of providing a detailed guide to explore rich spin-induced magnetoelectrics with strong ME effects. △ Less

Submitted 7 October, 2021; originally announced October 2021.

Comments: 3 figures

Journal ref: Phys. Rev. B 105, L020407 (2022)

Probing magnetic symmetry in antiferromagnetic Fe4Nb2O9 single crystals by linear magnetoelectric tensor

Authors: Jing Zhang, Na Su, Xinrun Mi, Maocai Pi, Haidong Zhou, Jinguang Cheng, Yisheng Chai

Abstract: In the present study, we investigated magnetodielectric, magnetoelectric (ME), and angular-dependent polarization in single-crystal Fe4Nb2O9. The magnetodielectric effects in epsilon(x) (x//[100]), epsilon(y) (y//[120]), and epsilon(z) (z//[001]) are found to be significant only around T-N approximate to 95 K when magnetic fields are applied along three orthogonal x-, y- (y//[120]), and z directio… ▽ More In the present study, we investigated magnetodielectric, magnetoelectric (ME), and angular-dependent polarization in single-crystal Fe4Nb2O9. The magnetodielectric effects in epsilon(x) (x//[100]), epsilon(y) (y//[120]), and epsilon(z) (z//[001]) are found to be significant only around T-N approximate to 95 K when magnetic fields are applied along three orthogonal x-, y- (y//[120]), and z directions. The finite polarization P-x, P-y, and P-z of 70, 100, and 30 mu C/m(2), respectively, can be induced in the antiferromagnetic phase when a finite magnetic field up to 9 T is applied along the three orthogonal directions. The angular-dependent polarization measurements verify the dominating linear ME effects below T-N. From the above experimental results, a linear ME tensor a(ij) with all nine nonzero components can be inferred, demonstrating a much lower magnetic point group of -1' for the canted antiferromagnetic configuration. △ Less

Submitted 23 September, 2021; originally announced September 2021.

Comments: 14 pages, 4 figures

Journal ref: Phys. Rev. B 103, L140401 (2021)

Stacking faults in $α$-RuCl$_3$ revealed by local electric polarization

Authors: Xinrun Mi, Xiao Wang, Hengrui Gui, Maochai Pi, Tingting Zheng, Kunya Yang, Yuhan Gan, Peipei Wang, Alei Li, Aifeng Wang, Liyuan Zhang, Yixi Su, Yisheng Chai, Mingquan He

Abstract: We present out-of-plane dielectric and magnetodielectric measurements of single crystallines $α$-RuCl$_3$ with various degrees of stack faults. A frequency dependent, but field independent, dielectric anomaly appears at $T_{A}\:(f=100\:\mathrm{kHz})\sim$ 4 K once both magnetic transitions at $T_{N1}\sim$ 7 K and $T_{N2}\sim$ 14 K set in. The observed dielectric anomaly is attributed to the emergen… ▽ More We present out-of-plane dielectric and magnetodielectric measurements of single crystallines $α$-RuCl$_3$ with various degrees of stack faults. A frequency dependent, but field independent, dielectric anomaly appears at $T_{A}\:(f=100\:\mathrm{kHz})\sim$ 4 K once both magnetic transitions at $T_{N1}\sim$ 7 K and $T_{N2}\sim$ 14 K set in. The observed dielectric anomaly is attributed to the emergency of possible local electric polarizations whose inversion symmetry is broken by inhomogeneously distributed stacking faults. A field-induced intermediate phase is only observed when a magnetic field is applied perpendicular to the Ru-Ru bonds for samples with minimal stacking faults. Less pronounced in-plane anisotropy is found in samples with sizable contribution from stacking imperfections. Our findings suggest that dielectric measurement is a sensitive probe in detecting the structural and magnetic properties, which may be a promising tool especially in studying $α$-RuCl$_3$ thin film devices. Moreover, the stacking details of RuCl$_3$ layers strongly affect the ground state both in the magnetic and electric channels. Such a fragile ground state against stacking faults needs to be overcome for realistic applications utilizing the magnetic and/or electric properties of Kitaev based physics in $α$-RuCl$_3$. △ Less

Submitted 28 April, 2021; originally announced April 2021.

Comments: 8 pages, 5 figures. Accepted for publication in Physical Review B

Journal ref: Phys. Rev. B 103, 174413(2021)

Dynamics of equilibration and collisions in ultradilute quantum droplets

Authors: Viktor Cikojević, Leandra Vranješ Markić, Martí Pi, Manuel Barranco, Francesco Ancilotto, Jordi Boronat

Abstract: Employing time-dependent density-functional theory, we have studied dynamical equilibration and binary head-on collisions of quantum droplets made of a $^{39}$K-$^{39}$K Bose mixture. The phase space of collision outcomes is extensively explored by performing fully three-dimensional calculations with effective single-component QMC based and two-components LHY-corrected mean-field functionals. We e… ▽ More Employing time-dependent density-functional theory, we have studied dynamical equilibration and binary head-on collisions of quantum droplets made of a $^{39}$K-$^{39}$K Bose mixture. The phase space of collision outcomes is extensively explored by performing fully three-dimensional calculations with effective single-component QMC based and two-components LHY-corrected mean-field functionals. We exhaustively explored the important effect -- not considered in previous studies -- of the initial population ratio deviating from the optimal mean-field value $N_2/N_1 = \sqrt{a_{11} / a_{22}}$. Both stationary and dynamical calculations with an initial non-optimal concentration ratio display good agreement with experiments. Calculations including three-body losses acting only on the $\left|F, m_{F}\right\rangle=|1,0\rangle$ state show dramatic differences with those obtained with the three-body term acting on the total density. △ Less

Submitted 19 April, 2021; originally announced April 2021.

Coexistence of vortex arrays and surface capillary waves in spinning prolate superfluid He-4 nanodroplets

Authors: M. Pi, J. M. Escartin, F. Ancilotto, M. Barranco

Abstract: Within Density Functional Theory, we have studied the interplay between vortex arrays and capillary waves in spinning prolate He-4 droplets made of several thousands of helium atoms. Surface capillary waves are ubiquitous in prolate superfluid He-4 droplets and, depending on the size and angular momentum of the droplet, they may coexist with vortex arrays. We have found that the equilibrium config… ▽ More Within Density Functional Theory, we have studied the interplay between vortex arrays and capillary waves in spinning prolate He-4 droplets made of several thousands of helium atoms. Surface capillary waves are ubiquitous in prolate superfluid He-4 droplets and, depending on the size and angular momentum of the droplet, they may coexist with vortex arrays. We have found that the equilibrium configuration of small prolate droplets is vortex-free, evolving towards vortex-hosting as the droplet size increases. This result is in agreement with a recent experiment [S.M. O'Connell et al., Phys. Rev. Lett. 124, 215301 (2020)], where it has been disclosed that vortex arrays and capillary waves coexist in the equilibrium configuration of very large drops. Contrarily to viscous droplets executing rigid body rotation, the stability phase diagram of spinning He-4 droplets cannot be universally described in terms of dimensionless angular momentum and angular velocity variables: instead, the rotational properties of superfluid helium droplets display a clear dependence on the droplet size and the number of vortices they host. △ Less

Submitted 10 September, 2021; v1 submitted 14 April, 2021; originally announced April 2021.

