Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter
For a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameters the emergence of a phase transition is reported and temperature dependences of energy and heat capacity are studied in detail. Estimations towards a possibility to check the obtained jumps in the speci...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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Rovenchak, A. 2017-05-31T10:23:28Z 2017-05-31T10:23:28Z 2013 Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter / A. Rovenchak // Физика низких температур. — 2013. — Т. 39, № 10. — С. 1141–1145. — Бібліогр.: 26 назв. — англ. 0132-6414 PACS: 05.30.Pr, 67.10.Fj,64.60.–i https://nasplib.isofts.kiev.ua/handle/123456789/118821 For a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameters the emergence of a phase transition is reported and temperature dependences of energy and heat capacity are studied in detail. Estimations towards a possibility to check the obtained jumps in the specific heat are made. I am grateful to Yuri Krynytskyi and Prof. Volodymyr Tkachuk for discussion and useful comments. Remarks from the anonymous referees are highly appreciated. This work was partly supported by Project ФФ-110Ф (registration No. 0112U001275) from the Ministry of Edu-cation and Sciences, Youth and Sports of Ukraine. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Поиск новых сверхтекучих систем Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter Article published earlier |
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Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter |
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Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter Rovenchak, A. Поиск новых сверхтекучих систем |
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Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter |
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Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter |
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Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter |
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Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter |
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phase transition in a system of 1d harmonic oscillators obeying polychronakos statistics with a complex parameter |
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Rovenchak, A. |
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Rovenchak, A. |
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Поиск новых сверхтекучих систем |
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Поиск новых сверхтекучих систем |
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2013 |
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English |
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Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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For a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameters the emergence of a phase transition is reported and temperature dependences of energy and heat capacity are studied in detail. Estimations towards a possibility to check the obtained jumps in the specific heat are made.
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0132-6414 |
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https://nasplib.isofts.kiev.ua/handle/123456789/118821 |
| citation_txt |
Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameter / A. Rovenchak // Физика низких температур. — 2013. — Т. 39, № 10. — С. 1141–1145. — Бібліогр.: 26 назв. — англ. |
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2025-11-26T08:16:49Z |
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2025-11-26T08:16:49Z |
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1850615306314055680 |
| fulltext |
Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 10, pp. 1141–1145
Phase transition in a system of 1D harmonic oscillators
obeying Polychronakos statistics with a complex
parameter
Andrij Rovenchak
Department for Theoretical Physics, Ivan Franko National University of Lviv
12 Drahomanov Str., Lviv, UA-79005, Ukraine
E-mail: andrij.rovenchak@gmail.com
Received February 25, 2013, revised March 28, 2013
For a system of 1D harmonic oscillators obeying Polychronakos statistics with a complex parameters
the emergence of a phase transition is reported and temperature dependences of energy and heat capacity are
studied in detail. Estimations towards a possibility to check the obtained jumps in the specific heat are made.
PACS: 05.30.Pr Fractional statistics systems (anyons, etc.);
67.10.Fj Quantum statistical theory;
64.60.–i General studies of phase transitions.
Keywords: Polychronakos statistics, phase transitions.
1. Introduction
In recent decades, quantum systems obeying fractional
statistics have been a subject of studies based on different
approaches [1]. In real physical systems a description
based on an intermediate statistics between the Fermi and
Bose cases appears in a number of possibilities: the so-
called exclusion statistics can be linked to the interaction in
1D systems [2]; ultracold gases can be described by means
of anyon–fermion mapping [3]; thermodynamics of finite
Bose-systems can be studied using the Gentile statistics
[4]; and this list can be expanded further.
In this work, a generalization of the Polychronakos
statistics [5–7] is studied. The parameter of statistics is
considered a complex number [8]. Complex-valued
physical quantities can occur effectively in some physical
systems: complex energy is connected to a dissipative
process [9]; complex chemical potential is also used, e.g.,
in quantum chromodynamics [10] or physics of semicon-
ductors [11]; complex external potential is applied to
describe the interaction between the light field and atoms
moving in crystals [12,13] and also can be used to study
-symmetry breaking [14].
