Evidence of multiband behavior in a new superconductor Ta0.8Zr0.2B with FeB-prototype structure

Article abstract of DOI:10.1016/j.jallcom.2019.06.186

We present results that show a relation between crystalline structure and superconductivity in the Ta_1-xZr_xB system. The substitution of Ta for Zr atoms induces a crystal structure change from CrB- to FeB-prototype structure in which superconductivity is present with higher superconducting critical temperature T_c. The interrelation between T_c and Zr content shows a dome-like behavior with maximum T_c at x = 0.2. In this sample, bulk superconductivity was observed and characterized by magnetization, electrical resistivity and specific heat measurements, all consistent with a superconducting critical temperature close to 7.8 K. The upper and lower critical field diagrams have no usual behavior which may be connected to a multiband scenario.

Full text of DOI:10.1016/j.jallcom.2019.06.186

Accepted Manuscript
Evidence of multiband behavior in a new superconductor Ta Zr B with FeB-
0.8 0.2
prototype structure
L.E. Correa, J.C.C. Nova, L.R. de Faria, F.B. Santos, B.S. de Lima, C.A. Nunes, G.C.
Coelho, P. Rogl, Z. Fisk, A.J.S. Machado
PII:
S0925-8388(19)32270-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.06.186
JALCOM 51084
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 25 March 2019
Revised Date: 11 June 2019
Accepted Date: 15 June 2019
Please cite this article as: L.E. Correa, J.C.C. Nova, L.R. de Faria, F.B. Santos, B.S. de Lima, C.A.
Nunes, G.C. Coelho, P. Rogl, Z. Fisk, A.J.S. Machado, Evidence of multiband behavior in a new
superconductor Ta Zr B with FeB-prototype structure, Journal of Alloys and Compounds (2019),
0.8 0.2
doi: https://doi.org/10.1016/j.jallcom.2019.06.186.
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ACCEPTED MANUSCRIPT
Evidence of multiband behavior in a new superconductor Ta0.8Zr0.2B with FeB-
prototype structure.
L. E. Correa1, J. C. C. Nova1, L. R. de Faria1, F. B. Santos1, B. S. de Lima1,2, C. A. Nunes1, G. C.
Coelho1, P. Rogl3, Z. Fisk4 and A. J. S. Machado1
1Escola de Engenharia de Lorena, University of Sao Paulo, P. O. Box 116, Lorena, 12602-810, Brazil
2Sao Carlos Institute of Physics, University of Sao Paulo, 565-905 São Carlos, SP, Brazil
3Institute of Materials Chemistry, University of Vienna, Wahringerstrasse 42, A-1090 Wien, Austria
4Department of Physics and Astronomy, University of California-Irvine, Irvine, CA 92697, USA
Abstract
We present results that show a relation between crystalline structure and
superconductivity in the Ta1-xZrxB system. The substitution of Ta for Zr
atoms induces a crystal structure change from CrB- to FeB-prototype
structure in which superconductivity is present with higher
superconducting critical temperature Tc. The interrelation between Tc and
Zr content shows a dome-like behavior with maximum Tc at x = 0.2. In this
sample, bulk superconductivity was observed and characterized by
magnetization, electrical resistivity and specific heat measurements, all
consistent with a superconducting critical temperature close to 7.8 K. The
upper and lower critical field diagrams have no usual behavior which may
be connected to a multiband scenario.
Introduction
In the most recent assessment of the Ta-B system, five compounds
are considered to be stable: TaB2, Ta3B4, TaB, Ta3B2 and Ta2B [1]. Among
them, superconductivity has been reported for TaB and Ta2B with
superconducting critical temperatures of 4.0 and 3.12 K, respectively [2-4].
Besides that, there is the case of TaB2, found non-superconducting in
earlier experiments [5] and more recently a superconducting transition
was observed in the vicinity of 9.5 K [6]. The binary TaB crystallizes in the
orthorhombic CrB-prototype structure with space group Cmcm. Failamani
et al. reported that the substitution of Ta atmos for Ti, Zr and Hf in TaB was
able to stabilize an orthorhombic FeB-prototype structure (space group
Pnma) with general formula Ta1-xRMxB [7]. More recently, Silva et al. have
shown that the TaB with FeB-prototype phase, is stable in the Ta-B binary
system [8], the FeB-prototype phase crystallizes from the melt but is not

