Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄

The Andreev reflection spectra dI/dV(V) of the magnetic superconductor Dy₀.6Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ have been investigated. Pronounced stimulation of superconductivity by an external magnetic field has been observed for the first time. The effect showed up as enhancement of the gap structure (and hence...

Ausführliche Beschreibung

Gespeichert in:

Bibliographische Detailangaben
Datum:	2012
Hauptverfasser:	Rybaltchenko, L.F., Khristenko, E.V., Ishchenko, L.A., Terekhov, A.V., Zolochevskii, I.V., Salenkova, T.V., Khlybov, E.P., Zaleski, A.J.
Format:	Artikel
Sprache:	English
Veröffentlicht:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2012
Schriftenreihe:	Физика низких температур
Schlagworte:	Свеpхпpоводимость, в том числе высокотемпеpатуpнаям
Online Zugang:	http://dspace.nbuv.gov.ua/handle/123456789/117971
Tags:	Tag hinzufügen Keine Tags, Fügen Sie den ersten Tag hinzu!
Назва журналу:	Digital Library of Periodicals of National Academy of Sciences of Ukraine
Zitieren:	Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ / L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, A.V. Terekhov, I.V. Zolochevskii, T.V. Salenkova, E.P. Khlybov, A.J. Zaleski // Физика низких температур. — 2012. — Т. 38, № 12. — С. 1403–1409 . — Бібліогр.: 31 назв. — англ.

Institution

Digital Library of Periodicals of National Academy of Sciences of Ukraine

id	irk-123456789-117971
record_format	dspace
spelling	irk-123456789-1179712017-05-28T03:05:13Z Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ Rybaltchenko, L.F. Khristenko, E.V. Ishchenko, L.A. Terekhov, A.V. Zolochevskii, I.V. Salenkova, T.V. Khlybov, E.P. Zaleski, A.J. Свеpхпpоводимость, в том числе высокотемпеpатуpнаям The Andreev reflection spectra dI/dV(V) of the magnetic superconductor Dy₀.6Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ have been investigated. Pronounced stimulation of superconductivity by an external magnetic field has been observed for the first time. The effect showed up as enhancement of the gap structure (and hence the gap itself) in the spectra and its shift towards higher voltages with an increasing field. In the intermediate fields the structure also behaved strangely: instead of the usual smooth decrease with a grooving field, the gap features dropped abruptly near the critical point Hc₂. Of interest is also the abnormally high relative gap value 2∆/kBTc ≈ 4 (as compared to conventional singlet superconductors) which was found for some contacts from a comparison of experimental spectra and the modified Blonder–Tinkham–Klapwiyk theory. We attribute the features revealed in the point-contact spectroscopic investigations of Dy₀.6Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ in a magnetic field to the triplet-type Cooper pairing in the compound because only in this case one can expect the stimulation of superconductivity in the stationary magnetic fields up to ~ 0.7Hc₂. The Andreev reflection spectra dI/dV(V) of the magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ have been investigated. Pronounced stimulation of superconductivity by an external magnetic field has been observed for the first time. The effect showed up as enhancement of the gap structure (and hence the gap itself) in the spectra and its shift towards higher voltages with an increasing field. In the intermediate fields the structure also behaved strangely: instead of the usual smooth decrease with a grooving field, the gap features dropped abruptly near the critical point Hc₂. Of interest is also the abnormally high relative gap value 2∆/kBTc ≈ 4 (as compared to conventional singlet superconductors) which was found for some contacts from a comparison of experimental spectra and the modified Blonder–Tinkham–Klapwiyk theory. We attribute the features revealed in the point-contact spectroscopic investigations of Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ in a magnetic field to the triplet-type Cooper pairing in the compound because only in this case one can expect the stimulation of superconductivity in the stationary magnetic fields up to ~ 0.7Hc₂. 2012 Article Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ / L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, A.V. Terekhov, I.V. Zolochevskii, T.V. Salenkova, E.P. Khlybov, A.J. Zaleski // Физика низких температур. — 2012. — Т. 38, № 12. — С. 1403–1409 . — Бібліогр.: 31 назв. — англ. 0132-6414 PACS: 74.45.+c, 74.70.Dd, 74.20.Rp http://dspace.nbuv.gov.ua/handle/123456789/117971 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
topic	Свеpхпpоводимость, в том числе высокотемпеpатуpнаям Свеpхпpоводимость, в том числе высокотемпеpатуpнаям
spellingShingle	Свеpхпpоводимость, в том числе высокотемпеpатуpнаям Свеpхпpоводимость, в том числе высокотемпеpатуpнаям Rybaltchenko, L.F. Khristenko, E.V. Ishchenko, L.A. Terekhov, A.V. Zolochevskii, I.V. Salenkova, T.V. Khlybov, E.P. Zaleski, A.J. Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ Физика низких температур
description	The Andreev reflection spectra dI/dV(V) of the magnetic superconductor Dy₀.6Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ have been investigated. Pronounced stimulation of superconductivity by an external magnetic field has been observed for the first time. The effect showed up as enhancement of the gap structure (and hence the gap itself) in the spectra and its shift towards higher voltages with an increasing field. In the intermediate fields the structure also behaved strangely: instead of the usual smooth decrease with a grooving field, the gap features dropped abruptly near the critical point Hc₂. Of interest is also the abnormally high relative gap value 2∆/kBTc ≈ 4 (as compared to conventional singlet superconductors) which was found for some contacts from a comparison of experimental spectra and the modified Blonder–Tinkham–Klapwiyk theory. We attribute the features revealed in the point-contact spectroscopic investigations of Dy₀.6Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ in a magnetic field to the triplet-type Cooper pairing in the compound because only in this case one can expect the stimulation of superconductivity in the stationary magnetic fields up to ~ 0.7Hc₂.