The one-dimensional problem is considered here for the
sake of simplicity. A system of 1D harmonic oscillators
effectively corresponds to a 2D homogenous system,
which opens a possibility to apply the obtained results for
studies of, e.g., critical phenomena in liquid-helium films
[15–17] or surface-electron systems [18].
The paper is organized as follows. In Sec. 2 statistics is
explained and calculation procedure is described. In Sec. 3
analytical expressions for the thermodynamic functions of
a 1D harmonic oscillator system are derived. Main results
are obtained in Sec. 4, where the critical temperature is
calculated and temperature dependence of energy and heat
capacity is analyzed. A discussion on possible experimen-
tal observation of the phase transition is given in Sec. 5.
2. Initial expressions
In the Polychronakos statistics [5,6] the occupation of
the jth level with energy jε is given by
/1
1= ,
e
j Tj
n
z
ε− − α
(1)
where z is fugacity and T is temperature. Here jn is a
formal characteristic and can take a complex values. Note
that this form for the occupation numbers was also
suggested a priori by Acharya and Narayana Swamy to
describe anyons [19].
To make a generalization, the parameter = e =iπνα
= i′ ′′α + α is put a complex number; = 0–1ν provides
a smooth transition between the bosonic and fermionic
© Andrij Rovenchak, 2013
Andrij Rovenchak
limits, while the value = 0α corresponding to the clas-
sical Boltzmann statistics is avoided, see Fig. 1 [8].
It can be shown that in the limit 0ν → a simple
correspondence can be established between a system with
the real excitation spectrum jε obeying the Polychronakos
statistics generalized as above and a system with a small
dissipative term in the excitation spectrum obeying the
Bose statistics. If the elementary excitation spectrum is
j jiε + γ , where jγ corresponds to small energy dissipation,
for high levels with 1j the link is =j jTγ πν ε [8].
Such systems are seen as most prospective candidates for
experimental observations of the effects discussed in the
present work.
Calculations are made using a simple scheme as
follows.
The number of particles N is given by
/1
= = ,
e
j
j j Tjj j
g
N g n
z
ε− − α
∑ ∑ (2)
where jg is the degeneration of the jth level. Elementary
excitation spectrum jε is assumed real. Since N is a real
number, fugacity =z z iz′ ′′+ must be a complex number.
Energy is then given by
/1
= = =
e
j j
j j j Tjj j
g
E g n i
z
ε−
ε
ε + Γ
− α
∑ ∑ (3)
with = ( , )z z N T defined from Eq. (2). Note that only such
values of the statistics parameter ν and other quantities
are physically reasonable which correspond to Γ ;
otherwise the system becomes essentially nonequilibrium
making the whole subsequent consideration incorrect.
Real and imaginary parts of the heat capacity equal
= , = .d dC
dT dT
Γ
Θ
(4)
The peculiarities of the temperature behavior are better
seen on the specific heat curves, and these will be given a
closer look further.
3. 1D oscillators
Let us consider a system of 1D harmonic oscillators
with frequency ω. The degeneracy of the jth level equals
unity, = 1jg . The excitation spectrum is
= , = 0, 1, 2, 3,j j jε ω (5)
The quantity ω is also used as a unit of temperature.
One-dimensional systems of oscillators can be realized
in highly anisotropic harmonic traps [20] or in optical
lattices [21,22].
To obtain results in an analytic form, the summation in
Eqs. (2), (3) is substituted with integration using the densi-
ty of state function ( )g ε : ( )j d g→ ∫ ε ε∑ . The accuracy
of this approximation increases for the number of particles
being large and if temperature is large comparing to the
level separation, T ω . Numerical comparison of the
results obtained using exact summation and integration
shows a very good agreement already for the number of
particles = 1000N .
For a system of 1D harmonic oscillators ( ) = constg ε ,
which makes it equivalent to, e.g., a system of free
particles trapped to a 1D harmonic potential (in the semi-
classical approach [23]) or a 2D system of free particles in
a box.
