Proximity-coupled nanostructures of conventional superconductors (SCs) and half-metallic ferromagnets (hmFs) are promising candidates as materials with unconventional superconductivity. The interrelated superposition of spin singlet-triplet and frequency even-odd superconducting condensates characterizes the superconducting state in such heterostructures. In a multi-band SC, the collective modes associated with the excitations of the relative phase between superconducting bands without perturbation of the Cooper pairs symmetry (Leggett modes) are allowed. In this report, we present the results of experimental investigations via the point-contact transport measurements of the Leggett-like collective excitations in the superconducting state of the nanocomposite of s-wave two-band superconductor MgB2 and half-metallic ferromagnet (La,Sr)MnO3. Two types of point contacts (PCs) have been used: the nanocomposite-nonmagnetic metal PCs and the nanocomposite–hmF PCs. The conductance equidistant peaks against the background of the gap structure were observed in both types of high-quality point junctions. Their distinctive feature was their period: two times shorter for the nanocomposite–hmF contacts compared to the nonmagnetic metal PCs. We attribute these spin-selective conductance periodic peaks to the relative phase Leggett’s excitations between “parents” MgB2 even-frequency singlet condensates and proximity-induced triplet superconducting condensates. The data obtained on the hmF PCs also demonstrate the features that may indicate a dynamic coupling between even-frequency condensates and odd-frequency gapless superconducting condensates.
REFERENCES
1.
I.
Žutić
, A.
Matos-Abiague
, B.
Scharf
, H.
Dery
, and K.
Belashchenko
, “Proximitized materials
,” Mater. Today
22
, 85
(2019
). 2.
I.
Žutić
, A.
Matos-Abiague
, B.
Scharf
, T.
Zhou
, H.
Dery
, and K.
Belashchenko
, “Nanoelectronics with proximitized materials
,” Solid State Electron.
155
, 93
(2019
). 3.
M.
Eschrig
, “Spin-polarized supercurrents for spintronics
,” Phys. Today
64
, 43
(2011
). 4.
Y. C.
Tao
and J. G.
Hu
, “Superconducting spintronics: Spin-polarized transport in superconducting junctions with ferromagnetic semiconducting contact,
” J. Appl. Phys.
107
, 041101
(2010
). 5.
M.
Sato
and Y.
Ando
, “Topological superconductors: A review,
” Rep. Prog. Phys.
80
, 076501
(2017
). 6.
F. S.
Bergeret
, A. F.
Volkov
, and K. B.
Efetov
, “Long-range proximity effects in superconductor-ferromagnet structures
,” Phys. Rev. Lett.
86
, 4096
(2001
). 7.
M.
Eschrig
and T.
Löfwander
, “Triplet supercurrents in clean and disordered half-metallic ferromagnets
,” Nature Phys.
4
, 138
(2008
). 8.
S. B.
Chung
, H.-J.
Zhang
, X.-L.
Qi
, and S.-Ch
.
Zhang
, “Topological superconducting phase and Majorana fermions in half-metal/superconductor heterostructures
,” Phys. Rev. B
84
, 060510(R)
(2011
). 9.
Y.
Tanaka
, M.
Sato
, and N.
Nagaosa
, “Symmetry and topology in superconductors. Odd-frequency pairing and edge states
,” J. Phys. Soc. Jpn.
81
, 011013
(2012
). 10.
M.
Kasai
, T.
Ohno
, and Y.
Kanke
, “Current-voltage characteristics of YBa2Cu3Oy/La0.7Ca0.3MnOz/YBa2Cu3Oy triayered-type junctions
,” Jpn. J. Appl. Phys.
29
, L2219
(1990
). 11.
M.
Kasai
, Y.
Kanke
, T.
Ohno
, and Y.
Kozono
, “Possible mechanism of proximity effect coupled to spin fluctuation in YBa2Cu3Oy/magnetic manganese oxide/YBa2Cu3Oy junctions
,” J. Appl. Phys.
72
, 5344
(1992
). 12.
M.
Eschrig
, J.
Kopu
, J. C.
Cuevas
, and G.
Schön
, “Theory of half-metal/superconductor heterostructures
,” Phys. Rev. Lett.
90
, 137003
(2003
). 13.
R. S.
Keizer
, S. T. B.
Goennenwein
, T. M.
Klapwijk
, G.
Miao
, G.
Xiao
, and A.