Comments: 11 pages, 6 figures

Journal ref: Phys. Rev. B 104, 094509 (2021)

Vortex properties in the extended supersolid phase of dipolar Bose-Einstein condensates

Authors: Francesco Ancilotto, Manuel Barranco, Marti Pi, Luciano Reatto

Abstract: We study the properties of singly-quantized linear vortices in the supersolid phase of a dipolar Bose-Einstein condensate at zero temperature modeling $^{164}$Dy atoms. The system is extended in the $x-y$ plane and confined by a harmonic trap in the the polarization direction $z$. Our study is based on a generalized Gross-Pitaevskii equation. We characterize the ground state of the system in terms… ▽ More We study the properties of singly-quantized linear vortices in the supersolid phase of a dipolar Bose-Einstein condensate at zero temperature modeling $^{164}$Dy atoms. The system is extended in the $x-y$ plane and confined by a harmonic trap in the the polarization direction $z$. Our study is based on a generalized Gross-Pitaevskii equation. We characterize the ground state of the system in terms of spatial order and superfluid fraction and compare the properties of a single vortex and of a vortex dipole in the superfluid phase (SFP) and in the supersolid phase (SSP). At variance with a vortex in the SFP, which is free to move in the superfluid, a vortex in the SSP is localized at the interstitial sites and does not move freely. We have computed the energy barrier for motion from an equilibrium site to another. The fact that the vortex is submitted to a periodic potential has a dramatic effect on the dynamics of a vortex dipole made of two counter rotating parallel vortices; instead of rigidly translating as in the SFP, the vortex and anti-vortex approach each other by a series of jumps from one site to another until they annihilate in a very short time and their energy is transferred to bulk excitations. △ Less

Submitted 16 March, 2021; v1 submitted 30 December, 2020; originally announced December 2020.

Comments: 13 pages, 16 figures

Journal ref: Phys. Rev. A 103, 033314 (2021)

Towards a QMC-based density functional including finite-range effects: excitation modes of a $^{39}$K quantum droplet

Authors: V. Cikojević, L. Vranješ Markić, M. Pi, M. Barranco, J. Boronat

Abstract: Some discrepancies between experimental results on quantum droplets made of a mixture of $^{39}$K atoms in different hyperfine states and their analysis within extended Gross-Pitaevskii theory (which incorporates beyond mean-field corrections) have been recently solved by introducing finite-range effects into the theory. Here, we study the influence of these effects on the monopole and quadrupole… ▽ More Some discrepancies between experimental results on quantum droplets made of a mixture of $^{39}$K atoms in different hyperfine states and their analysis within extended Gross-Pitaevskii theory (which incorporates beyond mean-field corrections) have been recently solved by introducing finite-range effects into the theory. Here, we study the influence of these effects on the monopole and quadrupole excitation spectrum of extremely dilute quantum droplets using a density functional built from first-principles quantum Monte Carlo calculations, which can be easily introduced in the existing Gross-Pitaevskii numerical solvers. Our results show differences of up to $20\%$ with those obtained within the extended Gross-Pitaevskii theory, likely providing another way to observe finite-range effects in mixed quantum droplets by measuring their lowest excitation frequencies. △ Less

Submitted 14 July, 2020; originally announced July 2020.

Rotating mixed $^3$He-$^4$He nanodroplets

Authors: Marti Pi, Francesco Ancilotto, Jose' Maria Escartin, Ricardo Mayol, Manuel Barranco

Abstract: Mixed $^3$He-$^4$He droplets created by hydrodynamic instability of a cryogenic fluid-jet may acquire angular momentum during their passage through the nozzle of the experimental apparatus. These free-standing droplets cool down to very low temperatures undergoing isotopic segregation, developing a nearly pure $^3$He crust surrounding a very $^4$He-rich superfluid core. Here, the stability and app… ▽ More Mixed $^3$He-$^4$He droplets created by hydrodynamic instability of a cryogenic fluid-jet may acquire angular momentum during their passage through the nozzle of the experimental apparatus. These free-standing droplets cool down to very low temperatures undergoing isotopic segregation, developing a nearly pure $^3$He crust surrounding a very $^4$He-rich superfluid core. Here, the stability and appearance of rotating mixed helium droplets are investigated using Density Functional Theory for an isotopic composition that highlights, with some marked exceptions related to the existence of the superfluid inner core, the analogies with viscous rotating droplets. △ Less

Submitted 10 March, 2020; v1 submitted 6 March, 2020; originally announced March 2020.

Comments: 6 pages, 4 figures (plus Supplemental Material). Revised version with corrections+missing figure

Journal ref: Phys. Rev. B 102, 060502 (2020)

Vortices in the supersolid phase of dipolar Bose-Einstein condensates

Authors: Francesco Ancilotto, Manuel Barranco, Marti Pi, Luciano Reatto

Abstract: Vortices are expected to exist in a supersolid but experimentally their detection can be difficult because the vortex cores are localized at positions where the local density is very low. We address here this problem by performing numerical simulations of a dipolar Bose-Einstein Condensate (BEC) in a pancake confinement at $T=0$ K and study the effect of quantized vorticity on the phases that can… ▽ More Vortices are expected to exist in a supersolid but experimentally their detection can be difficult because the vortex cores are localized at positions where the local density is very low. We address here this problem by performing numerical simulations of a dipolar Bose-Einstein Condensate (BEC) in a pancake confinement at $T=0$ K and study the effect of quantized vorticity on the phases that can be realized depending upon the ratio between dipolar and short-range interaction. By increasing this ratio the system undergoes a spontaneous density modulation in the form of an ordered arrangement of multi-atom "droplets". This modulated phase can be either a "supersolid" (SS) or a "normal solid" (NS). In the SS state droplets are immersed in a background of low-density superfluid and the system has a finite global superfluid fraction resulting in non-classical rotational inertia. In the NS state no such superfluid background is present and the global superfluid fraction vanishes. We propose here a protocol to create vortices in modulated phases of dipolar BEC by "freezing" into such phases a vortex-hosting superfluid (SF) state. The resulting system, depending upon the interactions strengths, can be either a SS or a NS To discriminate between these two possible outcome of a "freezing" experiment, we show that upon releasing of the radial harmonic confinement, the expanding vortex-hosting SS shows tell-tale quantum interference effects which display the symmetry of the vortex lattice of the originating SF, as opposed to the behavior of the NS which shows instead a ballistic radial expansion of the individual droplets. Such markedly different behavior might be used to prove the supersolid character of rotating dipolar condensates. △ Less

Submitted 14 July, 2021; v1 submitted 14 February, 2020; originally announced February 2020.

Comments: 7 pages, 4 figure + Supplemental Material (2 pages, 5 figures) Updated version with major revisions. arXiv admin note: text overlap with arXiv:2012.15157

Rotating He-3 droplets

Authors: Marti Pi, Francesco Ancilotto, Manuel Barranco

Abstract: Motivated by recent experiments, we study normal-phase rotating He-3 droplets within Density Functional Theory in a semi-classical approach. The sequence of rotating droplet shapes as a function of angular momentum are found to agree with those of rotating classical droplets, evolving from axisymmetric oblate to triaxial prolate to two-lobed shapes as the angular momentum of the droplet increases.… ▽ More Motivated by recent experiments, we study normal-phase rotating He-3 droplets within Density Functional Theory in a semi-classical approach. The sequence of rotating droplet shapes as a function of angular momentum are found to agree with those of rotating classical droplets, evolving from axisymmetric oblate to triaxial prolate to two-lobed shapes as the angular momentum of the droplet increases. Our results, which are obtained for droplets of nanoscopic size, are rescaled to the mesoscopic size characterizing ongoing experimental measurements, allowing for a direct comparison of shapes. The stability curve in the angular velocity-angular momentum plane shows small deviations from the classical rotating drop model predictions, whose magnitude increases with angular momentum. We attribute these deviations to effects not included in the simplified classical model description of a rotating fluid held together by surface tension, i.e. to surface diffuseness, curvature and finite compressibility, and to quantum effects associated with deformation of the He-3 Fermi surface. The influence of all these effects is expected to diminish as the droplet size increases, making the classical rotating droplet model a quite accurate representation of He-3 rotation. △ Less

Submitted 29 October, 2019; originally announced October 2019.