In the case under consideration, ( ) = 1/g ε ω . The num-
ber of particles is then
1 /
0
1= = ln (1 )
e T
d TN z
z
∞
− ε
ε
− − α
ω ωα− α∫
(6)
giving fugacity as
( )/1= 1 e .N Tz − ω α−
α
(7)
The energy equals
2
21 /
0
1= = ( ) =Li
e T
d TE z
z
∞
− ε
ε ε
α
ω ωα− α∫
( )
2
/
2= 1 e ,Li N TT − ω α−
ωα
(8)
where ( )Lis x is the polylogarithm:
=1
( ) = .Li
k
s s
k
xx
k
∞
∑ (9)
This series definition is valid for all complex s and
complex x with | | < 1x ; for other values of x an analytic
continuation is used.
Using the obvious property
1
( ) 1Li = ( )Lis
s
d x x
dx x −
Fig. 1. Statistics explanation.
+1
Bose
–1
Fermi
(Boltzmann)
′α
′′α α plane
e
iπν
1142 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 10
Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics
and taking into account that 1( ) = ln(1 )Li x x− − , for the
heat capacity one obtains
( )/
2/
/ 2= = 1 e .Li
1 e
N T
N T
dE N T TC i N N
dT N
− ω α
ω α
ω α
+ Θ + −
ω α−
(10)
As one should expect for extensive quantities, E N∝ and
C N∝ , which is not immediately seen from the above
expressions (8) and (10). To ensure extensive nature of
energy and heat capacity, thermodynamic limit must be
taken into consideration, which in the case of 1D harmonic
oscillators reads = constNω . Such a result can be obtained
in a number of different ways [4,20], cf. also [24]. The
easiest interpretation of such a thermodynamic limit can be
given as follows. If a D-dimensional system is contained in
a harmonic potential with frequency ω, classical turning
points confine the particles to some characteristic length
1/r ∝ ω . To keep an average density / DN r constant, the
requirement = constDNω must be satisfied.
Thus, in the case under consideration
const= = and = ,
const
T NT NT N
N T T
ω
ω ω
therefore, the conditions E N∝ and C N∝ hold.
4. Critical temperature and behavior
of thermodynamic functions
Discontinuities are observed in both energy and heat
capacity as functions of temperature. These discontinuities
are due to 2( )Li zα , where the argument is changing the
sign of the imaginary part.
Indeed, in the calculation of the integral one obtains
singularity “1/0” when
/ / /e = 0 = e 1 e =T T N Tzε ε − ω α− α − +
/= e 1 e cos sin
N
T T N Ni
T T
ω ′− αε ω ω ′′ ′′− + α − α
(11)
if
= and cos = ( 1) < 0.nN n n
T
ω ′′α π π −
A set of critical temperature values can thus be defined as
follows:
( ) = , = 0, 1, 2, 3, ,
(2 1)
k
cT N k
k
′′α
ω
+ π
(12)
cf. Fig. 2. For brevity, the index “(k)” will be dropped in
further derivations.
To estimate the contribution from the singularity into
thermodynamic function, the following procedure can be
applied. Let the singularity corresponds to 0ε such that
/0e =T zε α . The energy E can be expressed as follows:
0
/ /
0 0
= =
e eT T
z d z dE
z z
ε −∆∞
ε ε
ε ε ε ε
+
ω ω− α − α∫ ∫
0
/ // 0
0 0
,
e e eT TT
z d z d
z
ε +∆∞
ε εε
ε +∆ ε −∆
ε ε ε ε
+ +
ω ω− α −
∫ ∫
(13)
where ∆ is a small positive number. In the limit of 0∆ →
first two terms give a smooth function of temperature
smoothE , and the last term can be written only to the first
order of 0( )ε − ε :
0
/0smooth
0
0
= ( ) e .TzT dE E T
ε +∆
−ε
ε −∆
ε ε
+
ω ε − ε∫
(14)
While 0ε is real for = cT T , it becomes complex as soon as
T deviates from the critical point. The sign of the imagi-
nary part is defined by sin ( / )N T′′ω α , see (11). There-
fore, the denominator 0( )ε − ε must be substituted with
0( )iε − ε η for = 0cT T ± , where 0η → + . Using Sochocki's
formulas [25], one can write
0
0 0
1 1= v.p. ( ),i
i
± πδ ε − ε
ε − ε η ε − ε
(15)
where “v.p.” denotes principal value and 0( )δ ε − ε is
Dirac's delta-function.
