Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆
Direct evidence for superconductivity in the new magnetic compound PrAg₆In₆ is revealed for the first time. The distinct Andreev-reflection current is observed in metallic point contacts (PC) based on this compound. The data obtained provide reason enough to suggest that the rise of superconductiv...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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| Цитувати: | Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ / V.M. Dmitriev, L.F. Rybaltchenko, P. Wyder, A.G.M. Jansen, N.N. Prentslau, W. Suski // Физика низких температур. — 2005. — Т. 31, № 1. — С. 63-67. — Бібліогр.: 20 назв. — англ. |
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irk-123456789-1197982017-06-10T03:03:08Z Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ Dmitriev, V.M. Rybaltchenko, L.F. Wyder, P. Jansen, A.G.M. Prentslau, N.N. Suski, W. Свеpхпpоводимость, в том числе высокотемпеpатуpная Direct evidence for superconductivity in the new magnetic compound PrAg₆In₆ is revealed for the first time. The distinct Andreev-reflection current is observed in metallic point contacts (PC) based on this compound. The data obtained provide reason enough to suggest that the rise of superconductivity strongly depends on the local magnetic order varying over the sample volume. The triangular-shaped PC spectra (dV/dI V ( )) in the vicinity of the zero-bias voltage suggest an unconventional type of superconducting pairing. As follows from the temperature and magnetic field dependences of the PC spectra, the superconducting energy gap structure transforms into the pseudogap one as the temperature or the magnetic field increases. 2005 Article Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ / V.M. Dmitriev, L.F. Rybaltchenko, P. Wyder, A.G.M. Jansen, N.N. Prentslau, W. Suski // Физика низких температур. — 2005. — Т. 31, № 1. — С. 63-67. — Бібліогр.: 20 назв. — англ. 0132-6414 PACS: 74.70.Ad, 74.80.Fp http://dspace.nbuv.gov.ua/handle/123456789/119798 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| topic |
Свеpхпpоводимость, в том числе высокотемпеpатуpная Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| spellingShingle |
Свеpхпpоводимость, в том числе высокотемпеpатуpная Свеpхпpоводимость, в том числе высокотемпеpатуpная Dmitriev, V.M. Rybaltchenko, L.F. Wyder, P. Jansen, A.G.M. Prentslau, N.N. Suski, W. Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ Физика низких температур |
| description |
Direct evidence for superconductivity in the new magnetic compound PrAg₆In₆ is revealed for
the first time. The distinct Andreev-reflection current is observed in metallic point contacts (PC)
based on this compound. The data obtained provide reason enough to suggest that the rise of superconductivity
strongly depends on the local magnetic order varying over the sample volume. The
triangular-shaped PC spectra (dV/dI V ( )) in the vicinity of the zero-bias voltage suggest an unconventional
type of superconducting pairing. As follows from the temperature and magnetic field
dependences of the PC spectra, the superconducting energy gap structure transforms into the
pseudogap one as the temperature or the magnetic field increases. |
| format |
Article |
| author |
Dmitriev, V.M. Rybaltchenko, L.F. Wyder, P. Jansen, A.G.M. Prentslau, N.N. Suski, W. |
| author_facet |
Dmitriev, V.M. Rybaltchenko, L.F. Wyder, P. Jansen, A.G.M. Prentslau, N.N. Suski, W. |
| author_sort |
Dmitriev, V.M. |
| title |
Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ |
| title_short |
Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ |
| title_full |
Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ |
| title_fullStr |
Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ |
| title_full_unstemmed |
Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ |
| title_sort |
evidence for superconductivity and a pseudogap in the new magnetic compound prag₆in₆ |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2005 |