Comments: 8 pages, 8 figures

Angular momentum in rotating superfluid droplets

Authors: Sean M. O. OConnell, Rico Mayro P. Tanyag, Deepak Verma, Charles Bernando, Weiwu Pang, Camila Bacellar, Catherine A. Saladrigas, Johannes Mahl, Benjamin W. Toulson, Yoshiaki Kumagai, Peter Walter, Francesco Ancilotto, Manuel Barranco, Marti Pi, Christoph Bostedt, Oliver Gessner, Andrey F. Vilesov

Abstract: The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated sub-micrometer superfluid 4He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vo… ▽ More The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated sub-micrometer superfluid 4He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host. In capsule-shaped droplets, vortices form a distorted triangular lattice, whereas they arrange along elliptical contours in ellipsoidal droplets. The combined action of vortices and capillary waves results in droplet shapes close to those of classical droplets rotating with the same angular velocity. The findings are corroborated by density functional theory calculations describing the velocity fields and shape deformations of a rotating superfluid cylinder. △ Less

Submitted 28 October, 2019; originally announced October 2019.

Comments: submitted to Physical Review Letters

Vorticity and quantum turbulence in the merging of superfluid Helium nanodroplets

Authors: José María Escartín, Francesco Ancilotto, Manuel Barranco, Martí Pi

Abstract: We have studied the merging of two $^4$He droplets at zero temperature, caused by their Van der Waals mutual attraction. During the early stages of the merging, density structures appear which closely match the experimental observations by Vicente et al. [J. Low Temp. Phys. 121, 627 (2000)]. When the droplets are merging, quantized vortex-antivortex ring pairs nucleate at the surface and annihilat… ▽ More We have studied the merging of two $^4$He droplets at zero temperature, caused by their Van der Waals mutual attraction. During the early stages of the merging, density structures appear which closely match the experimental observations by Vicente et al. [J. Low Temp. Phys. 121, 627 (2000)]. When the droplets are merging, quantized vortex-antivortex ring pairs nucleate at the surface and annihilate inside the merged droplet producing a roton burst. We also observe the nucleation of quantized vortex-antivortex rings that wrap the droplet surface and remain localized on the surface until they eventually decay into short-wavelength surface waves. Analysis of the kinetic energy spectrum discloses the existence of a regime where turbulence caused by vortex interaction and annihilation is characterized by a Kolmogorov power law. This is followed by another regime where roton radiation (produced by vortex-antivortex annihilation) dominates, whose hallmark is a weak, turbulent surface dynamics. We suggest that similar processes might appear in superfluid helium droplets after they capture impurities or if they are produced by hydrodynamic instability of a liquid jet. Experiments on collisions between recently-discovered self-bound Bose-Einstein condensates should display a similar phenomenology. △ Less

Submitted 23 April, 2019; v1 submitted 7 February, 2019; originally announced February 2019.

Comments: 6 pages, 5 figures; revised version

Journal ref: Phys. Rev. B 99, 140505(R) (2019)

Self-bound ultra dilute Bose mixtures within Local Density Approximation

Authors: Francesco Ancilotto, Manuel Barranco, Montserrat Guilleumas, Marti Pi

Abstract: We have investigated self-bound binary ultra dilute bosonic mixtures at zero temperature within Density Functional Theory using a Local Density Approximation. We provide the explicit expression of the Lee-Huang-Yang correction in the general case of heteronuclear mixtures, and investigate the general thermodynamic conditions which lead to the formation of self-bound systems. We have determined the… ▽ More We have investigated self-bound binary ultra dilute bosonic mixtures at zero temperature within Density Functional Theory using a Local Density Approximation. We provide the explicit expression of the Lee-Huang-Yang correction in the general case of heteronuclear mixtures, and investigate the general thermodynamic conditions which lead to the formation of self-bound systems. We have determined the conditions for stability against the evaporation of one component, as well as the mechanical and diffusive spinodal lines. We have also calculated the surface tension of the self-bound state as a function of the inter-species interaction strength. We find that relatively modest changes of the latter result in order-of-magnitude changes in the calculated surface tension. We suggest experimental realizations which might display the metastability and phase separation of the mixture when entering regions of the phase diagram characterized by negative pressures. Finally, we show that these droplets may sustain stable vortex and vortex dimers. △ Less

Submitted 9 September, 2018; v1 submitted 28 August, 2018; originally announced August 2018.

Comments: 12 pages, 11 figures, improved version with additional material

Journal ref: Phys. Rev. A 98, 053623 (2018)

Spinning superfluid helium-4 nanodroplets

Authors: Francesco Ancilotto, Manuel Barranco, Marti Pi

Abstract: We have studied spinning superfluid $^4$He nanodroplets at zero temperature using Density Functional theory. Due to the irrotational character of the superfluid flow, the shapes of the spinning nanodroplets are very different from those of a viscous normal fluid drop in steady rotation. We show that when vortices are nucleated inside the superfluid droplets, their morphology, which evolves from ax… ▽ More We have studied spinning superfluid $^4$He nanodroplets at zero temperature using Density Functional theory. Due to the irrotational character of the superfluid flow, the shapes of the spinning nanodroplets are very different from those of a viscous normal fluid drop in steady rotation. We show that when vortices are nucleated inside the superfluid droplets, their morphology, which evolves from axisymmetric oblate to triaxial prolate to two-lobed shapes, is in good agreement with experiments. The presence of vortex arrays confers to the superfluid droplets the rigid-body behavior of a normal fluid in steady rotation, and this is the ultimate reason of the surprising good agreement between recent experiments and the classical models used for their description. △ Less

Submitted 29 May, 2018; v1 submitted 10 November, 2017; originally announced November 2017.

Comments: 5 pages, 3 figure

Journal ref: Phys. Rev. B 97, 184515 (2018)

Density Functional Theory of doped superfluid liquid helium and nanodroplets

Authors: Francesco Ancilotto, Manuel Barranco, Francois Coppens, Jussi Eloranta, Nadine Halberstadt, Alberto Hernando, David Mateo, Marti Pi

Abstract: During the last decade, density function theory (DFT) in its static and dynamic time dependent forms, has emerged as a powerful tool to describe the structure and dynamics of doped liquid helium and droplets. In this review, we summarize the activity carried out in this field within the DFT framework since the publication of the previous review article on this subject [M. Barranco et al., J. Low T… ▽ More During the last decade, density function theory (DFT) in its static and dynamic time dependent forms, has emerged as a powerful tool to describe the structure and dynamics of doped liquid helium and droplets. In this review, we summarize the activity carried out in this field within the DFT framework since the publication of the previous review article on this subject [M. Barranco et al., J. Low Temp. Phys. 142, 1 (2006)]. Furthermore, a comprehensive presentation of the actual implementations of helium DFT is given, which have not been discussed in the individual articles or are scattered in the existing literature. This is an Accepted Manuscript of an article published on August 2, 2017 by Taylor & Francis Group in Int. Rev. Phys. Chem. 36, 621 (2017), available online: http://dx.doi.org/10.1080/0144235X.2017.1351672 △ Less

Submitted 5 August, 2017; originally announced August 2017.