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stable at low temperatures. It has been shown that Ta0.7Hf0.3B is a new
superconducting material with Tc close to 6.7 K, (x = 0.3 being the phase
composition with the highest Tc) and presents a strong signature of
multiband superconductivity [9], based on Hall effect data and heat
capacity measurements. Although the main motivation to investigate
refractory metals borides has been their application at temperatures above
1000°C in extreme environments [10, 11], superconductivity is frequently
observed in these phases. Within this context, this work presents results of
a systematic experimental study on the effect of Zr substitutions for Ta in
the Ta1-xZrxB (x = 0.1 to 0.3) compound. This material is another example of
multiband superconductors in the Ta1-xRMxB phase family, showing a
maximum critical temperature of ~ 7.8 K at Ta0.7Zr0.2B stoichiometry.
Experimental Procedure.
Polycrystalline samples of Ta1-xZrxB (with 0.1 ≤ x ≤ 0.3, ranging from 0.05)
were synthesized by arc melting pieces of high-purity Zr, Ta and B on a
water-cooled Cu hearth in high purity Ar atmosphere and using Ti sponge
as getter. The weight loss during the arc melting was negligible (<0,2%).
Crystallographic characterization was performed using powder samples in
a PANalytical model Empyrean diffractometer with Cu Kα radiation. The
phases were identified using crystallographic information reported in the
literature [7, 12], and the lattice parameters were determined by Rietveld
refinement using PANalytical software HighScore Plus. Magnetization,
electrical resistivity and heat capacity were measured using Quantum
Design VSM-PPMS EverCool II equipment. Magnetization as a function of
temperature was measured in Zero Field Cooled (ZFC) and Field Cooled
(FC) regime under 20 Oe. Magnetization as a function of applied magnetic
field was measured at 2.5 K. Electrical resistivity as a function of
temperature was determined using the standard four point geometry
between 2.0 and 300 K, both with and without applied magnetic field in
order to estimate the upper critical field at zero Kelvin. The specific heat
measurements was performed on polished flat samples by the relaxation
method.
Results and Discussion.

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Figure 1 shows both experimental and refined X-ray diffraction
patterns for a sample of Ta0.8Zr0.2B nominal composition. An excellent fit
between the experimental and simulated diffraction patterns is observed
using FeB as prototype, indicating that the sample is X-ray due to the
secondary FeB prototype structure as in reference 7. This secondary phase
is richer in Zr and it is not superconductor phase. The lattice parameters
obtained after Rietveld refinement are a = 6.140 Å, b = 3.159 Å and c =
4.620 Å. X-ray samples were obtained for compositions between 0.1 ≤ x ≤
0.3.
Figure 1 - X-ray diffraction pattern of Ta0.8Zr0.2B showing good agreement between
experimental and simulated data.
All the samples presented a single phase pattern in the range of 0.1 ≤
x ≤ 0.3, but here we focus on the results for the sample with highest Tc,
Ta0.8Zr0.2B, whose X-ray diffraction pattern is shown in Figure 1. The
magnetization as function of temperature in the ZFC and FC regimes for
this composition is displayed in Figure 2.

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Figure 2 - M versus T for sample of composition Ta0.8Zr0.2B showing a superconducting
transition close to 7.8 K. The inset shows the M versus H dependence at 2.5 K, which
suggests type II superconductivity
A sharp superconducting transition with critical temperature at near
7.8 K can be observed in this Figure. The estimated superconducting
fraction from the magnitude of the magnetization signal is around 95%.
The inset shows the M vs H behavior at T = 2.5 K, which reveals a type-II
superconductor with a relatively high upper critical field. The critical
temperatures as a function of Zr concentration for Ta1-xZrxB are shown in
Figure 3.

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Figure 3 – Tc as a function of Zr content showing a dome with the maximum Tc close to x
= 0.20.
In this Figure a dome-like behavior is seen with a maximum critical
temperature reached with x =0.2. However, the first point (x= 0) refers to
undoped TaB which possess a CrB-prototype structure and, therefore,
different from the samples Zr-doped samples with FeB-prototype
structure. This dome-like behavior was also observed for the Ta1-xHfxB
compounds [8] and, therefore, seems to be a characteristic behavior of the
FeB-prototype phase stabilized with either Hf or Zr.
The electrical resistivity as a function of temperature in absence of
magnetic field is shown in Figure 4. A sharp transition (ΔTc < 0.5 K) with
onset temperature close to 7.8 K can be observed. The transition width is
smaller than 0.5 K, revealing a high quality polycrystalline sample. This
result is totally consistent with the magnetization measurement. The inset
in Figure 4 shows the electrical resistivity in applied magnetic field up to
8.0 T.