| topic_facet |
Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/119798 |
| citation_txt |
Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg₆In₆ / V.M. Dmitriev, L.F. Rybaltchenko, P. Wyder, A.G.M. Jansen,
N.N. Prentslau, W. Suski // Физика низких температур. — 2005. — Т. 31, № 1. — С. 63-67. — Бібліогр.: 20 назв. — англ. |
| series |
Физика низких температур |
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2025-07-08T16:37:30Z |
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2025-07-08T16:37:30Z |
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| fulltext |
Fizika Nizkikh Temperatur, 2005, v. 31, No. 1, p. 63–67
Evidence for superconductivity and a pseudogap
in the new magnetic compound PrAg In6 6
V.M. Dmitriev1,2,3, L.F. Rybaltchenko1,2, P. Wyder1, A.G.M. Jansen1,
N.N. Prentslau2, and W. Suski3,4
1Grenoble High Magnetic Field Laboratory, Max-Planck-Institut für Festkörperforschung and
Centre National de la Recherche Scientifique, B.P. 166, F-38042 Grenoble Cedex 9, France
2B. Verkin Institute for Low Temperature Physics and Engineering
of the National Akademy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: dmitriev@ilt.kharkov.ua
3International Laboratory of High Magnetic Fields and Low Temperatures, 53-421 Wroclaw, Poland
4W. Trzebiatowski Institute of Low Temperatures and Structure Research,
Polish Academy of Sciences, 50-950 Wroclaw, Poland
Direct evidence for superconductivity in the new magnetic compound PrAg6In6 is revealed for
the first time. The distinct Andreev-reflection current is observed in metallic point contacts (PC)
based on this compound. The data obtained provide reason enough to suggest that the rise of super-
conductivity strongly depends on the local magnetic order varying over the sample volume. The
triangular-shaped PC spectra (dV/dI V( )) in the vicinity of the zero-bias voltage suggest an uncon-
ventional type of superconducting pairing. As follows from the temperature and magnetic field
dependences of the PC spectra, the superconducting energy gap structure transforms into the
pseudogap one as the temperature or the magnetic field increases.
PACS: 74.70.Ad, 74.80.Fp
According to the conventional views, only a perfect
antiferromagnetic (AFM) order is well compatible
with superconductivity in a quite broad temperature
range. New magnetic superconductors whose magnetic
structures are far from having perfect AFM order pro-
vide a new insight into the problem of interplay be-
tween magnetism and superconductivity. A few years
ago, a new class of magnetic superconductors with the
ThMn12-type crystal structure was perceived to exist.
Radio-frequency impedance and heat capacity mea-
surements carried out on several compounds of this
family have revealed distinct features in the corre-
sponding characteristics which might be associated
with superconductivity in some regions of the sam-
ples. Such indications of superconductivity were
found in LuFe Al4 8, ScFe Al4 8, YCr4Al8, YFe Al4 8,
and PrAg In6 6 [1,2]. These compounds crystalize with
the comparatively simple tetragonal body-centered
structure of space group I /mmm D h4 4
17( , nr. 139). In
spite of the AFM transition well above 100 K, the
magnetic structure remains very complicated [3,4].
So far, only very restricted information about the elec-
tronic structure is available [5]. Recently [6], we have
proved an existence of a superconducting phase in
YFe Al4 8 by the point-contact (PC) Andreev-reflec-
ton technique. The character of the measured PC spec-
tra (differential resistance vs voltage, dV/dI V( )) im-
plied an unconventional type of superconductivity in
this compound.
Here, we present for the first time the direct evi-
dence for superconductivity found in Andreev-reflec-
tion experiments on another recently synthesized com-
pound PrAg In6 6 [7]. This fact seems to be very
impressive, because the Pr component is very antago-
nistic to superconductivity and can destroy it entirely.