format	Article
author	Rybaltchenko, L.F. Khristenko, E.V. Ishchenko, L.A. Terekhov, A.V. Zolochevskii, I.V. Salenkova, T.V. Khlybov, E.P. Zaleski, A.J.
author_facet	Rybaltchenko, L.F. Khristenko, E.V. Ishchenko, L.A. Terekhov, A.V. Zolochevskii, I.V. Salenkova, T.V. Khlybov, E.P. Zaleski, A.J.
author_sort	Rybaltchenko, L.F.
title	Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄
title_short	Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄
title_full	Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄
title_fullStr	Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄
title_full_unstemmed	Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄
title_sort	point-contact andreev reflection spectroscopy of a magnetic superconductor dy₀.₆y₀.₄rh₃.₈₅ru₀.₁₅b₄
publisher	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate	2012
topic_facet	Свеpхпpоводимость, в том числе высокотемпеpатуpнаям
url	http://dspace.nbuv.gov.ua/handle/123456789/117971
citation_txt	Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄ / L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, A.V. Terekhov, I.V. Zolochevskii, T.V. Salenkova, E.P. Khlybov, A.J. Zaleski // Физика низких температур. — 2012. — Т. 38, № 12. — С. 1403–1409 . — Бібліогр.: 31 назв. — англ.
series	Физика низких температур
work_keys_str_mv	AT rybaltchenkolf pointcontactandreevreflectionspectroscopyofamagneticsuperconductordy06y04rh385ru015b4 AT khristenkoev pointcontactandreevreflectionspectroscopyofamagneticsuperconductordy06y04rh385ru015b4 AT ishchenkola pointcontactandreevreflectionspectroscopyofamagneticsuperconductordy06y04rh385ru015b4 AT terekhovav pointcontactandreevreflectionspectroscopyofamagneticsuperconductordy06y04rh385ru015b4 AT zolochevskiiiv pointcontactandreevreflectionspectroscopyofamagneticsuperconductordy06y04rh385ru015b4 AT salenkovatv pointcontactandreevreflectionspectroscopyofamagneticsuperconductordy06y04rh385ru015b4 AT khlybovep pointcontactandreevreflectionspectroscopyofamagneticsuperconductordy06y04rh385ru015b4 AT zaleskiaj pointcontactandreevreflectionspectroscopyofamagneticsuperconductordy06y04rh385ru015b4
first_indexed	2025-07-08T13:05:41Z
last_indexed	2025-07-08T13:05:41Z
_version_	1837084119650009088
fulltext	© L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, A.V. Terekhov, I.V. Zolochevskii, T.V. Salenkova, E.P. Khlybov, and A.J. Zaleski, 2012 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 12, pp. 1403–1409 Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4 L.F. Rybaltchenko1, E.V. Khristenko1, L.A. Ishchenko1, A.V. Terekhov1,3, I.V. Zolochevskii1, T.V. Salenkova1, E.P. Khlybov2,4, and A.J. Zaleski3 1B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine 47 Lenin Ave., Kharkov 61103, Ukraine E-mail: rybal@ilt.kharkov.ua 2L.F. Vereshchagin Institute for High-Pressure Physics, RAN, Troitsk 142190, Russia 3W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, Wroclaw 50-950, Poland 4International Laboratory for High Magnetic Fields and Low Temperatures, Gajowicka 95, Wroclaw 53-421, Poland Received May 14, 2012, revised August 6, 2012 The Andreev reflection spectra dI/dV(V) of the magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4 have been investigated. Pronounced stimulation of superconductivity by an external magnetic field has been observed for the first time. The effect showed up as enhancement of the gap structure (and hence the gap itself) in the spectra and its shift towards higher voltages with an increasing field. In the intermediate fields the structure also behaved strangely: instead of the usual smooth decrease with a grooving field, the gap features dropped abruptly near the critical point Hc2. Of interest is also the abnormally high relative gap value 2∆/kBTc ≈ 4 (as compared to conven- tional singlet superconductors) which was found for some contacts from a comparison of experimental spectra and the modified Blonder–Tinkham–Klapwiyk theory. We attribute the features revealed in the point-contact spectroscopic investigations of Dy0.6Y0.4Rh3.85Ru0.15B4 in a magnetic field to the triplet-type Cooper pairing in the compound because only in this case one can expect the stimulation of superconductivity in the stationary magnetic fields up to ~ 0.7Hc2. PACS: 74.45.