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Figure 4 – Resistivity versus temperature for the sample Ta0,8Zr0,2B showing the sharp
transition with onset close to 7.8 K. Inset shows resistance in applied magnetic field
between 0 and 8.0 T.
The μoHc2 versus reduced temperature (tr = T/Tc) phase diagram can be
built using the mid-point of the transition as a criterion for Tc. The upper
critical field at zero temperature can be estimated using the WHH formula
in the dirty limit [13],
μoHc2(0)= −0.693Tc(dμ
o
H
c
/dT)T=T
(1)
c
Using this equation the upper critical magnetic field at zero Kelvin
(μoHc2(0)) was estimated to be ~ 5.5 T. Figure 5 displays a clear deviation of
the experimental data (red dashed line), indicating that the empirical
quadratic conventional relation, [μoHc2(T)= μoHc2(0)(1−tr2)], cannot be applied
for this material. It indicates that this material is not a single gap
superconductor. Indeed, other multiband superconductors also display
similar divergence. For example, the multiband LaFeAsO0.89F0.11 phase,
exhibits this divergence in the upper critical field [14]. Many others
examples can be found in the literature, LuNi2B2C [15] and YNi2B2C [16]

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among others [17-19]. Micnas et al. [20] suggest that another equation
better describes a superconductor with more than one gap:
μoHc2(T)=μoHc2(0)(1−t1.5)1.5
(2)
Figure 5 – μ0Hc2 versus reduced temperature (t) (black symbols). Red dashed line is the
fit estimated by WHH theory [11]. Dashed black line is the fit using the equation
suggested by Micnas et al. [18] for a superconductor with two gaps.
In Figure 5, the experimental points seems to fit very well with this
equation (black dashed line), and the upper critical field value in zero
Kelvin could be estimated to be around 8.8 T. Thus, these results suggest
that this material may be another example of a boride multiband
superconductor. Another deviation from conventional single band behavior
occurs lower critical magnetic field phase diagram. In order to verify the
existence of this in Ta0.8Zr0.2B, M vs H was measured at several
temperatures between 2.0 and 7.8 K as shown in Figure 6 (a). Figure 6 (b)
shows ΔM vs H, Hc1 being defined using ΔM = 10-3 as a criterion and plotted
against tr in Figure 6 (c). The existence of two gaps in this superconductor
can be observed, which is similar to those of other superconducting
materials recognized as multiband superconductors.

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Figure 6 - a – M versus H at different temperatures, with isotherms
between 1.8 K and 7.8 K. b - ΔM vs H and the Hc1 was defined using ΔM = 10-
3 as criterion for lower critical field definition. c – Hc1 versus t (T/Tc) (black
symbols). Dashed red line is fit estimated by the conventional single band
superconductor.

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Figure 7 - Cp/T versus T showing the superconducting anomaly at ∼ 7.8 K. Inset shows
electronic contribution to specific heat, which reveals that the ΔC/γTc value is close to
the BCS prediction.
In Figure 7 a clear jump around 7.8 K can be observed in the specific
heat measurement, indicating that this material is unambiguously a new
bulk superconductor. This is consistent with the conclusions derived from
the results of the present investigation using other measurement
techniques (magnetization and resistivity). Using the Debye approach at
low temperatures, given by Cp/T = γ + βT2, the values of γ ~ 3.067
mJ/molK2 and β ~ 0.1511 mJ/molK4 are obtained. From this β value, the
Debye temperature θD ~ 295 K is estimated, which is slightly smaller than
that obtained for Ta0.7Hf0.3B, θD ~ 325.93 K) [8], suggesting different
phonon spectra for these two compounds of the same family. The γ value is
proportional to the Density of States at the Fermi Level. In this case, the γ
value for Ta0.8Zr0.2B suggests a higher Density of States than that for
Ta0.7Hf0.3B (γ ~ 2.0 mJ/molK2) [8]. The subtraction of the phonon

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contribution allows us to evaluate the electronic contribution for this
material. The inset of Figure 7 shows just the electronic contribution for
specific heat. An analysis of the jump yields ΔC/γnTc ~ 1.44, which is close
to that predicted by BCS, and indicates that the superconducting gap is in
the weak-coupling limit. The exponential behavior of the electronic specific
heat suggests an excess entropy, which indicates that another gap can be
opened at lower temperatures, confirming that Ta0.8Zr0.2B is another
example of multiband superconductor.
Conclusion
This article presents a systematic study of Zr substitutions for Ta in
Ta1-xZrxB. The results show that Zr additions to TaB change its crystal
structure from CrB-prototype to FeB-prototype in Ta1-xZrxB with
composition range 0.1 ≤ x ≤ 0.3. The FeB-prototype structure displays
superconducting behavior with a maximum in the superconducting critical
temperature close to 7.8 K for x=0.2 nominal composition. The present set
of results shows that this new superconductor phase can be understood in
a multiband scenario through anomalies observed in the upper and lower
critical fields and the heat capacity.
Acknowledgments
This material is based upon work supported by the FAPESP (2018/08819-2,
2016/11565-7, 2016/11774-5) and CNPq (130322/2016-4 and 302149/2017-1)

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Hightlight
•
This paper show that Zr additions to TaB change its crystal structure from CrB-
prototype to FeB-prototype in Ta1-xZrxB with composition range 0.1 ≤ x ≤ 0.3
Ta1-xZrxB has been found to be a new superconductor at 7.8 K.
Magnetization, specfic-heat and electrical resistivity measurements points to a
multiband superconducting state.
•
•