This is what occurs in PrBa2Cu3Ox unless special
preparation techniques are used. We studied point
contacts with the metallic conductivity of the nee-
dle–anvil geometry between Ag and freshly fractured
surfaces of the PrAg In6 6 polycrystal prepared by arc
melting. According to the standard x-ray examina-
tion, the sample had the proper ThMn12-type lattice
© V.M. Dmitriev, L.F. Rybaltchenko, P. Wyder, A.G. M. Jansen, N.N. Prentslau, and W. Suski, 2005
without noticeable traces of additional phases. The
PC method employed enables one to measure on N–S
contacts the Andreev-reflection current whose energy
and temperature dependences allow to infer the basic
superconducting characteristics. The high reliability
of PC Andreev-reflection method can be seen by com-
paring the gap values of high-Tc superconductors mea-
sured by this method and by tunneling spectroscopy
[8] as well as by the photoemission technique [9].
The contact sizes varied within 5–70 nm. The stand-
ard modulation technique was used to register the
dV/dI V( ) characteristics.
Figure 1 shows the typical PC spectra of the
PrAg In6 6 compound measured at different tempera-
tures, which furnish direct evidence for superconduc-
tivity in this material. Indeed, as is well known, the
resistance decrease of metallic contacts near zero bias
voltage is due to the Andreev reflection of quasi-
particles, which is always occurring at an N–S inter-
face. Thus, the zero-bias resistance minimum which
first arises on the 3 K curve and whose amplitude then
increases with lowering temperature (Fig. 1) is a dis-
tinct indication of probing the superconducting re-
gion. Such indications of superconductivity with a
critical temperature Tc varying within 1.7–3.3 K were
found in many parts of the fractured sample surface
(sometimes, up to 10% of the total surface). Notice-
ably, the radio-frequency experiments carried out on
the same sample resulted in Tc � 8 K in the first mea-
surements and in the irregular reduction of Tc in fol-
lowing tests. Perhaps, the reason is connected with
some variations of magnetic structure when the AFM
transitions happen. The critical magnetic field could
reach about 0.5 T in some cases, which essentially ex-
cluded the possibility of superconductivity arising in
the In clusters, which could appear, for instance, due
to composition variations. Then, if the superconduct-
ing features in our spectra were caused by the Andreev
reflection from In clusters, the shape of spectra would
agree with that typical for N–S contacts based on con-
ventional superconductors [10]. Because such spectra
were never observed, this is reliable proof that super-
conductivity in PrAg6In6 is not due to a free In com-
ponent. Moreover, the In clusters should result in
wide variations of gap-feature voltages in different
contacts [11], and that was not observed as well.
The PC spectra have very unusual shape as com-
pared to the conventional BCS superconductors and
show a number of striking features. When the tempe-
rature goes down, the dV/dI V( ) characteristic deve-
lops into a triangular-shaped structure at low volt-
ages. This behavior is not consistent with the fully
gapped Fermi surface expected for conventional BCS
superconductors and may be taken as an indication
of the presence of nodes or lines of nodes in the gap
function. Indeed, according to the Blonder–Tin-
kham–Klapwijk (BTK) theory [10] of N–S contacts
based on conventional superconductors, nearV � 0 the
PC spectra should display either a double minimum
structure if a potential barrier occurs at the N–S
boundary or a flat bottom if this barrier is absent. The
spectra measured are characterized by the horn struc-
ture which is often observed in low-ohmic contacts al-
though its origin is not understood fully even for con-
ventional superconductors. Besides, it is seen that
the width of the superconducting structure does not
change strongly with temperature and remains non-
zero up to T � 3 05. K, where no structure can be seen
at all. Because of the extremely unusual type of PC
spectra measured, the standard BTK model cannot be
used for finding the gap parameter. Hence, the spectra
may be characterized only phenomenologically. Each
curve is described by the depth of the resistance mini-
mum R R RN S0 � � , where RN and RS are the con-
tact resistances at V � 0 in the normal and supercon-
ducting states, respectively, and by the width of the
zero-bias minimum at its half depth, 2�.