+c Proximity effects; Andreev reflection; SN and SNS junctions; 74.70.Dd Ternary, quaternary, and multinary compounds (including Chevrel phases, borocarbides, etc.); 74.20.Rp Pairing symmetries (other than s-wave) . Keywords: point contact, Andreev reflection spectroscopy, magnetic superconductors, triplet pairing. Introduction It is well known [1] that in many compounds antiferromagnetism (AFM) coexists readily with singlet superconductivity in a wide temperature region because the magnetic moments compensate each other appreciably at distances comparable to the superconducting coherence length. Theoretically [2–4], in ferromagnetic (FM) sub- stances the FM state and singlet superconductivity can coexist in a limited temperature region because in struc- tures with a disturbed regularity of magnetic moments a change in their relative orientation minimizes the total magnetic moment. With the advent of the microscopic the- ory of superconductivity many researchers pointed imme- diately to a basic possibility of triplet conductivity, i.e., the Cooper pairing of electrons with parallel spins. According to the latest data, this type of ordering is expected in newly synthesized uranium-containing FM superconductors UGe2 [5], URhGe [6], UCoGe [7] in which 5f-electrons cause both types of cooperative phenomena. Convincing evidence in favor of the triplet Cooper pairing was ob- tained in direct experiments on the strontium ruthenate Sr2RuO4 [8], only layered perovskite that becomes super- conducting without the presence of Cu. These findings have drawn much attention to other magnetic compounds in which coexistence of supercon- ductivity and FM could be possible on a microscopic scale. These were the families of rare-earth molybdenum chalco- L.F. Rybaltchenko et al. 1404 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 12 genides ReMo6X8 (Re is a rare-earth element and X is a chalcogene) and rare-earth rhodium borides ReRh4B4 which possess a wide diversity of magnetic and supercon- ducting properties [1]. In some of these compounds FM and superconductivity coexist in a rather narrow tempera- ture region below the Curie point. The effect was most pronounced in ErRh4B4 [9] in which superconductivity appears at ~ 8.7 K and persists after the FM transition, TC ~ 1.2 K, down to ~ 0.8 K demonstrating thus a conside- rable (~ 0.4 K) region of ferromagnetism–superconducti- vity coexistence. A similar behavior was also observed in HoMo6X8. Of equal interest is another rare-earth rhodium boride DyRh4B4 which can exist in several phase modifications but only one of them (most complex technologically) can be superconducting. Employing special technologies, the au- thors of [10] succeeded in synthesizing and investigating this phase. Besides, the close atomic radii of Dy and Y made it possible to prepare a number of Dy1–xYxRh4B4 deriva- tives (0 ≤ x ≤1). It was found that in this system the critical temperature of the superconducting transition Tc changes smoothly from 4 to 10 K as x increases from 0 to 1. The system with x ≥ 0.4 was found to undergo two magnetic transitions: a FM transition during which the Curie temperature TC decreased from ~ 40 to ~ 12 K as the index x changed from 0 to 0.4 and an AFM transition at T < Tc. Note that compounds with higher yttrium-content (x > 0.4) are not magnetic. The authors analyzed the mag- netic and resistive characteristics of some samples and concluded that the triplet type pairing was quite possible at certain temperatures. Later [11,12] the first transition was identified as ferrimagnetic, in which case the magnetic structure consists of two sublattices with unequal and op- posite directed magnetic moments. This however does not prohibit its coexistence with superconductivity. A compound of this family (Dy0.8Y0.2Rh4B4) was used to form a point contact (PC) with Au, and the Andreev reflection spectra dI/dV(V) [11–13] and the dependence Hc2(T) were measured on it in a wide range of tempera- tures and magnetic fields. By analyzing the measured spec- tra the authors obtained the temperature, ∆(T), and magne- tic field, ∆(H), dependences of the order parameter. They differed considerably from the classical dependences of conventional type II superconductors. The difference was particularly striking in ∆(H) at T < TN (TN is a temperature of AFM transition). In our opinion, this deviation is in fa- vor of the previous assumption [10] of the triplet mecha- nism of Cooper paring in the system Dy1–xYxRh4B4. The analysis of the magnetic field characteristics of the Dy0.8Y0.2Rh4B4 compound prompts a similar conclusion. In this work we have investigated a compound of some- what different composition — Dy0.6Y0.4Rh3.85Ru0.15B4. The effect of the magnetic field upon the PC Andreev re- flection spectra dI/dV(V) was investigated mainly at 1.6 and 4.2 K. In a certain range ~ (0.5–0.7)Hc2 the magnetic field was found to enhance superconductivity rather than suppress it. We attribute the effect to the spin-triplet type of pairing in this compound because superconductivity stimulation by a stationary magnetic field is only possible when spins of the electrons in pairs are oriented in parallel. Experiment The samples of Dy0.6Y0.4Rh3.85Ru0.15B4 were prepared by arc-melting the starting components and subsequent annealing for several days. According to the x-ray phase and structural analyses, the resulting objects were single- phase polycrystals with the LuRu4B4 type structure (space group I4/mmm). The critical superconducting transition temperature was about 7.0 K (as counted off from the