Comments: 113 pages, 42 figures

Journal ref: Int. Rev. Phys. Chem. 36, 621 (2017)

The onset of nanoscale dissipation in superfluid He-4 at zero temperature: the role of vortex shedding and cavitation

Authors: Francesco Ancilotto, Manuel Barranco, Marti Pi, Jussi Eloranta

Abstract: Two-dimensional flow past an infinitely long cylinder of nanoscopic radius in superfluid He-4 at zero temperature is studied by time-dependent density functional theory. The calculations reveal two distinct critical phenomena for the onset of dissipation: 1) vortex-antivortex pair shedding from the periphery of the moving cylinder and 2) appearance of cavitation in the wake, which possesses simila… ▽ More Two-dimensional flow past an infinitely long cylinder of nanoscopic radius in superfluid He-4 at zero temperature is studied by time-dependent density functional theory. The calculations reveal two distinct critical phenomena for the onset of dissipation: 1) vortex-antivortex pair shedding from the periphery of the moving cylinder and 2) appearance of cavitation in the wake, which possesses similar geometry as observed experimentally for fast moving micrometer-scale particles in superfluid He-4. Vortex pairs with the same circulation are occasionally emitted in the form of dimers, which constitute the building blocks for the Benard-von Karman vortex street structure observed in classical turbulent fluids and Bose-Einstein condensates. The cavitation induced dissipation mechanism should be common to all superfluids that are self-bound and have a finite surface tension, which include the recently discovered self-bound droplets in ultracold Bose gases. △ Less

Submitted 9 May, 2017; originally announced May 2017.

Comments: 5 pages, 6 figures

Journal ref: Phys. Rev. B 96, 064503 (2017)

Vortex arrays in nanoscopic superfluid helium droplets

Authors: Francesco Ancilotto, Marti Pi, Manuel Barranco

Abstract: We have studied the appearance of vortex arrays in a rotating helium-4 nanodroplet at zero temperature within density functional theory. Our results are compared with those for classical rotating fluid drops used to analyze the shape and vorticity in recent experiments [L.F. Gomez et al., Science 345, 906 (2014)], where vortices have been directly seen in superfluid droplets for the first time. In… ▽ More We have studied the appearance of vortex arrays in a rotating helium-4 nanodroplet at zero temperature within density functional theory. Our results are compared with those for classical rotating fluid drops used to analyze the shape and vorticity in recent experiments [L.F. Gomez et al., Science 345, 906 (2014)], where vortices have been directly seen in superfluid droplets for the first time. In agreement with the experiments, we have found that the shape of the droplet changes from pseudo-spheroid, oblate-like for a small number of vortices to a peculiar "wheel-like" shape, delimited by nearly flat upper and lower surfaces, when the number of vortices is large. Also in agreement with the experiments, we have found that the droplet remains stable well above the stability limit predicted by classical theories. △ Less

Submitted 26 January, 2015; originally announced January 2015.

Comments: 5 pages, 5 figures

Vortex arrays in a rotating superfluid He-4 nanocylinder

Authors: Francesco Ancilotto, Marti Pi, Manuel Barranco

Abstract: Within Density Functional theory, we investigate stationary many-vortex structures in a rotating $^4$He nanocylinder at zero temperature. We compute the stability diagram and compare our results with the classical model of vortical lines in an inviscid and incompressible fluid. Scaling the results to millimeter-size buckets, they can be compared with experiments on vortex arrays conducted in the p… ▽ More Within Density Functional theory, we investigate stationary many-vortex structures in a rotating $^4$He nanocylinder at zero temperature. We compute the stability diagram and compare our results with the classical model of vortical lines in an inviscid and incompressible fluid. Scaling the results to millimeter-size buckets, they can be compared with experiments on vortex arrays conducted in the past. Motivated by recent experiments that have used atomic impurities as a means of visualizing vortices in superfluid $^4$He droplets, we have also considered the formation of chains of xenon atoms along a vortex line and the interaction between xenon atoms inside the same vortex and on different neighboring vortex lines. △ Less

Submitted 17 October, 2014; originally announced October 2014.

Propagation of collective modes in non-overlapping dipolar Bose-Einstein condensates

Authors: A. Gallemí, M. Guilleumas, R. Mayol, M. Pi

Abstract: We investigate long-range effects of the dipolar interaction in Bose-Einstein condensates by solving the time-dependent 3D Gross-Pitaevskii equation. We study the propagation of excitations between non-overlapping condensates when a collective mode is excited in one of the condensates. We obtain the frequency shifts due to the long-range character of the dipolar coupling for the bilayer and also t… ▽ More We investigate long-range effects of the dipolar interaction in Bose-Einstein condensates by solving the time-dependent 3D Gross-Pitaevskii equation. We study the propagation of excitations between non-overlapping condensates when a collective mode is excited in one of the condensates. We obtain the frequency shifts due to the long-range character of the dipolar coupling for the bilayer and also the trilayer system when the dipolar mode is excited in one condensate. The propagation of the monopolar and quadrupolar modes are also investigated. The coupled-pendulum model is proposed to qualitatively explain the long range effects of the dipolar coupling. △ Less

Submitted 18 March, 2016; v1 submitted 15 November, 2013; originally announced November 2013.

Comments: 10 pages, 8 figures

Journal ref: J. Phys.: Conf. Ser. 497 012035 (2014)

Desorption of alkali atoms from 4He nanodroplets

Authors: Alberto Hernando, Manuel Barranco, Martí Pi, Evgeniy Loginov, Marina Langlet, Marcel Drabbels

Abstract: The dynamics following the photoexcitation of Na and Li atoms located on the surface of helium nanodroplets has been investigated in a joint experimental and theoretical study. Photoelectron spectroscopy has revealed that excitation of the alkali atoms via the (n+1) -> ns transition leads to the desorption of these atoms. The mean kinetic energy of the desorbed atoms, as determined by ion imaging,… ▽ More The dynamics following the photoexcitation of Na and Li atoms located on the surface of helium nanodroplets has been investigated in a joint experimental and theoretical study. Photoelectron spectroscopy has revealed that excitation of the alkali atoms via the (n+1) -> ns transition leads to the desorption of these atoms. The mean kinetic energy of the desorbed atoms, as determined by ion imaging, shows a linear dependence on excitation frequency. These experimental findings are analyzed within a three-dimensional, time-dependent density functional approach for the helium droplet combined with a Bohmian dynamics description of the desorbing atom. This hybrid method reproduces well the key experimental observables. The dependence of the observables on the impurity mass is discussed by comparing the results obtained for the 6Li and 7Li isotopes. The calculations show that the desorption of the excited alkali atom is accompanied by the creation of highly non-linear density waves in the helium droplet that propagate at supersonic velocities. △ Less

Submitted 15 January, 2012; originally announced January 2012.