Gupta
, “A spin-triplet supercurrent through the half-metallic ferromagnet CrO2
,” Nature (London)
439
, 825
(2006
). 14.
V. N.
Krivoruchko
and V. Yu.
Tarenkov
, “Local triplet superconductivity of La0.65Ca0.35MnO3-X point contacts (X = Pb, MgB2)
,” Phys. Rev. B
75
, 214508
(2007
). 15.
V. N.
Krivoruchko
and V. Yu.
Tarenkov
, “Anomalous superconductivity of Pb/La0.7Sr0.3MnO3 point contacts
,” Phys. Rev. B
78
, 054522
(2008
). 16.
M. S.
Anwar
, F.
Czeschka
, M.
Hesselberth
, M.
Porcu
, and J.
Aarts
, “Long-range supercurrents through half-metallic ferromagnetic CrO2
,” Phys. Rev. B
82
, 100501(R)
(2010
). 17.
T. S.
Khaire
, M. A.
Khasawneh
, W. P.
Pratt
Jr., and N. O.
Birge
,
“Observation of spin-triplet superconductivity in co-based josephson junctions
,” Phys. Rev. Lett.
104
, 137002
(2010
). 18.
J. W. A.
Robinson
, J. D. S.
Witt
, and M. G.
Blamire
, “Controlled injection of spin-triplet supercurrents into a strong ferromagnet
,” Science
329
, 59
(2010
). 19.
D.
Sprungmann
, K.
Westerholt
, H.
Zabel
, M.
Weides
, and H.
Kohlstedt
, “Evidence for triplet superconductivity in josephson junctions with barriers of the ferromagnetic heusler alloy Cu2MnAl
,” Phys. Rev. B
82
, 060505(R)
(2010
). 20.
Y.
Kalcheim
, T.
Kirzhner
, G.
Koren
, and O.
Millo
, “Long-range proximity effect in La2/3Ca1/3MnO3/(100)YBa2Cu3O7–δ ferromagnet/superconductor bilayers: Evidence for induced triplet superconductivity in the ferromagnet
,” Phys. Rev. B
83
, 064510
(2011
). 21.
C.
Visani
, Z.
Sefrioui
, J.
Tornos
, C.
Leon
, J.
Briatico
, M.
Bibes
, A.
Barthélémy
, J.
Santamaría
, and J. E.
Villegas
, “Equal-spin Andreev reflection and long-range coherent transport in high-temperature superconductor/half-metallic ferromagnet junctions
,” Nature Phys.
8
, 539
(2012
). 22.
V. N.
Krivoruchko
, “Local spin-triplet superconductivity in half-metallic manganites: A perspective platform for high-temperature topological superconductivity
,” Fiz. Nizk. Temp.
47
, 981
(2021
) [Low Temp. Phys. 47, 901 (2021)]. 23.
A. J.
Leggett
, “Number-phase fluctuations in two-band superconductors,
” Prog. Theor. Phys.
36
, 901
(1966
). 24.
X. X.
Xi
, “Two-band superconductor magnesium diboride,
” Rep. Prog. Phys.
71
, 116501
(2008
). 25.
Ya. G.
Ponomarev
, S. A.
Kuzmichev
, M. G.
Mikheev
, M. V.
Sudakova
, S. N.
Tchesnokov
, N. Z.
Timergaleev
, A. V.
Yarigin
, E. G.
Maksimov
, S. I.
Krasnosvobodtsev
, A. V.
Varlashkin
, M. A.
Hein
, G.
Müller
, H.
Piel
, L. G.
Sevastyanova
, O. V.
Kravchenko
, K. P.
Burdina
, and B. M.
Bulychev
, “Evidence for a two-band behavior of MgB2 from point-contact and tunneling spectroscopy
,” Solid State Commun.
129
, 85
(2004
). 26.
G.
Blumberg
, A.
Mialitsin
, B. S.
Dennis
, M. V.
Klein
, N. D.
Zhigadlo
, and J.
Karpinski
, “Observation of Leggett's collective mode in a multiband MgB2 superconductor
,” Phys. Rev. Lett.
99
, 227002
(2007
). 27.
S. G.
Sharapov
, V. P.
Gusynin
, and H.
Beck
, “Effective action approach to the Leggett's mode in two-band superconductors
,” Eur. Phys. J. B
30
, 45
(2002
). 28.