mid- point of the resistive transition) (Fig. 1). A partial substitu- tion of Ru for Rh permitted synthesis under the normal pressure, which would be impossible otherwise. According to the electron microscopic analysis, the samples had a close-packed structure consisting of approximately equi- axial crystallites whose sizes varied from several units to several tens of micrometers. Many of the crystallites had submicron-thick layers at their boundaries which might be non-identified inclusions. The PC Andreev reflection spectra, dI/dV(V)-characte- ristics, of N–S contacts were investigated in a wide range of voltage biases much exceeding the gap sizes. This per- mitted us to control the excess (Andreev) current and to exclude unstable contacts from consideration. The spectra were taken on fresh fractures of small (2–3 mm across) samples broken off a bulk ingot. A counterelectrode was an Au wire sharpened mechanically and etched chemically. The measurements were made mostly at 1.6 and 4.2 K in magnetic fields varying from zero to the critical value. A reasonable electrical and mechanical stability was achieved only on the contacts whose resistance RN was within seve- ral tens of Ohms (RN is the contact resistance in the normal state). Gauging the sizes of the Dy0.6Y0.4Rh3.85Ru0.15B4- based contacts is rather a challenge for the lack of infor- 0 100 200 300 0.4 0.8 1.2 H = 0 I f = 10 mA, = 133 Hz T, K R es is tiv ity , 10 O hm ·c m –4 Fig. 1. The resistive transition of the Dy0.6Y0.4Rh3.85Ru0.15B4 sample into the superconducting state. Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 12 1405 mation about their basic properties in literature. We believe that the high excess current Iexp in the contacts selected indicated for spectroscopic conditions in our experiments, i.e., the contact sizes were smaller or at least comparable to the inelastic mean free path of electrons. The PC spectra dI/dV(V) were measured using the standard modulation method and synchronous detection with simultaneous computer recording. They were then processed in terms of the modified Blonder–Tinkham– Klapwijk (BTK) theory [14–16] which is practiced widely for parameterization of N–S point contacts. Despite some serious simplifications, the theory ensures adequate de- scriptions of the superconducting characteristics of con- ventional s-wave superconductors with an isotropic gap function ∆(k). Besides, the theory is efficient at estimating qualitatively the angular dependence ∆(k) in anisotropic single-crystalline or at least coarse-grained superconduc- tors from directed PC spectroscopy data provided that the Fermi momenta in the contacting electrodes are signifi- cantly different. This is possible because the raster of the quasiparticles injected from the normal metal narrows con- siderably to the extent of the Fermi momenta ratio kFN/kFS [17,18]. The effect of narrowing is favored by the contact geometry (elongated channel) which we expect from our preparation technique. In addition to two basic parameters — gap ∆ and barrier Z, characterizing the penetrability of the N–S boundary, the modified BTK theory includes the spectrum smearing parameter Г which describes both the pair-breaking processes and the nonuniform distribution of ∆ over the contact area. Results and discussion The typical magnetic field set of PC spectra dI/dV(V) for the contact Au−Dy0.6Y0.4Rh3.85Ru0.15B4 (normal resis- tance RN ≈ 3.7 Ω) taken in various magnetic fields (0–Hc2) at 1.6 K is shown in Fig. 2. Similar sets were also regis- tered within the temperature range ~ 1.6–2.0 K on the sta- ble contacts permitting a complete cycle of measurement. They were little more than ten altogether. The unstable contacts also demonstrated similar spectra but they were influenced by electric and mechanical perturbing factors, which prohibited measuring a complete set. The high quality of the investigated contacts is attested by the large excess (Andreev) currents Iexc that changed but little in the overgap region of voltages (V >> ∆/e). For the contacts whose spectra are illustrated in Figs. 2 and 5 Iexs makes about 50 and 80% of the BTK value for a one- dimensional model of a contact. It is obvious that Iexc of a three-dimensional contact should be higher but only slight- ly on account of the difference between the Fermi momen- ta in the contacting electrodes and the expected shape of the contact area (elongated channel). We also measured temperature sets of spectra on sever- al contacts in a zero magnetic field (not discussed here). They had no features. The onset of the superconducting transition on cT evidenced by an appreciable zero-bias max- imum in the curve dI/dV(V) was within 6.7–6.9 K, which is slightly different from the corresponding value for a bulk sample (Fig. 1) and is further proof of the high quality of our contacts. The obtained Tc was about 1 K higher than Tc of Dy0.8Y0.2Rh4B4 [12]. This