Phase-slippage and self-trapping in a self-induced bosonic Josephson junction

Authors: M. Abad, M. Guilleumas, R. Mayol, M. Pi, D. M. Jezek

Abstract: A dipolar condensate confined in a toroidal trap constitutes a self-induced Josepshon junction when the dipoles are oriented perpendicularly to the trap symmetry axis and the s-wave scattering length is small enough. The ring-shaped double-well potential coming from the anisotropic character of the mean-field dipolar interaction is robust enough to sustain self-trapping dynamics, which takes place… ▽ More A dipolar condensate confined in a toroidal trap constitutes a self-induced Josepshon junction when the dipoles are oriented perpendicularly to the trap symmetry axis and the s-wave scattering length is small enough. The ring-shaped double-well potential coming from the anisotropic character of the mean-field dipolar interaction is robust enough to sustain self-trapping dynamics, which takes place when the initial population imbalance between the two wells is large. We show that in this system the self-trapping regime is directly related to a vortex-induced phase-slip dynamics. A vortex and antivortex are spontaneously nucleated in the low density regions, before a minimum of the population imbalance is reached, and then cross the toroidal section in opposite directions through the junctions.This vortex dynamics yields a phase slip between the two weakly linked condensates causing an inversion of the particle flux. △ Less

Submitted 26 September, 2011; originally announced September 2011.

Comments: 4 pages

Journal ref: Phys. Rev. A 84, 035601 (2011)

Towards a density functional description of liquid pH2

Authors: J. Navarro, F. Ancilotto, M. Barranco, M. Pi

Abstract: A finite-temperature density functional approach to describe the properties of parahydrogen in the liquid-vapor coexistence region is presented. The first proposed functional is zero-range, where the density-gradient term is adjusted so as to reproduce the surface tension of the liquid-vapor interface at low temperature. The second functional is finite-range and, while it is fitted to reproduce bu… ▽ More A finite-temperature density functional approach to describe the properties of parahydrogen in the liquid-vapor coexistence region is presented. The first proposed functional is zero-range, where the density-gradient term is adjusted so as to reproduce the surface tension of the liquid-vapor interface at low temperature. The second functional is finite-range and, while it is fitted to reproduce bulk pH2 properties only, it is shown to yield surface properties in good agreement with experiments. These functionals are used to study the surface thickness of the liquid-vapor interface, the wetting transition of parahydrogen on a planar Rb model surface, and homogeneous cavitation in bulk liquid pH2. △ Less

Submitted 20 December, 2010; originally announced December 2010.

Comments: 24 pages and 8 figures

A dipolar self-induced bosonic Josephson junction

Authors: M. Abad, M. Guilleumas, R. Mayol, M. Pi, D. M. Jezek

Abstract: We propose a new scheme for observing Josephson oscillations and macroscopic quantum self-trapping phenomena in a toroidally confined Bose-Einstein condensate: a dipolar self-induced Josephson junction. Polarizing the atoms perpendicularly to the trap symmetry axis, an effective ring-shaped, double-well potential is achieved which is induced by the dipolar interaction. By numerically solving the t… ▽ More We propose a new scheme for observing Josephson oscillations and macroscopic quantum self-trapping phenomena in a toroidally confined Bose-Einstein condensate: a dipolar self-induced Josephson junction. Polarizing the atoms perpendicularly to the trap symmetry axis, an effective ring-shaped, double-well potential is achieved which is induced by the dipolar interaction. By numerically solving the three-dimensional time-dependent Gross-Pitaevskii equation we show that coherent tunneling phenomena such as Josephson oscillations and quantum self-trapping can take place. The dynamics in the self-induced junction can be qualitatively described by a two-mode model taking into account both s-wave and dipolar interactions. △ Less

Submitted 8 March, 2011; v1 submitted 23 November, 2010; originally announced November 2010.

Comments: Major changes. Accepted for publication in EPL

Journal ref: EPL 94, 10004 (2011)

Excited electron-bubble states in superfluid helium-4: a time-dependent density functional approach

Authors: David Mateo, Dafei Jin, Manuel Barranco, Martí Pi

Abstract: We present a systematic study on the excited electron-bubble states in superfluid helium-4 using a time-dependent density functional approach. For the evolution of the 1P bubble state, two different functionals accompanied with two different time-development schemes are used, namely an accurate finite-range functional for helium with an adiabatic approximation for electron versus an efficient zero… ▽ More We present a systematic study on the excited electron-bubble states in superfluid helium-4 using a time-dependent density functional approach. For the evolution of the 1P bubble state, two different functionals accompanied with two different time-development schemes are used, namely an accurate finite-range functional for helium with an adiabatic approximation for electron versus an efficient zero-range functional for helium with a real-time evolution for electron. We make a detailed comparison between the quantitative results obtained from the two methods, which allows us to employ with confidence the optimal method for suitable problems. Based on this knowledge, we use the finite-range functional to calculate the time-resolved absorption spectrum of the 1P bubble, which in principle can be experimentally determined, and we use the zero-range functional to real-time evolve the 2P bubble for several hundreds of picoseconds, which is theoretically interesting due to the break down of adiabaticity for this state. Our results discard the physical realization of relaxed, metastable 2P electron-bubbles △ Less

Submitted 9 November, 2010; originally announced November 2010.

Comments: 16 pages, 12 figures

Journal ref: J. Chem. Phys. 134, 044507 (2011)

Evolution of the excited electron bubble in liquid 4He and the appearance of fission-like processes

Authors: David Mateo, Martí Pi, Manuel Barranco

Abstract: We have studied the evolution of an excited electron bubble in superfluid 4He for several tens of picoseconds combining the dynamics of the liquid with an adiabatic evolution for the electron. The path followed by the excited bubble in its decay to the ground state is shown to strongly depend on pressure. While for pressures below 1 bar the 1P excited electron bubble has allowance for radiatively… ▽ More We have studied the evolution of an excited electron bubble in superfluid 4He for several tens of picoseconds combining the dynamics of the liquid with an adiabatic evolution for the electron. The path followed by the excited bubble in its decay to the ground state is shown to strongly depend on pressure. While for pressures below 1 bar the 1P excited electron bubble has allowance for radiatively decay to the deformed ground state, evolving then non-radiatively towards the ground state of the spherical electron bubble, we have found that above 1 bar two distinct baby bubbles appear in the course of the dynamical evolution, pointing to a different relaxation path in which the electron may be localized in one of the baby bubbles while the other collapses, allowing for a pure radiationless de-excitation. Our calculations are in agreement with experiments indicating that relaxed 1P bubbles are only observed for pressures smaller than a critical one, of the order of 1 bar, and that above this value the decay of the excited bubble has to proceed differently. A similar analysis carried out for the 2P bubble shows that the adiabatic approximation fails at an early stage of its dynamical evolution due to the crossing of the 2P and 1F states. △ Less

Submitted 28 April, 2010; originally announced April 2010.

Comments: 29 pages, 10 figures, to be published in Phys. Rev. B

Journal ref: Phys. Rev. B 81, 174510 (2010)

Dipolar condensates confined in a toroidal trap: ground state and vortices

Authors: M. Abad, M. Guilleumas, R. Mayol, M. Pi, D. M. Jezek

Abstract: We study a Bose-Einstein condensate of 52Cr atoms confined in a toroidal trap with a variable strength of s-wave contact interactions. We analyze the effects of the anisotropic nature of the dipolar interaction by considering the magnetization axis to be perpendicular to the trap symmetry axis. In the absence of a central repulsive barrier, when the trap is purely harmonic, the effect of reducin… ▽ More We study a Bose-Einstein condensate of 52Cr atoms confined in a toroidal trap with a variable strength of s-wave contact interactions. We analyze the effects of the anisotropic nature of the dipolar interaction by considering the magnetization axis to be perpendicular to the trap symmetry axis. In the absence of a central repulsive barrier, when the trap is purely harmonic, the effect of reducing the scattering length is a tuning of the geometry of the system: from a pancake-shaped condensate when it is large, to a cigar-shaped condensate for small scattering lengths. For a condensate in a toroidal trap, the interaction in combination with the central repulsive Gaussian barrier produces an azimuthal dependence of the particle density for a fixed radial distance. We find that along the magnetization direction the density decreases as the scattering length is reduced but presents two symmetric density peaks in the perpendicular axis. For even lower values of the scattering length we observe that the system undergoes a dipolar-induced symmetry breaking phenomenon. The whole density becomes concentrated in one of the peaks, resembling an origin-displaced cigar-shaped condensate. In this context we also analyze stationary vortex states and their associated velocity field, finding that this latter also shows a strong azimuthal dependence for small scattering lengths. The expectation value of the angular momentum along the z direction provides a qualitative measure of the difference between the velocity in the different density peaks. △ Less

Submitted 20 January, 2010; originally announced January 2010.