Thus, in the vicinity of critical points energy equals
/0smooth 0( 0) = ( ) ( ) e .Tc cc c c
T
E T E T i z T −ε± ± π ε
ω
(16)
For the real part one obtains
/00( ) = ( 0) ( 0) = 2 ( ) e .Tc cc c c c
T
T T T z T −ε′′∆ + − − − π ε
ω
(17)
Taking into account the definition of critical tempera-
ture (12) and writing the statistics parameter in the form
Fig. 2. Real part of the specific heat C/N at = 0.25ν . Vertical
lines show critical temperatures. The right-most one corresponds
to = 0k .
0.2
0.4
0.6
0.8
1.0
0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
C
N
/
/T Nω
Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 10 1143
Andrij Rovenchak
= cos siniα πν + πν, after simple transformations the fol-
lowing results can be obtained for jumps of the real parts
of energy and heat capacity:
( )
3
(2 1) cot
2
( )1 2sin= ln 1 e
(2 1)
kcT
N N k
− + π πν∆ πν
+
ω + π
, (18)
( )
2
(2 1) cot1 4sin( ) = ln 1 e
(2 1)
k
cC T
N k
− + π πνπν
∆ +
+
. (19)
In the bosonic limit 0ν → :
2 3
(2 1)/
2
( )1 2= e
(2 1)
kcT
N N k
− + ν∆ π ν
ω +
, (20)
2 2
(2 1)/1 4( ) = e
(2 1)
k
cC T
N k
− + νπ ν
∆
+
. (21)
The jumps are finite as soon as > 0ν , but the most pro-
nounced one corresponds to = 0k and then they rapidly
vanish for > 0k , see Table 1. Note that for values 0.35ν
the oscillations of ( )C T in the low-temperature domain
increase and lead to some unphysical regions with negative
specific heat.
Table 1. Jumps of the specific heat /C N at the critical
temperature
ν = 0k = 1k = 2k
0.01 461.5 10−⋅ 1347.5 10−⋅ 2216.6 10−⋅
0.05 102.4 10−⋅ 284.7 10−⋅ 451.7 10−⋅
0.10 52.4 10−⋅ 143.2 10−⋅ 237.7 10−⋅
0.15 31.7 10−⋅ 92.6 10−⋅ 156.7 10−⋅
0.20 21.8 10−⋅ 61.1 10−⋅ 101.1 10−⋅
0.25 28.5 10−⋅ 55.4 10−⋅ 86.0 10−⋅
0.30 12.5 10−⋅ 49.3 10−⋅ 65.8 10−⋅
0.34 15.0 10−⋅ 35.7 10−⋅ 41.1 10−⋅
Notes: While the jump values of 10–14 and smaller are expe-
rimentally unobservable, they are preserved in the Table to
show how rapidly C∆ vanish as 0ν → and/or k increases.
Low- and high-temperature limits of the heat capacity
are easily calculated using series expansions in Eqs. (8) or
(10). A linear dependence C T∝ observed in Fig. 3 in the
low-temperature domain is
2
2
2= (1) = (cos sin ),Li 3
C i T T i
N N N
+ Θ π
πν − πν
ω α ω
(22)
where the link between the polylogarithm and Riemann's
zeta-function is taken into account: 2
2(1) = (2) = / 6Li ζ π .