is because of the lower con- tent of magnetic Dy and fits the data obtained in the first study of the electric and magnetic characteristics of the Dy1–xYxRh4B4 system [10]. It was rather hard to detect significant visual distinc- tions between the temperature PC spectra taken in a zero magnetic field and the spectra of conventional supercon- ductors. However, the difference was drastic when the spectra were measured in a magnetic field near T = 1.6 K (Fig. 2). An example of a trivial spectrum is illustrated in Fig. 3 of Asen and Keck [19]. The magnetic field spectra of our contacts have two significant distinctions. Firstly, Fig. 2. Representative set of the Andreev spectra (dI/dV(V)) for a typical contact with RN ≈ 3.7 Ω exhibiting a considerable en- hancement of the gap structure in a magnetic field at T = 1.6 K. The BTK fitting of the spectra is shown by dash curves. The magnetic field is specified at each curve. The fitting revealed the tendency of the dimensionless barrier parameter Z to grow with the field, kOe: 0.1 (0); 0.13 (2.63); 0.16 (3.29); 0.26 (3.95); 0.34 (4.48); 0.42 (5.21); 0.34 (6.06), the smearing parameter Г ≈ ≈ 0.1 meV being invariant. For clearness, the curves are arbitrari- ly displaced vertically. –10 –8 –6 –4 –2 0 2 4 6 8 10 6.2 5.9 5.8 5.3 4.7 4.2 3.7 6.6 3.3 1.3 0 T = 1.6 K Experiment Theory 0.05 S dI /d V, re la t. un its V, mV Au–Dy Y Rh Ru B0.6 0.4 3.85 0.15 4 1/ = 0.27 SRN H, kOe L.F. Rybaltchenko et al. 1406 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 12 the spectra taken in a zero magnetic field have no double gap maxima near V = 0 that are expected when the contact- ing electrodes have different Fermi momenta or a thin die- lectric layer appears at the N–S boundary [20] which is natural for conventional superconductors. However, when nonconventional superconductors (cup- rates, heavy-fermion compounds and more recent iron pnic- tides and chalcogenides) come into contact with a N-electrode whose kFN is much higher, the correlation between the gap maxima intensity and the ratio kFN/kFS is rather weak, if any. This contradicts the classical theory. We also ob- served this in the PC spectra of EuAsFeO0.85F0.15 [21] in which the Fermi velocities vF in the contacting electrodes differed up to eightfold. This should suggest a tunnel re- gime with high gap maxima and practically zero Andreev current. Nevertheless, these were pure Andreev-type spec- tra with weak gap maxima or without them at all. This dis- crepancy was first noted and interpreted by Deutscher and Nozieres [22] who assumed that the electron mass renor- malization responsible for the effective Fermi velocity vF is much weaker in the N–S contact area than that in the bulk material, which caused a significant departure of the gap structure of exotic superconductors from the classical BTK predictions [20]. The other and more essential distinction of our spectra measured at 1.6 K (Fig. 2) from classical ones is an en- hancement of the gap structure in an increasing magnetic field. Initially, in a low magnetic field, the central maxi- mum caused by the Andreev reflection is broadened. It should be emphasized that the width of this maximum is directly related to the magnitude of the gap in any of the existing models for the time being, which can be used to calculate the electrical characteristics of N–S contacts. At a certain moment classical double maxima form in the spec- tra, just like in the N–S contacts based on conventional superconductors. In this case, their position on the energy scale accurately determines the magnitude of the gap itself, provided a small smearing of the spectra (Г << ∆). As the field grows further, the maxima intensity increases to a certain level and then the processes reverses ending in al- most complete suppression of the maxima. The gap maxi- ma voltages also grow up to a certain stable value which persists until the critical point Hc2 is reached. This surpris- ing behavior is clear evidence of superconductivity stimu- lation by a stationary magnetic field. There is one more spectroscopy-unrelated feature in our PC spectra — dips of differential conductivity at voltages exceeding noticeably those of the gap. The dips account for the excessive resistance of the N–S boundary. The ex- cessive resistance has been known for decades since [23,24] but its first adequate explanation appeared in [25] where it was attributed to disturbance of the balance be- tween the chemical potentials of the Cooper pairs and normal quasiparticles due to significant current injection to the N–S structure. Later the interpretation was supported in numerous independent studies. Equalization of the poten- tials is commonly described simply and rigorously in terms of the relaxation times τQ of charge imbalance between the quasi-electron and quasi-hole branches in the energy exci- tation spectrum of superconductors (see, e.g., [26]). The