As is seen in Figs. 1 and 2, with rising temperature
the resistance parameter R0 decreases and slowly ap-
64 Fizika Nizkikh Temperatur, 2005, v. 31, No. 1
V.M. Dmitriev, L.F. Rybaltchenko, P. Wyder, A.G. M. Jansen, N.N. Prentslau, and W. Suski
–10 –5 0 5 10
RN =1.4 �
H=0
1.4 K
1.7 K
1.95 K
2.2 K
2.4 K
2.5 K
2.6 K
2.7 K
2.8 K
2.9 K
2.95 K
3.0 K
3.05 K
0.3 �
0.01 �
V, mV
d
V
/d
I
Fig. 1. dV/dI V( ) characteristics of the point contact Ag–
PrAg In6 6 measured at different temperatures indicated at
each curve. For clarity, the curves are shifted vertically
and the upper part of the spectra is shown on an enlarged
scale.
proaches zero at 3.05 K. Correspondingly, the relative
parameter RS increases and goes to RN . Such behavior
does not agree with the BTK theory (see inset in
Fig. 2). Both of these dependences change their slopes
near 2.8 K showing that the amplitude variations are
about 98% below this temperature and only about 2%
above it. Near T � 2 8. K, the R TS ( ) curve looks very
much like the smeared resistive N–S transition. There-
fore, we may quite reasonably take this temperature as
the critical temperature Tc. In the vicinity of and
above 2.8 K, the behavior of the R T0( ) and R TS ( )
dependences is probably determined by superconduct-
ing fluctuations, i.e., Cooper pairs arising above Tc. It
is worth noting that the width of the whole supercon-
ducting dV/dI V( ) structure (see arrows on the 1.4 K
curve in Fig. 1) and the distance between horns de-
crease slowly (by � 40%) as the temperature increases
up to 2.8 K and then remain practically unchanged in
the temperature range from 2.8 to 3.05 K.
The width 2� of the resistance minimum has an-
other remarkable temperature dependence. When the
temperature increases, 2� first decreases (by � 25%)
but then, near Tc, comes back practically to its initial
value, thus passing through a minimum near 2.2 K.
The � parameter of the dV/dI V( ) structure is usually
associated with the energy gap or order parameter in
superconductors. On the other hand, the resistance
parameters R0 and RS are associated with the number
of superconducting quasiparticles involved in the
Andreev-reflection processes. So, it is reasonable to
suppose that near 2.8 K we see the smooth transforma-
tion of the superconducting gap (or order parameter)
into a pseudogap with the same energy scale at this
temperature, and that only a small amount of fluctu-
ating paired quasiparticles persists in the pseudogap
regime.
Our scenario is in qualitative accordance with the
results of tunneling spectroscopy of Bi Sr CaCu O2 2 2 8+�
single crystals [12], and with the measurements
of a coherent boson current in the normal state of
the high-Tc superconductor YBa2Cu3Oy [13]. If the
pseudogap model is valid, this may imply that we are
dealing with the d-wave type of superconductivity
proved for high-Tc superconductors, for which the
pseudogap is known to be typical. In this case the
Fermi surface is not gapped entirely and this explains
the unusual shape of our dV/dI V( ) characteristics.
The triangular-shaped PC Andreev-reflection spectra
were recently observed by us in YFe Al4 8 [6]. Earlier,
similar behavior of the PC spectra was reported in
[14] for the heavy-fermion UPt3. This behavior was
attributed to the d-wave symmetry of the order param-
eter in the superconductor indicated. The BTK theory
extended for N–S contacts based on the dx y2 2
�
super-
conductors also predicts a triangular shape of the PC
spectra for most crystallographic directions if the con-
tact barrier transparency is enough high, i.e., the
dimensionless barrier strength factor Z � 1 [15].
Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg In6 6
Fizika Nizkikh Temperatur, 2005, v. 31, No. 1 65
1.2 1.6 2.0 2.4 2.8 3.2
0 0
0.2 0.2
0.4 0.4
0.6 0.6
0.8
0.8
1.0
1.0
T, K
T, K
0 0.2 0.4
H, T
2 3
0.6
0.8
1.0
0 0.2 0.4
BTK
R
/R
S
N
R (T)0
R (H)0
R (T)S
R (H)S
�(T)
�(H)
R
(T
)/
R
(1
.4
K
),
0
0
�
�
(T
)/
(1
.4
K
),
R
(H
)/
R
(0
)
0
0
�
�
(H
)/
(0
)
H, T
Fig. 2. Temperature and magnetic field dependences of the
main parameters for the dV/dI V( ) structure: the depth
R R RN S0 � � , and the width � of the zero-bias resistance
minimum (squares and triangles, respectively) for the con-
tact presented in Figs. 1 and 3. Inset: the analogous
dependences of the zero-bias contact resistance RS (cir-
cles) together with the dependence expected from BTK
theory, R TS( ) (solid line).
–10 –5 0 5 10
1, 2, 3, 4
5, 6
7,
8, 9
10,
11, 12
13
14
RN =1.4�
T=1.4 K
0.45 T
0.42 T
0.38, 0.4 T
0.35,
0.25,
0.28, 0.3 T
0.15, 0.2 T
0.04, 0.08 T
H=0, 0.02,
0.3 �
0.01 �
V, mV
d
V
/d
I
Fig. 3. dV/dI V( ) characteristics of the point contact Ag–
PrAg In6 6 measured at different magnetic fields indicated
at each curve. For clarity, the curves are shifted vertically
and the upper part of the spectra is shown on an enlarged
scale.
The effect of the magnetic field on the PC spectra
of PrAg In6 6 is very surprising, resulting in the step-
like behavior of the spectra in varying field (see Fig. 3
for the same contact as in Fig. 1). Without additional
experiments it is difficult to explain this phenomenon
clearly. Nonuniform distribution of the superconduct-
ing parameters over the sample volume (cf. Figs. 3
and 4) gives evidence for intrinsic structural or/and
magnetic inhomogeneities in the given materials, as
was earlier seen in contacts with YFe Al4 8 single crys-
tals [6]. In this case, a lot of uncompensated magnetic
moments of irregular ordering should appear, which
may result in an exotic symmetry of the Cooper pair-
ing and the unconventional behavior of PC spectra in
the field.
Remarkably, the main tendencies in magnetic field
dependences of the resistance parameters, characteriz-
ing the Andreev-reflection structure, remain practi-
cally the same as in the case of temperature variations.
As is seen in Figs. 2 and 3, the resistance minimum
(parameters R0 and RS) vanishes at H � 0 45. T. How-
ever, the main changes of the parameters (about 98%)
take place below 0.25 T (we may take this value as a
critical field), showing that only a very small number
of paired quasiparticles persist in the field range of
0.25–0.45 T. In this field range, zero-bias minima may
be considered as an exhibition of the pseudogap state.
The magnetic field dependence of the energy pa-
rameter �, mirroring the gap or the order parameter, is
presented in Fig. 2. Neglecting the steps, one can see
that the �( )H dependence resembles the �( )T one,
passing through a minimum within the 0.15–0.20 T
range. Note that the magnetic field dependences of the
width of the whole superconducting structure in
dV/dI V( ) and the distance between the horns, which
also correlate with the superconducting gap (order pa-
rameter), resemble very much the temperature depen-
dences discussed above.
On some of the point contacts we observed quite in-
teresting behavior of the PC spectra in an applied
magnetic field (Fig. 4). In these spectra, after practi-
cally full suppression of the low-magnetic-field
Andreev-reflection structure typical for PrAg6In6 (at
0.08 T for the contact in Fig. 4), there arises another
pseudogap-like structure. The latter first increases
with field but then disappears in a higher field,
slightly above 0.35 T for the given contact. In some
cases this high-field structure could persist up to
H � 2 5. T.