Different approaching to the asymptotic value
/ 1C N → (from below at < 1/ 4ν and from above at
> 1/ 4ν ) is determined by the following limiting behavior
of the specific heat:
2 2cos 2 sin 2= 1 ,
36 36
C i N Ni
N T T
+ Θ πν ω πν ω − −
(23)
where the temperature-dependent term in the real part
changes sign at = 1/ 4ν , while the imaginary part remains
negative for all ν , see Fig. 3.
At = 1/ 4ν for the real part of the specific heat the
leading deviation from the asymptotic value is given by
41= 1 .
1200
C N
N T
ω −
(24)
5. Discussion
Since a discontinuity is found in the energy as a func-
tion of temperature, the first-order phase transitions occur
at ( )k
cT . It is worth to estimate whether this effect can be
tested experimentally.
As it was mentioned in Secs. 1 and 2, complex para-
meter in the statistics can be obtained as an effective
influence of external laser field or dissipation in the ele-
mentary excitation spectrum. It is expected, therefore, that
the value of ν is small, 1ν . Hence, only the highest
critical temperature corresponding to the value = 0k
sin=cT N πν
ω
π
(25)
Fig. 3. (Color online) The real (a) and imaginary (b) parts of the
specific heat C/N for different values of the parameter = 0.0ν
(black, ); 0.1 (red, – – –); 0.2 (green, ); 0.25 (blue, );
0.3 (magenta, -- --); 0.34 (cyan, ).
–0.8
–0.7
–0.6
–0.5
–0.4
–0.3
–0.2
–0.1
0
0 0.1
0.1
0.2
0.2
0.3
0.3
0.4
0.4
0.5
0.5
0.6
0.6
0.7
0.70
0.2
0.4
0.6
0.8
1.0
1.2
C
/
N
Θ
/
N
/T Nω
/T Nω
(a)
(b)
1144 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 10
Phase transition in a system of 1D harmonic oscillators obeying Polychronakos statistics
is relevant for the analysis since the jumps of the specific
heat become experimentally negligible at > 1k , see Table 1.
Recovering the Boltzmann constant Bk in order to
obtain the value of cT in Kelvins:
sin= ,B ck T N πν
ω
π
(26)
one can make simple estimations as follows. Assuming
experimental precision in the specific heat measurements
as high as 0.01%, which corresponds to = 0.10–0.15ν (see
Table 1), trapping potential frequency = 1ω kHz, and the
number of particles 4= 10N , critical temperature is ob-
tained as 510cT −
K. The effect thus is available for
experimental observations as soon as a proper system,
which can be treated within the Polychronakos statistics
with a complex parameter, is prepared.
It is interesting to derive from the above results some
estimation for a 2D homogenous system. Its density of
states (for spin-0 particles of mass m on the area S) is
easily calculated as 2( ) = / 2 = constg mSε π , which,
comparing to the oscillator result ( ) = 1/g ε ω , leads to an
effective correspondence:
2
2= .mNN
Sm
π
ω
(27)
The obtained expression for critical temperature appears to
have a structure similar to that for the Kosterlitz–Thouless
transition KTT [26]:
2 2
2 22 2
2sin= versus = ,
2 2
B c B KT sk T k T
m m
π πν π
ρ ρ
π
(28)
where the 2D density 2 = /mN Sρ and 2sρ is the superfluid
density. With a 2D concentration 188 10⋅ m–2 correspon-
ding to the bulk liquid helium density of 0.146 g/cm3 [17],
the value of the critical temperature is 1cT K. Note that
this estimation is relevant to an ideal homogenous Bose-
gas with parameters corresponding to liquid 4He. To apply
this approach to a realistic planar system, one can proceed
from studying phonons — excitations with a linear dis-
persion — thus a 2D oscillator problem must be analyzed,
which is yet to be considered in detail.
Acknowledgments
I am grateful to Yuri Krynytskyi and Prof. Volodymyr
Tkachuk for discussion and useful comments. Remarks
from the anonymous referees are highly appreciated.
This work was partly supported by Project ФФ-110Ф
(registration No. 0112U001275) from the Ministry of Edu-
cation and Sciences, Youth and Sports of Ukraine.
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