equalization is achieved mainly through the interaction between nonequilibrium quasiparticles and phonons. The latter are rather scanty at low temperatures and low excita- tion energies DeV ћω<< ( Dω is the Debye frequency), which accounts for the relatively long time of energy re- laxation τE of quasiparticles (up to 10–9 s). In the hierarchy of characteristic relaxation times of superconductors τQ is significantly higher than τE, which makes the reason for the excessive resistance at the N–S boundary quite obvi- ous. The problem was analyzed for N–S point contacts and an expression was derived to describe the excessive re- sistance in such structures [27]. As previously, we found the magnetic field dependence of the order parameter ∆(H) by matching our experimental spectra (Figs. 2 and 5) with the modified BTK theory in- cluding the smearing parameter Г [15]. Usually, in the case of conventional superconductors the barrier parameter Z, estimated for the lowest-temperature zero-field dI/dV(V) curve of each set of spectra, was practically invariant for curves measured in higher fields. This occurred to be im- proper for our contacts Au-Dy0.6Y0.4Rh3.85Ru0.15B4 be- cause magnetic field caused significant transformations in the gap structure, characterized to a large extent by the parameter Z (an example of Z-variations is illustrated in the caption to Fig. 2). This Z-growth can be explained by the electron mass increase in a magnetic field as it was multi- ply observed in U-based ferromagnetic superconductors [28]. Similarly, such phenomenon is quite possible in the magnetic compound studied here. So, the initial weakness of renormalization effects in the contact area (according to Deutscher and Nozieres [22]) could be compensated by the electron mass enhancement in a magnetic field. This Fig. 3. Dynamic conductance versus the applied voltage for a Ta–Ag point contact (R = 3.56 Ω, T = 1.5 K) in different magnetic fields (0–899 mT) [19]. 0 20 40 60 80 120 140 160 200 250 300 350 400 450 500 600 700 800 mT –6.0 6.0–4.0 –2.0 0 2.0 4.0 V, mV dI /d V, a rb . u ni ts Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 12 1407 should result in an increase of the gap maxima intensity in the field as it really takes place in our experiments. The ∆-values found from the BTK-analysis of the spec- tra in Fig. 2 are plotted as a function of the magnetic field ∆(H) in Fig. 4. This figure also carries two possible theo- retical dependences ∆(H) commonly used for a comparison with experimental results. They were calculated for the bulk state [28,29] and the thin films in a parallel magnetic field [30] of conventional type II superconductors. Either of them is applicable to describe PC data depending on the geometry of the experiment (the relative orientation of the contact and magnetic field axes). An intermediate sort of dependence is also possible. According to the analysis of PC spectra, most of the contacts measured in a zero magnetic field at T ~ 1.6 K characterized by the gap within 0.6–1.2 meV (2∆/kTc = = 2.0–4.0 in reduced units). The upper limit of the range is indicative of an exotic character of the Cooper pairing in the compound investigated. In conventional superconduc- tors the above ratio is close to 3.52 (in conformity with the BCS theory) and reaches 4 only in substances with a strong electron-phonon interaction (e.g., Hg and Pb). Moreover, in conventional superconductors a high characteristic ratio 2∆/kBTc is possible only in the nonmagnetic state. Mean- while our compound contains rare-earth element Dy with a relatively large magnetic moment (~ 8μB). As is well known, the order parameter decreases rapidly when intrinsic mag- netic moments or external fields affect the singlet super- conductors. We observed an opposite effect in our experi- ments. The PC spectra of Dy0.6Y0.4Rh3.85Ru0.15B4 exhibited an anomalous behavior in a magnetic field in the whole range of the temperatures used, T = 1.6–4.2 K (spectra in Fig. 5 are measured at 4.2 K and generally we did not go above this temperature). It is therefore hardly reasonable to attribute the phenomenon observed to a magnetic transition below 4.2 K where some compounds of the Dy1–xYxRh4B4 family experience certain magnetic transformations. The magnetic field does not stimulate gap maxima in the spec- tra at 4.2 K (Fig. 5) but they always appear in the spectra at 1.6 K (Fig. 2). However, the fact that the central maximum in Fig. 5 does not become narrower with an increasing field (its width correlates directly with ∆) and decreases sharply near Hc2 agree basically with the data at 1.6 K. The effect of the magnetic field at T = 4.2 K is seen more clearly in the dependence ∆(H) (Fig. 6) derived from the BTK analysis of the spectra in Fig. 5. The observed effect becomes weaker as the temperature increases (of Figs. 4 and 6). We suggest that the anomalous behavior of a PC spec- trum in a magnetic field is caused by the triplet-type Cooper pairing in the Dy0.6Y0.4Rh3.85Ru0.15B4 compound. The concept makes it easy to explain the enhancement of the gap structure in the PC spectra. Indeed, when the elec- tron spins of the Cooper pairs are parallel, the applied field Fig. 4. The dependence of the order parameter upon the magnetic field ∆(H) at T ≈ 1.6 K for the contact whose spectra are illustrat- ed in Fig. 2. For comparison, two theoretical dependences (bro- ken lines) are shown, which are possible in contacts based on conventional superconductors when the contact axis is along or perpendicular to the field. 