546 research outputs found
Optical realization of the two-site Bose-Hubbard model in waveguide lattices
A classical realization of the two-site Bose-Hubbard Hamiltonian, based on
light transport in engineered optical waveguide lattices, is theoretically
proposed. The optical lattice enables a direct visualization of the
Bose-Hubbard dynamics in Fock space.Comment: to be published, J Phys. B (Fast Track Communication
Low-temperature lattice effects in the spin-liquid candidate -(BEDT-TTF)Cu(CN)
The quasi-two-dimensional organic charge-transfer salt
-(BEDT-TTF)Cu(CN) is one of the prime candidates for a
quantum spin-liquid due the strong spin frustration of its anisotropic
triangular lattice in combination with its proximity to the Mott transition.
Despite intensive investigations of the material's low-temperature properties,
several important questions remain to be answered. Particularly puzzling are
the 6\,K anomaly and the enigmatic effects observed in magnetic fields. Here we
report on low-temperature measurements of lattice effects which were shown to
be particularly strongly pronounced in this material (R. S. Manna \emph{et
al.}, Phys. Rev. Lett. \textbf{104}, 016403 (2010)). A special focus of our
study lies on sample-to-sample variations of these effects and their
implications on the interpretation of experimental data. By investigating
overall nine single crystals from two different batches, we can state that
there are considerable differences in the size of the second-order phase
transition anomaly around 6\,K, varying within a factor of 3. In addition, we
find field-induced anomalies giving rise to pronounced features in the sample
length for two out of these nine crystals for temperatures 9 K. We
tentatively assign the latter effects to -induced magnetic clusters
suspected to nucleate around crystal imperfections. These -induced effects
are absent for the crystals where the 6\,K anomaly is most strongly pronounced.
The large lattice effects observed at 6\,K are consistent with proposed pairing
instabilities of fermionic excitations breaking the lattice symmetry. The
strong sample-to-sample variation in the size of the phase transition anomaly
suggests that the conversion of the fermions to bosons at the instability is
only partial and to some extent influenced by not yet identified
sample-specific parameters
Nonlinear Self-Trapping of Matter Waves in Periodic Potentials
We report the first experimental observation of nonlinear self-trapping of
Bose-condensed 87Rb atoms in a one dimensional waveguide with a superimposed
deep periodic potential . The trapping effect is confirmed directly by imaging
the atomic spatial distribution. Increasing the nonlinearity we move the system
from the diffusive regime, characterized by an expansion of the condensate, to
the nonlinearity dominated self-trapping regime, where the initial expansion
stops and the width remains finite. The data are in quantitative agreement with
the solutions of the corresponding discrete nonlinear equation. Our results
reveal that the effect of nonlinear self-trapping is of local nature, and is
closely related to the macroscopic self-trapping phenomenon already predicted
for double-well systems.Comment: 5 pages, 4 figure
Quantum dynamics of a binary mixture of BECs in a double well potential: an Holstein-Primakoff approach
We study the quantum dynamics of a binary mixture of Bose-Einstein
condensates (BEC) in a double-well potential starting from a two-mode
Bose-Hubbard Hamiltonian. Focussing on the regime where the number of atoms is
very large, a mapping onto a SU(2) spin problem together with a
Holstein-Primakoff transformation is performed. The quantum evolution of the
number difference of bosons between the two wells is investigated for different
initial conditions, which range from the case of a small imbalance between the
two wells to a coherent spin state. The results show an instability towards a
phase-separation above a critical positive value of the interspecies
interaction while the system evolves towards a coherent tunneling regime for
negative interspecies interactions. A comparison with a semiclassical approach
is discussed together with some implications on the experimental realization of
phase separation with cold atoms.Comment: 12 pages, 7 figures, accepted for publication in J. Phys.