The data presented above strongly imply that the
arising of the pseudogap state is attributable to super-
conductivity. Indeed, nothing other than the Andreev
reflection associated with fluctuating Cooper pairs
can result in the zero-bias resistance minimum on the
PC spectra above Tc or Hc2. If the pseudogap were not
caused by superconductivity, its sign would be op-
posite to that observed in our experiments, as was
observed earlier in the electron-doped cuprate super-
conductor Pr2–xCexCuO4, where the pseudogap was
shown to be governed by nonsuperconducting factors
[16]. As was shown in [17], the Andreev reflection in
the pseudogap state can occur from the phase-incoher-
ent preformed Cooper pairs whose possible existence
above Tc was suggested by Emery and Kivelson [18].
The effect of magnetic field on the pseudogap struc-
ture (see Figs. 3 and 4) indicates that the interior
magnetic structure of PrAg In6 6 interacts with pre-
formed pairs above Hc2 in some unusual way and that
interaction is specified strongly by the variations of
magnetic ordering over the sample volume. A closely
similar situation was discussed for Bi-2201 crystals of
different doping levels [19], which could be compared
with the spin variations in our sample. However, at
the moment the machanism of this interaction cannot
be explained properly.
66 Fizika Nizkikh Temperatur, 2005, v. 31, No. 1
V.M. Dmitriev, L.F. Rybaltchenko, P. Wyder, A.G. M. Jansen, N.N. Prentslau, and W. Suski
–10 –5 0 5 10
0.08 T
0.35 T
0.27 T
0.20 T
0.15 T
0.10 T
0.085 T
0.04 T
H=0
RN=0.13 �
T=1.4 K
0.005 �
0.001 �
V, mV
d
V
/d
I
Fig. 4. Another type of dV/dI V( ) characteristics for the
point contact Ag–PrAg In6 6 measured at different magnetic
fields indicated at each curve. For clarity, the curves are
shifted vertically and the upper part of the spectra is
shown on an enlarged scale. As is seen, the new
pseudogap-like structure arises after practically full disap-
pearance of the superconducting Andreev-reflection struc-
ture near H � 008. T.
In conclusion, direct evidence for superconductiv-
ity in the new ternary magnetic compound PrAg In6 6
has been obtained for the first time in point-contact
Andreev-reflection experiments. Observation of the
pseudogap and step-like structures in the Pr com-
pound is a further (besides the triangular-shaped
Andreev features of the same width in different con-
tacts and the high critical magnetic field indicated
above) strong proof that the superconductivity cannot
be connected with hypothetical chemically free In
atoms, but rather originates from the primary crystal
structure. The small amount of superconducting phase
(� 10% of total sample volume) may result from some
distortions of the crystal structure, e.g., when the
slight displacements of magnetic Pr atoms may pro-
vide significant changes of the local magnetic struc-
ture. Hereby, the superconducting phase can appear
only in separate sample regions with certain magnetic
order. The unconventional shape of the point-contact
characteristics dV/dI V( ) strongly implies non-s-wave
symmetry of the Cooper pairing. The basic parameters
of the dV/dI V( ) structure which undoubtedly are as-
sociated with the energy gap or the order parameter in
superconductors do not become zero when the temper-
ature or magnetic field increases up to the critical
value. Herewith, the superconducting spectra are
transformed smoothly into another gap-like structure,
demonstrating gradual conversion of the supercon-
ducting gap (order parameter) into the pseudogap.
This means that between the superconducting and nor-
mal states there exists an unusual intermediate
(pseudogap) state which possesses properties of both
the normal and superconducting phases [20]. Accord-
ing to our knowledge, this is the first observation of
the pseudogap regime in a non-high-Tc superconduc-
tor. Finally, the exciting step-like transitions of the
dV/dI V( ) Andreev-reflection spectra caused by the
magnetic field have been discovered in PrAg6In6.
We would like to thank G.A. Gogadze for very
helpful discussions and V. Zaremba for preparing the
PrAg In6 6 samples.
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Evidence for superconductivity and a pseudogap in the new magnetic compound PrAg In6 6
Fizika Nizkikh Temperatur, 2005, v. 31, No. 1 67
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