P. C.
Canfield
and G. W.
Crabtree
, “Magnesium diboride: Better late than never
,” Phys Today
56
, 34
(2003
). 29.
A.
Brinkman
, S. H. W.
van der Ploeg
, A. A.
Golubov
, H.
Rogalla
, T. H.
Kim
, and J. S.
Moodera
, “Charge transport in normal metal-magnesium diboride junctions
,” J. Phys. Chem. Solids
67
, 407
(2006
). 30.
F.
Giorgianni
, T.
Cea
, C.
Vicario
, C. P.
Hauri
, W. K.
Withanage
, X.
Xi
, and L.
Benfatto
, “Leggett mode controlled by light pulses
,” Nature Phys.
15
, 341
(2019
). 31.
M. V.
Klein
, “Theory of Raman scattering from Leggett's collective mode in a multiband superconductor: Application to MgB2
,” Phys. Rev. B
82
, 014507
(2010
). 32.
T.
Cea
and L.
Benfatto
, “Signature of the Leggett mode in the A1g Raman response: From MgB2 to iron-based superconductors
,” Phys. Rev. B
94
, 064512
(2016
). 33.
S. N.
Klimin
, H.
Kurkjian
, and J.
Tempere
, “Leggett collective excitations in a two-band Fermi superfluid at finite temperatures
,” New. J. Phys.
21
, 113043
(2019
). 34.
A.
Di Bernardo
, S.
Diesch
, Y.
Gu
, J.
Linder
, G.
Divitini
, C.
Ducati
, E.
Scheer
, M. G.
Blamire
, and J. W. A.
Robinson
, “Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces
,” Nat. Commun.
6
, 8053
(2015
). 35.
Y. G.
Naidyuk
and I. K.
Yanson
, Point-Contact Spectroscopy, Springer Series in Solid State Science
(Springer
, New York
, 2005
), Vol. 145
.36.
B.
Keimer
and J. E.
Moore
, “The physics of quantum materials
,” Nat. Phys.
13
, 1045
(2017
). 37.
F.
Giustino
, J. H.
Lee
, F.
Trier
, M.
Bibes
, S. M.
Winter
, R.
Valentí
, Y.-W.
Son
, L.
Taillefer
, Ch.
Heil
, A. I.
Figueroa
, B.
Plaçais
, Q. S.
Wu
, O. V.
Yazyev
, E.
Bakkers
, J.
Nygård
, P.
Forn-Díaz
, S.
De Franceschi
, J. W.
McIver
, L.
Torres
, T.
Low
, A.
Kumar
, R.
Galceran
, S. O.
Valenzuela
, M. V.
Costache
, A.
Manchon
, E.-A.
Kim
, G. R.
Schleder
, A.
Fazzio
, and S.
Roche
, “The 2021 quantum materials roadmap
,” J. Phys. Mater.
3
, 042006
(2020
). 38.
I. I.
Mazin
, O. K.
Andersen
, O.
Jepsen
, O. V.
Dolgov
, J.
Kortus
, A. A.
Golubov
, A. B.
Kuz’menko
, and D.
van der Marel
, “Superconductivity in MgB2: Clean or dirty?
,” Phys. Rev. Lett.
89
, 107002
(2002
). 39.
E.
Dagotto
, T.
Hotta
, and A.
Moreo
, “Colossal magnetoresistant materials: The key role of phase separation
,” Phys. Rep.
344
, 1
(2001
). 40.
V.
Krivoruchko
and V.
Tarenkov
, “Triple-gap superconductivity of MgB2-(La,Sr)MnO3 composite
,” Phys. Rev. B
86
, 104502
(2012
). 41.
D.
Daghero
and R. S.
Gonnelli
, “Probing multiband superconductivity by point-contact spectroscopy
,” Supercond. Sci. Technol.
23
, 043001
(2010
). 42.
M.
Eschrig
, T.
Löfwander
, T.
Champel
, J. C.
Cuevas
, J.
Kopu
, and G.
Schön
, “Symmetries of pairing correlations in superconductor–ferromagnet nanostructures,
” J. Low Temp. Phys.
147
, 457
(2007
). 43.
A. M.
Black-Schaffer
and A. V.
Balatsky
, “Odd-frequency superconducting pairing in multiband superconductors
,” Phys. Rev. B
88
, 104514
(2013
). 44.
E.
Abrahams
, A.