Comments: 9 pages, 12 figures

Journal ref: Phys. Rev. A 81, 043619 (2010)

Energy barriers for vortex nucleation in dipolar condensates

Authors: M. Abad, M. Guilleumas, R. Mayol, M. Pi, D. M. Jezek

Abstract: We consider singly-quantized vortex states in a condensate of 52Cr atoms in a pancake trap. We obtain the vortex solutions by numerically solving the Gross-Pitaevskii equation in the rotating frame with no further approximations. The behavior of the condensate is studied under three different situations concerning the interactions: only s-wave, s-wave plus dipolar and only dipolar. The energy ba… ▽ More We consider singly-quantized vortex states in a condensate of 52Cr atoms in a pancake trap. We obtain the vortex solutions by numerically solving the Gross-Pitaevskii equation in the rotating frame with no further approximations. The behavior of the condensate is studied under three different situations concerning the interactions: only s-wave, s-wave plus dipolar and only dipolar. The energy barrier for the nucleation of a vortex is calculated as a function of the vortex displacement from the rotation axis in the three cases. These results are compared to those obtained for contact interaction condensates in the Thomas-Fermi approximation, and to a pseudo-analytical model, showing this latter a very good agreement with the numerical calculation. △ Less

Submitted 29 October, 2009; originally announced October 2009.

Comments: Submitted to Laser Physics

Vortices in dipolar condensates with dominant dipolar interactions

Authors: M. Abad, M. Guilleumas, R. Mayol, M. Pi, D. M. Jezek

Abstract: We present full three-dimensional numerical calculations of single vortex states in rotating dipolar condensates. We consider a Bose-Einstein condensate of 52Cr atoms with dipole-dipole and s-wave contact interactions confined in an axially symmetric harmonic trap. We obtain the vortex states by numerically solving the Gross-Pitaevskii equation in the rotating frame with no further approximation… ▽ More We present full three-dimensional numerical calculations of single vortex states in rotating dipolar condensates. We consider a Bose-Einstein condensate of 52Cr atoms with dipole-dipole and s-wave contact interactions confined in an axially symmetric harmonic trap. We obtain the vortex states by numerically solving the Gross-Pitaevskii equation in the rotating frame with no further approximations. We investigate the properties of a single vortex and calculate the critical angular velocity for different values of the s-wave scattering length. We show that, whereas the standard variational approach breaks down in the limit of pure dipolar interactions, exact solutions of the Gross-Pitaevskii equation can be obtained for values of the s-wave scattering length down to zero. The energy barrier for the nucleation of a vortex is calculated as a function of the vortex displacement from the rotation axis for different values of the angular velocity of the rotating trap. △ Less

Submitted 23 June, 2009; originally announced June 2009.

Comments: 10 pages with 9 figures

Journal ref: Phys. Rev. A 79, 063622 (2009)

Isospin phases of vertically coupled double quantum rings under the influence of perpendicular magnetic fields

Authors: M. Royo, F. Malet, M. Barranco, M. Pi, J. Planelles

Abstract: Vertically coupled double quantum rings submitted to a perpendicular magnetic field $B$ are addressed within the local spin-density functional theory. We describe the structure of quantum ring molecules containing up to 40 electrons considering different inter-ring distances and intensities of the applied magnetic field. When the rings are quantum mechanically strongly coupled, only bonding stat… ▽ More Vertically coupled double quantum rings submitted to a perpendicular magnetic field $B$ are addressed within the local spin-density functional theory. We describe the structure of quantum ring molecules containing up to 40 electrons considering different inter-ring distances and intensities of the applied magnetic field. When the rings are quantum mechanically strongly coupled, only bonding states are occupied and the addition spectrum of the artificial molecules resembles that of a single quantum ring, with some small differences appearing as an effect of the magnetic field. Despite the latter has the tendency to flatten the spectra, in the strong coupling limit some clear peaks are still found even when $B\neq 0$ that can be interpretated from the single-particle energy levels analogously as at zero applied field, namely in terms of closed-shell and Hund's-rule configurations. Increasing the inter-ring distance, the occupation of the first antibonding orbitals washes out such structures and the addition spectra become flatter and irregular. In the weak coupling regime, numerous isospin oscillations are found as a function of $B$. △ Less

Submitted 18 September, 2008; originally announced September 2008.

Comments: 27 pages, 11 figures. To be published in Phys. Rev. B

Magnesium doped helium nanodroplets

Authors: Alberto Hernando, Manuel Barranco, Ricardo Mayol, Marti Pi, Francesco Ancilotto

Abstract: We have studied the structure of $^4$He droplets doped with magnesium atoms using density functional theory. We have found that the solvation properties of this system strongly depend on the size of the $^4$He droplet. For small drops, Mg resides in a deep surface state, whereas for large size drops it is fully solvated but radially delocalized in their interior. We have studied the $3s3p$ $^1$P… ▽ More We have studied the structure of $^4$He droplets doped with magnesium atoms using density functional theory. We have found that the solvation properties of this system strongly depend on the size of the $^4$He droplet. For small drops, Mg resides in a deep surface state, whereas for large size drops it is fully solvated but radially delocalized in their interior. We have studied the $3s3p$ $^1$P$_1 \leftarrow 3s^2$ $^1$S$_0$ transition of the dopant, and have compared our results with experimental data from laser induced fluorescence (LIF). Line broadening effects due to the coupling of dynamical deformations of the surrounding helium with the dipole excitation of the impurity are explicitly taken into account. We show that the Mg radial delocalization inside large droplets may help reconcile the apparently contradictory solvation properties of magnesium as provided by LIF and electron-impact ionization experiments. The structure of $^4$He drops doped with two magnesium atoms is also studied and used to interpret the results of resonant two-photon-ionization (R2PI) and LIF experiments. We have found that the two solvated Mg atoms do not easily merge into a dimer, but rather form a weakly-bound state due to the presence of an energy barrier caused by the helium environment that keep them some 9.5 Å apart, preventing the formation of the Mg$_2$ molecule. From this observation, we suggest that Mg atoms in $^4$He drops may form, under suitable conditions, a soft ``foam''-like aggregate rather than coalesce into a compact metallic cluster. Our findings are in qualitative agreement with recent R2PI experimental evidences. We predict that, contrarily, Mg atoms adsorbed in $^3$He droplets do not form such metastable aggregates. △ Less

Submitted 11 July, 2008; originally announced July 2008.