0 1 2 3 4 5 6 7 0 0.4 0.8 1.2 1.6 T = 1.6 K O rd er p ar am et er , m eV H, kOe Experiment points and guide for eye Theory for type II superconductors Ginzburg Landau theory for thin films– Au–Dy Y Rh Ru B0.6 0.4 3.85 0.15 4 Fig. 5. A typical set of magnetic field PC spectra for one of the contacts with RN ≈ 5.5 Ω (solid lines). An acceptable coincidence of experimental and BTK-calculated spectra (broken lines) was ob- tained using invariant fitting parameters Z ≈ 0.1 and Г ≈ 0.1 meV. For clearness, the curves are arbitrarily displaced vertically. –10 –8 –6 –4 –2 0 2 4 6 8 10 T = 4.2 K H, kOe 10.5 8.7 10.1 7.9 6.3 4.6 3.3 2.0 0 0.12 S Experiment Theory dI /d V, re la t. un its Au–Dy Y Rh Ru B0.6 0.4 3.85 0.15 4 1/ = 0. SRN 18 V, mV L.F. Rybaltchenko et al. 1408 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 12 stabilizes the parallel orientation of them resulting in the enhance of superconducting parameters. In this case the pair potential (order/gap parameter) determining the ener- gy of the electron coupling in the Cooper pairs can hold its intensity up to the critical point where superconductivity is destroyed by the orbital magnetic moments. It is precisely these moments are responsible for the progressive reduc- tion of the Cooper pairs and hence the intensity of the gap features in an increasing magnetic field. If superconductivity stimulation occurred only in weak fields, it could be attributed, within a singlet model of pair- ing, to a suppression of possible disturbance of the magnet- ic order at H << Hc2. This would enhance the condensate stability and somewhat increase the gap voltage. However, the assumption is hardly reasonable because the effect ex- ists in a wide range of fields and no smooth decrease in ∆ occurs near Hc2. Other possible factors (extraneous inclu- sions of different phase compositions or dielectric layers in the PC region) are meaningless for this consideration as the critical parameters of all the contacts were practically inva- riant. Moreover, as the BTK-estimates show, in some cases the excess current Iexc can reach ~ 80% of the correspond- ing theoretical value and never decreases below ~ 25%. It should be noted that the superconducting characteris- tics of the related compound Dy0.8Y0.2Rh4B4 [12,13] had some features that could be attributed to the triplet-type pairing, at least below the magnetic transition point near 3.5 K. But those PC spectra had no striking anomalies (like in our spectra) though the compounds have close elemental compositions. The lower content of Dy in our sample only reduces the magnetic effect and the partial substitution of Ru for Rh (for technical reason) can hardly influence its properties because these elements occupy neighboring po- sitions in the periodic table and differ only in one electron in the 4d-shell. It is obvious that further broader research by various techniques is necessary to clear up the origin of the strong anomalies in the PC spectra of Dy0.6Y0.4Rh3.85Ru0.15B4 in a magnetic field and to substantiate the possibility of the triplet-type pairing in this compound. Conclusions 1. The PC Andreev reflection spectra dI/dV(V) have been investigated in N–S contacts based on the magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4 in different mag- netic fields, the critical temperature of the onset of the su- perconducting transition being on cT = 6.7–6.9 K. 2. When the magnetic field grows, the gap features of the spectra (and hence the gap/order parameter) do not shift towards lower energies, as in classical spectra; on the contrary, they move in the opposite direction and gain in- tensity. After reaching a maximum and the following loss of their intensity they still remain practically non-shifted on the energy axis up to the critical point Hc2 where the superconducting state disappears in a stepwise manner. 3. We suggest that a triplet mechanism of Cooper pair- ing operates in the compound investigated because stimu- lation of superconductivity by an external stationary mag- netic field is possible only in this case. The assumption permits a reasonable explanation of the high (up to 4) rati- os 2∆/kBTc unusual for singlet magnetic superconductors. 