Balatsky
, D. J.
Scalapino
, and J. R.
Schrieffer
, “Properties of odd-gap superconductors
,” Phys. Rev. B
52
, 1271
(1995
). 45.
T.
Yokoyama
, Y.
Tanaka
, and A. A.
Golubov
, “Manifestation of the odd-frequency spin-triplet pairing state in diffusive ferromagnet/superconductor junctions
,” Phys. Rev. B
75
, 134510
(2007
). 46.
J.
Linder
and A. V.
Balatsky
, “Odd-frequency superconductivity
,” Rev. Mod. Phys.
91
, 045005
(2019
). 47.
R.
Shimano
and N.
Tsuji
, “Higgs mode in superconductors,
” Annu. Rev. Condens. Matter Phys.
11
, 103
(2020
). 48.
H.
Krull
, N.
Bittner
, G. S.
Uhrig
, D.
Manske
, and A. P.
Schnyder
, “Coupling of Higgs and Leggett modes in non-equilibrium superconductors
,” Nat. Commun.
7
, 11921
(2016
). 49.
R.
Osborn
, E. A.
Goremychkin
, A. I.
Kolesnikov
, and D. G.
Hinks
, “Phonon density of states in MgB2
,” Phys. Rev. Lett.
87
, 017005
(2001
). 50.
T. H.
Kim
and J. S.
Moodera
, “Demonstration of all in situ magnesium diboride superconductor thin-film tunnel junctions
,” Appl. Phys. Lett.
85
, 434
(2004
). 51.
V. V.
Fisun
, L. Yu.
Triputen
, and I. K.
Yanson
, “Low-frequency phonons in the point-contact spectrum of MgB2
,” Fiz. Nizk. Temp.
31
, 1104
(2005
) [Low Temp. Phys.
31, 842 (2005)]. 52.
J.
Mitra
, A. K.
Raychaudhuri
, N.
Gayathri
, and Ya. M.
Mukovskii
, “Point-contact spectroscopy of single crystal La0.75Sr0.25MnO3 and resistivity due to electron-phonon interaction
,” Phys. Rev. B
65
, 140406(R)
(2002
). 53.
M. A.
Belogolovski
, Yu. F.
Revenko
, A. Yu.
Gerasimenko
, Yu. V.
Medvedev
,
O. I.
Chernyak
, V. M.
Svistunov
, and G.
Plitnik
, “Spectroscopy of bosonic excitations in manganite-based heterostructures
,” Fiz. Nizk. Temp.
29
, 889
(2003
) [Low Temp. Phys.
29, 666 (2003)].54.
V. N.
Krivoruchko
, A. I.
D’yachenko
, and V. Yu.
Tarenkov
, “Andreev-spectroscopy study of unconventional superconductivity in MgB2:(La,Sr)MnO3 nanocomposite
,” Fiz. Nizk. Temp.
40
, 1147
(2014
) [Low Temp. Phys.
40, 895 (2014)]. 55.
A.
Aperis
, P.
Maldonado
, and P. M.
Oppeneer
, “Ab initio theory of magnetic-field-induced odd-frequency two-band superconductivity in MgB2
,” Phys. Rev. B
92
, 054516
(2015
). 56.
S. V.
Kul’bachinskii
, Ya. G.
Ponomarev
, L. M.
Fisher
, and O. V.
Belyaeva
, “Leggett mode in cuprate superconductors and the electromagnetic properties of Josephson junctions
,” JETP Lett.
96
, 35
(2012
). 57.
Y.
Ota
, M.
Machida
, T.
Koyama
, and H.
Aoki
, “Collective modes in multiband superfluids and superconductors: Multiple dynamical classes
,” Phys. Rev. B
83
, 060507(R)
(2011
). 58.
V.
Stanev
and Z.
TeŠanović
, “Three-band superconductivity and the order parameter that breaks time-reversal symmetry
,” Phys. Rev. B
81
, 134522
(2010
). 59.
H.
Asai
, Y.
Ota
, S.
Kawabata
, M.
Machida
, and F.
Nori
, “Theory of macroscopic quantum tunneling with Josephson-Leggett collective excitations in multiband superconducting Josephson junctions
,” Phys. Rev. B
89
, 224507
(2014
). © 2024 Author(s). Published under an exclusive license by AIP Publishing.
2024
Author(s)
You do not currently have access to this content.