Absorption spectrum of Ca atoms attached to $^4$He nanodroplets

Authors: Alberto Hernando, Manuel Barranco, Marek Krośnicki, Ricardo Mayol, Martí Pi

Abstract: Within density functional theory, we have obtained the structure of $^4$He droplets doped with neutral calcium atoms. These results have been used, in conjunction with newly determined {\it ab-initio} $^1Σ$ and $^1Π$ Ca-He pair potentials, to address the $4s4p$ $^1$P$_1 \leftarrow 4s^2$ $^1$S$_0$ transition of the attached Ca atom, finding a fairly good agreement with absorption experimental dat… ▽ More Within density functional theory, we have obtained the structure of $^4$He droplets doped with neutral calcium atoms. These results have been used, in conjunction with newly determined {\it ab-initio} $^1Σ$ and $^1Π$ Ca-He pair potentials, to address the $4s4p$ $^1$P$_1 \leftarrow 4s^2$ $^1$S$_0$ transition of the attached Ca atom, finding a fairly good agreement with absorption experimental data. We have studied the drop structure as a function of the position of the Ca atom with respect of the center of mass of the helium moiety. The interplay between the density oscillations arising from the helium intrinsic structure and the density oscillations produced by the impurity in its neighborhood plays a role in the determination of the equilibrium state, and hence in the solvation properties of alkaline earth atoms. In a case of study, the thermal motion of the impurity within the drop surface region has been analyzed in a semi-quantitative way. We have found that, although the atomic shift shows a sizeable dependence on the impurity location, the thermal effect is statistically small, contributing by about a 10% to the line broadening. The structure of vortices attached to the calcium atom has been also addressed, and its effect on the calcium absorption spectrum discussed. At variance with previous theoretical predictions, we conclude that spectroscopic experiments on Ca atoms attached to $^4$He drops will be likely unable to detect the presence of quantized vortices in helium nanodrops. △ Less

Submitted 12 November, 2007; originally announced November 2007.

Comments: 33 pages with 11 figures

The structure and energetics of $^3$He and $^4$He nanodroplets doped with alkaline earth atoms

Authors: A. Hernando, R. Mayol, M. Pi, M. Barranco, F. Ancilotto, O. Bünermann, F. Stienkemeier

Abstract: We present systematic results, based on density functional calculations, for the structure and energetics of $^3$He and $^4$He nanodroplets doped with alkaline earth atoms. We predict that alkaline earth atoms from Mg to Ba go to the center of $^3$He drops, whereas Ca, Sr, and Ba reside in a deep dimple at the surface of $^4$He drops, and Mg is at their center. For Ca and Sr, the structure of th… ▽ More We present systematic results, based on density functional calculations, for the structure and energetics of $^3$He and $^4$He nanodroplets doped with alkaline earth atoms. We predict that alkaline earth atoms from Mg to Ba go to the center of $^3$He drops, whereas Ca, Sr, and Ba reside in a deep dimple at the surface of $^4$He drops, and Mg is at their center. For Ca and Sr, the structure of the dimples is shown to be very sensitive to the He-alkaline earth pair potentials used in the calculations. The $5s5p\leftarrow5s^2$ transition of strontium atoms attached to helium nanodroplets of either isotope has been probed in absorption experiments. The spectra show that strontium is solvated inside $^3$He nanodroplets, supporting the calculations. In the light of our findings, we emphasize the relevance of the heavier alkaline earth atoms for analyzing mixed $^3$He-$^4$He nanodroplets, and in particular, we suggest their use to experimentally probe the $^3$He-$^4$He interface. △ Less

Submitted 8 May, 2007; originally announced May 2007.

Comments: Typeset using Revtex, 20 pages and 8 Postscript files

Optical response of two-dimensional few-electron concentric double quantum rings: A local-spin-density-functional theory study

Authors: Francesc Malet, Martí Pi, Manuel Barranco, Enrico Lipparini, Llorenç Serra

Abstract: We have investigated the dipole charge- and spin-density response of few-electron two-dimensional concentric nanorings as a function of the intensity of a perpendicularly applied magnetic field. We show that the dipole response displays signatures associated with the localization of electron states in the inner and outer ring favored by the perpendicularly applied magnetic field. Electron locali… ▽ More We have investigated the dipole charge- and spin-density response of few-electron two-dimensional concentric nanorings as a function of the intensity of a perpendicularly applied magnetic field. We show that the dipole response displays signatures associated with the localization of electron states in the inner and outer ring favored by the perpendicularly applied magnetic field. Electron localization produces a more fragmented spectrum due to the appearance of additional edge excitations in the inner and outer ring. △ Less

Submitted 30 October, 2006; originally announced October 2006.

Comments: To be published in Physical Review B

Journal ref: Phys. REv. B74 193309 (2006)

Spin-orbit effects in GaAs quantum wells: Interplay between Rashba, Dresselhaus, and Zeeman interactions

Authors: Enrico Lipparini, Manuel Barranco, Francesc Malet, Martí Pi, Llorenç Serra

Abstract: The interplay between Rashba, Dresselhaus and Zeeman interactions in a quantum well submitted to an external magnetic field is studied by means of an accurate analytical solution of the Hamiltonian, including electron-electron interactions in a sum rule approach. This solution allows to discuss the influence of the spin-orbit coupling on some relevant quantities that have been measured in inelas… ▽ More The interplay between Rashba, Dresselhaus and Zeeman interactions in a quantum well submitted to an external magnetic field is studied by means of an accurate analytical solution of the Hamiltonian, including electron-electron interactions in a sum rule approach. This solution allows to discuss the influence of the spin-orbit coupling on some relevant quantities that have been measured in inelastic light scattering and electron-spin resonance experiments on quantum wells. In particular, we have evaluated the spin-orbit contribution to the spin splitting of the Landau levels and to the splitting of charge- and spin-density excitations. We also discuss how the spin-orbit effects change if the applied magnetic field is tilted with respect to the direction perpendicular to the quantum well. △ Less

Submitted 8 May, 2006; originally announced May 2006.

Comments: 26 pages (with 3 figures included)

Journal ref: Phys. Rev. B74 115303 (2006)

Electronic structure of few-electron concentric double quantum rings

Authors: J. I. Climente, J. Planelles, M. Barranco, F. Malet, M. Pi

Abstract: The ground state structure of few-electron concentric double quantum rings is investigated within the local spin density approximation. Signatures of inter-ring coupling in the addition energy spectrum are identified and discussed. We show that the electronic configurations in these structures can be greatly modulated by the inter-ring distance: At short and long distances the low-lying electron… ▽ More The ground state structure of few-electron concentric double quantum rings is investigated within the local spin density approximation. Signatures of inter-ring coupling in the addition energy spectrum are identified and discussed. We show that the electronic configurations in these structures can be greatly modulated by the inter-ring distance: At short and long distances the low-lying electron states localize in the inner and outer rings, respectively, and the energy structure is essentially that of an isolated single quantum ring. However, at intermediate distances the electron states localized in the inner and the outer ring become quasi-degenerate and a rather entangled, strongly-correlated system is formed. △ Less

Submitted 22 March, 2006; originally announced March 2006.

Comments: 16 pages (preprint format), 6 figures

Vertically coupled double quantum rings at zero magnetic field

Authors: Francesc Malet, Manuel Barranco, Enrico Lipparini, Ricardo Mayol Martí Pi, Juan Ignacio Climente, Josep Planelles

Abstract: Within local-spin-density functional theory, we have investigated the `dissociation' of few-electron circular vertical semiconductor double quantum ring artificial molecules at zero magnetic field as a function of inter-ring distance. In a first step, the molecules are constituted by two identical quantum rings. When the rings are quantum mechanically strongly coupled, the electronic states are… ▽ More Within local-spin-density functional theory, we have investigated the `dissociation' of few-electron circular vertical semiconductor double quantum ring artificial molecules at zero magnetic field as a function of inter-ring distance. In a first step, the molecules are constituted by two identical quantum rings. When the rings are quantum mechanically strongly coupled, the electronic states are substantially delocalized, and the addition energy spectra of the artificial molecule resemble those of a single quantum ring in the few-electron limit. When the rings are quantum mechanically weakly coupled, the electronic states in the molecule are substantially localized in one ring or the other, although the rings can be electrostatically coupled. The effect of a slight mismatch introduced in the molecules from nominally identical quantum wells, or from changes in the inner radius of the constituent rings, induces localization by offsetting the energy levels in the quantum rings. This plays a crucial role in the appearance of the addition spectra as a function of coupling strength particularly in the weak coupling limit. △ Less

Submitted 2 March, 2006; originally announced March 2006.