Control of unstable macroscopic oscillations in the dynamics of three coupled Bose condensates
We study the dynamical stability of the macroscopic quantum oscillations
characterizing a system of three coupled Bose-Einstein condensates arranged
into an open-chain geometry. The boson interaction, the hopping amplitude and
the central-well relative depth are regarded as adjustable parameters. After
deriving the stability diagrams of the system, we identify three mechanisms to
realize the transition from an unstable to stable behavior and analyze specific
configurations that, by suitably tuning the model parameters, give rise to
macroscopic effects which are expected to be accessible to experimental
observation. Also, we pinpoint a system regime that realizes a
Josephson-junction-like effect. In this regime the system configuration do not
depend on the model interaction parameters, and the population oscillation
amplitude is related to the condensate-phase difference. This fact makes
possible estimating the latter quantity, since the measure of the oscillating
amplitudes is experimentally accessible.Comment: 25 pages, 12 figure
Analisis Perbandingan Kapasitas Balok Komposit Baja-Beton dengan Steel Headed Stud dan UNP Stud
Penghubung geser atau stud memiliki peranan penting dalam menghasilkan perilaku komposit baja-beton. Terdapat dua jenis stud yang direkomendasikan, yaitu steel headed stud atau paku berkepala dan UNP stud atau kanal. Penelitian ini menganalisis perbandingan kapasitas balok komposit baja-beton dengan penghubung geser jenis steel headed stud dan UNP stud. Pemodelan menggunakan program elemen hingga dipilih sebagai metode dalam analisis perilaku non-linier. Luas permukaan kedua jenis penghubung geser dibuat sama sehingga kuat nominal, jumlah, dan jarak pemasangan penghubung geser pun juga akan sama untuk kedua jenis pemodelan ini. Kapasitas yang ditinjau meliputi tegangan pada balok baja WF, tegangan pelat beton, tegangan pada stud, konsentrasi tegangan pada balok baja dan pelat beton, serta defleksi pada balok komposit. Sebelum melakukan pemodelan dengan program elemen hingga, proses desain sesuai SNI 1727:2015 dilakukan terlebih dahulu untuk menentukan dimensi struktur yang akan digunakan. Validasi diperlukan untuk melihat tingkat keakuratan pemodelan yang dilakukan. Metode validasi dilakukan dengan membandingkan nilai tegangan lentur pada serat bagian bawah dan atas balok komposit baja-beton. Persentase pemodelan mencapai 86,95% untuk model steel headed stud dan 87,4% untuk UNP stud. Hasil pemodelan menunjukkan bahwa balok komposit dengan UNP stud memilik kapasitas yang lebih baik karena menghasilkan nilai tegangan-tegangan dan defleksi yang lebih kecil. Tegangan lentur balok baja UNP stud dan steel headed stud adalah 19,129 MPa dan 19,556 MPa (perbedaan 2,18 %). Tegangagan lentur pelat beton UNP stud dan steel headed stud adalah 1,21 MPa dan 1,194 MPa (perbedaan 1,34 %). Defleksi balok komposit dengan UNP stud dan steel headed stud adalah 0.478 mm dan 0,435 mm (perbedaan 8,99 %)
Quantum Bose Josephson Junction with binary mixtures of BECs
We study the quantum behaviour of a binary mixture of Bose-Einstein
condensates (BEC) in a double-well potential starting from a two-mode
Bose-Hubbard Hamiltonian. We focus on the small tunneling amplitude regime and
apply perturbation theory up to second order. Analytical expressions for the
energy eigenvalues and eigenstates are obtained. Then the quantum evolution of
the number difference of bosons between the two potential wells is fully
investigated for two different initial conditions: completely localized states
and coherent spin states. In the first case both the short and the long time
dynamics is studied and a rich behaviour is found, ranging from small amplitude
oscillations and collapses and revivals to coherent tunneling. In the second
case the short-time scale evolution of number difference is determined and a
more irregular dynamics is evidenced. Finally, the formation of Schroedinger
cat states is considered and shown to affect the momentum distribution.Comment: 14 pages, 4 figure
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