4. The high Andreev current (up to ~ 80% of the BTK estimate for a one-dimensional case) in some contacts makes the presence of extraneous inclusions in the PC ar- ea, resulting in the destructive modification of the spectra, improbable. This is also supported by the close critical parameters of our point contacts and the bulk material. 5. To clear up the origin of the effect observed, it is necessary to have information about the electron and mag- netic structures of the object studied. This calls for com- prehensive investigations by various techniques of its elec- tric and magnetic characteristics, including the PC pro- perties, in a wide range of temperatures and magnetic fields. Some of us thank the partially financial support of this study by the RFFI, grant No. 12-02-01193. 1. M.B. Maple and O. Fischer, Superconductivity in Ternary Compounds II, Superconductivity and Magnetism, Springer- Verlag Berlin, Heidelberg, New York (1982). 2. P. Fulde and R.A. Ferrel, Phys. Rev. 135, A550 (1964). 3. A.I. Larkin and J.N. Ovchinnikov, Sov. Phys. JETP 20, 762 (1965). 4. L.N. Bulaevskii, A.I. Buzdin, M.L. Kulic’, and S.V. Pankov, Adv. Phys. 34, 175 (1985). 5. S.S. Saxena, K. Ahilan, P. Agarwal, F.M. Grosche, R.K. Ha- selwimmer, M. Steiner, E. Pugh, I.R. Walker, S.R. Julian, P. Monthoux, G.G. Lonzarich, A.D. Huxley, I. Sheikin, D. Braithwaite, and J. Flouquet, Nature (London) 406, 587 (2000). Fig. 6. The BTK-calculated dependence of the order parameter on the magnetic field for the spectra shown in Fig. 5. For compar- ison, the figure illustrates two theoretical dependences (broken lines) expected for contacts based on conventional singlet super- conductors. 0 2 4 6 8 10 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 T = 4.2 K O rd er p ar am et er , m eV H, kOe Experiment points and guide for eye Theory for type II superconductors Ginzburg Landau theory for thin films– Au–Dy Y Rh Ru B0.6 0.4 3.85 0.15 4 Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 12 1409 6. D. Aoki, A.D. Huxley, E. Ressouche, D. Braithwaite, J. Flouquet, J.P. Brison, E. Lhotel, and C. Paulsen, Nature (London) 413, 613 (2001). 7. A. Gasparini, Y.K. Huang, N.T. Huy, J.C.P. Klaasse, T. Naka, E. Slooten, and A. deVisser, J. Low Temp. Phys. 161, 134 (2010). 8. K.D. Neison, Z.Q. Mao, Y. Maeno, and Y. Liu, Science 12, 1151 (2004). 9. W.A. Fertig, D.C. Johnston, L.E. DeLong, R.W. McCallum, and B.T. Matthias, Phys. Rev. Lett. 38, 987 (1977). 10. A.J. Zaleski, A.V. Tswyashchenko, E.P. Khlybov, I.N. Fo- michova, I.E. Kostyleva, S.A. Lachenkov, and O.G. Zamo- lodchikov, in: Proc. 24th Int. Conf. on Low Temp. Phys., Orlando, Florida, USA, August 10-17 (2005), p. 691. 11. V.M. Dmitriev, A.J. Zaleskii, E.P. Khlybov, L.F. Rybalt- chenko, E.V. Khristenko, L.A. Ishchenko, and A.V. Tere- khov, Acta Phys. Polon. A 114, 83 (2008). 12. V.M. Dmitriev, A. Zaleski, E.P. Khlybov, L.F. Rybaltchen- ko, E.V. Khristenko, L.A. Ishchenko, A.V. Terekhov, I.E. Kostyleva, and S.A. Lachenkov, Fiz. Nizk. Temp. 34, 1152 (2008) [Low Temp. Phys. 34, 909 (2008)]. 13. V.M. Dmitriev, I.E. Kostyleva, E.P. Khlybov, A.J. Zaleski, A.V. Terekhov, L.F. Rybaltchenko, E.V. Khristenko, L.A. Ish- chenko, O.E. Omel'yanovskiy, and A.V. Sadakov, Fiz. Nizk. Temp. 35, 659 (2009) [Low Temp. Phys. 35, 517 (2009)]. 14. G.E. Blonder, M. Tinkham, and T.M. Klapwijk, Phys. Rev. B 27, 112 (1982). 15. Y. DeWilde, T.M. Klapwijk, A.G.M. Jansen, J. Heil, and P. Wyder, Physica B 218, 165 (1996). 16. R.C. Dynes, V. Narayanamurti, and J.P. Garno, Phys. Rev. Lett. 41, 1509 (1978). 17. A.V. Zaitsev, Sov. Phys. JETP 59, 1015 (1984). 18. H.U. Baranger, A.H. MacDonald, and C.R. Leavens, Phys. Rev. B 31, 6197 (1985). 19. M. Asen and K. Keck, Fiz. Nizk. Temp. 18, 602 (1992) [Sov. J. Low Temp. Phys. 18, 421 (1992)]. 20. G.E. Blonder and M. Tinkham, Phys. Rev. B 27, 112 (1983). 21. V.M. Dmitriev, E.P. Khlybov, D.S. Kondrashov, A.V. Tere- khov, L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, I.E. Kostyleva, and A.J. Zaleski, Fiz. Nizk. Temp. 37, 360 (2011) [Low Temp. Phys. 37, 280 (2011)]. 22. G. Deutscher and P. Nozières, Phys. Rev. Lett. 50, 13557 (1994). 23. I.L. Landau, JETP Lett. 11, 295 (1970). 24. A.B. Pippard, J.G. Shepherd, and D.A. Tindal, Proc. Roy. Soc. London, Ser. A 324, 17 (1971). 25. T.J. Rieger, D.J. Scalapino, and J.E. Mercereau, Phys. Rev. Lett. 27, 1787 (1971). 26. M.L. Yu and J.E. Mercereau, Phys. Rev. B 12, 4909 (1975). 27. M.A. Peshkin and R.A. Buhrman, Phys. Rev. B 28, 161 (1983). 28. D. Aoki, T.D. Matsuda, F. Hardy, C. Meingast, V. Taufour, E. Hassinger, I. Sheikin, C. Paulsen, G. Knebel, H. Kote- gawa, and J. Flouquet, J. Phys. Soc. Jpn. 80, SA008 (2011). 29. A.L. Fetter and P.C. Hohenberg, in: Superconductivity, R.D. Parks (ed.), Marcel Dekker, New York (1969), vol. 2, p. 817. 30. H. Kusunose, Phys. Rev. B 70, 054509 (2004). 31. V.L. Ginzburg and L.D. Landau, Zh. Eksp. Teor. Fiz. 20, 1064 (1950).

Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy₀.₆Y₀.₄Rh₃.₈₅Ru₀.₁₅B₄

Institution

Ähnliche Einträge