Comments: 18 pages, 8 figures, submitted to Physical Review B

Journal ref: Phys. Rev. B73 245324 (2006)

Condensation of helium in nanoscopic alkali wedges at zero temperature

Authors: E. S. Hernández, F. Ancilotto, M. Barranco, R. Mayol, Martí Pi

Abstract: We present a complete calculation of the structure of liquid $^4$He confined to a concave nanoscopic wedge, as a function of the opening angle of the walls. This is achieved within a finite-range density functional formalism. The results here presented, restricted to alkali metal substrates, illustrate the change in meniscus shape from rather broad to narrow wedges on weak and strong alkali adso… ▽ More We present a complete calculation of the structure of liquid $^4$He confined to a concave nanoscopic wedge, as a function of the opening angle of the walls. This is achieved within a finite-range density functional formalism. The results here presented, restricted to alkali metal substrates, illustrate the change in meniscus shape from rather broad to narrow wedges on weak and strong alkali adsorbers, and relate this change to the wetting behavior of helium on the corresponding planar substrate. As the wedge angle is varied, we find a sequence of stable states that, in the case of cesium, undergo one filling and one emptying transition at large and small openings, respectively. A computationally unambiguous criterion to determine the contact angle of $^4$He on cesium is also proposed. △ Less

Submitted 21 February, 2006; originally announced February 2006.

Journal ref: Phys. Rev. B73 245406 (2006)

Spin-orbit effects on the Larmor dispersion relation in GaAs quantum wells

Authors: Francesc Malet, Enrico Lipparini, Manuel Barranco, Martí Pi

Abstract: We have studied the relevance of spin-orbit coupling to the dispersion 00009 relation of the Larmor resonance observed in inelastic light scattering and electron-spin resonance experiments on GaAs quantum wells. We show that the spin-orbit interaction, here described by a sum of Dresselhaus and Bychkov-Rashba terms, couples Zeeman and spin-density excitations. We have evaluated its contribution… ▽ More We have studied the relevance of spin-orbit coupling to the dispersion 00009 relation of the Larmor resonance observed in inelastic light scattering and electron-spin resonance experiments on GaAs quantum wells. We show that the spin-orbit interaction, here described by a sum of Dresselhaus and Bychkov-Rashba terms, couples Zeeman and spin-density excitations. We have evaluated its contribution to the spin splitting as a function of the magnetic field $B$, and have found that in the small $B$ limit, the spin-orbit interaction does not contribute to the spin splitting, whereas at high magnetic fields it yields a $B$ independent contribution to the spin splitting given by $2(λ_R^2-λ_D^2)$, with $λ_{R,D}$ being the intensity of the Bychkov-Rashba and Dresselhaus spin-orbit terms. △ Less

Submitted 27 January, 2006; originally announced January 2006.

Comments: To be published in Physical Review B

Journal ref: Phys. Rev. B73 125302 (2006)

Electron bubbles in liquid helium: infrared-absorption spectrum

Authors: Víctor Grau, Manuel Barranco, Ricardo Mayol, Martí Pi

Abstract: Within Density Functional Theory, we have calculated the energy of the transitions from the ground state to the first two excited states in the electron bubbles in liquid helium at pressures from zero to about the solidification pressure. For $^4$He at low temperatures, our results are in very good agreement with infrared absorption experiments. Above a temperature of $\sim 2$ K, we overestima… ▽ More Within Density Functional Theory, we have calculated the energy of the transitions from the ground state to the first two excited states in the electron bubbles in liquid helium at pressures from zero to about the solidification pressure. For $^4$He at low temperatures, our results are in very good agreement with infrared absorption experiments. Above a temperature of $\sim 2$ K, we overestimate the energy of the $1s-1p$ transition. We attribute this to the break down of the Franck-Condon principle due to the presence of helium vapor inside the bubble. Our results indicate that the $1s-2p$ transition energies are sensitive not only to the size of the electron bubble, but also to its surface thickness. We also present results for the infrared transitions in the case of liquid $^3$He, for which we lack of experimental data. △ Less

Submitted 20 January, 2006; originally announced January 2006.

Comments: 17 pages and 9 figures. Accepted Phys. Rev. B

Ground state structure and conductivity of quantum wires of infinite length and finite width

Authors: Francesc Malet, Marti Pi, Manuel Barranco, Enrico Lipparini

Abstract: We have studied the ground state structure of quantum strips within the local spin-density approximation, for a range of electronic densities between $\sim$ 5$\times10^4$ and 2$\times10^6$ cm$^{-1}$ and several strengths of the lateral confining potential. The results have been used to address the conductance $G$ of quantum strips. At low density, when only one subband is occupied, the system is… ▽ More We have studied the ground state structure of quantum strips within the local spin-density approximation, for a range of electronic densities between $\sim$ 5$\times10^4$ and 2$\times10^6$ cm$^{-1}$ and several strengths of the lateral confining potential. The results have been used to address the conductance $G$ of quantum strips. At low density, when only one subband is occupied, the system is fully polarized and $G$ takes a value which is close to 0.7(2e$^2/h$), decreasing with increasing electron density in agreement with experiments. At higher densities the system becomes paramagnetic and $G$ takes a value near (2e$^2/h$), showing a similar decreasing behaviour with increasing electron density. In both cases, the physical parameter that determines the value of the conductance is the ratio $K/K_0$ of the compressibility of the system over the free one. △ Less

Submitted 15 November, 2005; originally announced November 2005.

Comments: Accepted for publication in Physical Review B

Bose-Fermi Mixtures in Optical Lattices

Authors: M. Centelles, M. Guilleumas, M. Barranco, R. Mayol, M. Pi

Abstract: Using mean field theory, we have studied Bose-Fermi mixtures in a one-dimensional optical lattice in the case of an attractive boson-fermion interaction. We consider that the fermions are in the degenerate regime and that the laser intensities are such that quantum coherence across the condensate is ensured. We discuss the effect of the optical lattice on the critical rotational frequency for vo… ▽ More Using mean field theory, we have studied Bose-Fermi mixtures in a one-dimensional optical lattice in the case of an attractive boson-fermion interaction. We consider that the fermions are in the degenerate regime and that the laser intensities are such that quantum coherence across the condensate is ensured. We discuss the effect of the optical lattice on the critical rotational frequency for vortex line creation in the Bose-Einstein condensate, as well as how it affects the stability of the boson-fermion mixture. A reduction of the critical frequency for nucleating a vortex is observed as the strength of the applied laser is increased. The onset of instability of the mixture occurs for a sizeably lower number of fermions in the presence of a deep optical lattice. △ Less

Submitted 7 November, 2005; originally announced November 2005.

Comments: 7 pages, 6 figures, revtex4, 14th International Laser Physics Workshop (LPHYS'05)

Journal ref: Laser Physics 16, 360 (2006)