Nanowire bolometer using a 2D high-temperature superconductor Sanat Ghosh1Digambar A. Jangade1and Mandar M. Deshmukh1y 1Department of Condensed Matter Physics and Materials Science

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Nanowire bolometer using a 2D high-temperature superconductor

Sanat Ghosh,1, ∗Digambar A. Jangade,1and Mandar M. Deshmukh1, †

1Department of Condensed Matter Physics and Materials Science,

Tata Institute of Fundamental Research,

Homi Bhabha Road, Mumbai 400005, India

arXiv:2210.11254v2 [cond-mat.supr-con] 24 Feb 2023

Abstract

Superconducting nanowires are very important due to their applications ranging from quantum

technology to astronomy. In this work, we implement a non-invasive process to fabricate nanowires

of high-Tcsuperconductor Bi2Sr2CaCu2O8+δ(BSCCO). We demonstrate that our nanowires can be

used as bolometers in the visible range with very high responsivity of 9.7 ×103V/W. Interestingly,

in a long (30 µm) nanowire of 9 nm thickness and 700 nm width, we observe bias current-dependent

localized spots of maximum photovoltage. Moreover, the scalability of the bolometer responsivity

with the normal state resistance of the nanowire could allow further performance improvement by

increasing the nanowire length in a meander geometry. We observe phase slip events in nanowires

with small cross-sections (12 nm thick, 300 nm wide, and 3µmlong) at low temperatures. Our

study presents a scalable method for realizing sensitive bolometers working near the liquid-nitrogen

temperature.

I. Introduction

Superconducting nanowires have attracted increasing interest in recent years because of

their technological applications, as well as for addressing fundamental questions of super-

conductivity in reduced dimensions. Because of the ultra-small form factor, currently, they

are one of the most sensitive photon detectors [1], reaching single photon detection limit [2],

having large bandwidth [3], low dark count [4], and limited dead time [5]. They have been

realized as a nonlinear inductor [6] for a prototypical qubit system in the ﬁeld of quantum

information and technology. Using superconducting nanowire detectors, signiﬁcant advance-

ments in the ﬁeld of astronomy have also been possible [7].

In spite of superior qualities, the applicability of superconducting detectors is limited

as they require very low temperature for operation, usually liquid helium temperature. A

superconducting detector working above the boiling point of liquid nitrogen, therefore, has a

distinct advantage. High-Tccuprate superconductors with a particular interest in exfoliable

van der Waals material because of their high quality could pave the way in that direction.

For a long time, superconductivity in layered cuprate superconductors, where supercon-

∗sanatghosh1996@gmail.com

†deshmukh@tifr.res.in

ducting CuO2planes are separated by insulating charge reservoir layers, was considered

a bulk phenomenon. Recent developments in the exfoliation of van der Waals materials,

however, show that the superconductivity is achievable down to 0.5 unit cell thick (∼1.5

nm) Bi2Sr2CaCu2O8+δ(BSCCO)–a high-Tcsuperconducting material while maintaining Tc

similar to its bulk counterpart [8,9]. This suggests superconductivity in cuprate is of 2D

nature, entirely arising from single CuO2planes. Isolating few unit cells thick BSCCO and

the ability to modify superconductivity locally can, in principle, allow us to realize nanowire

of extremely small form factor. The small form factor of the nanowire is desirable as it has

low heat capacity and low thermal conductance (because of the small contact area between

bolometer and substrate), which is expected to result in both a fast and a sensitive bolometer

[10].

However, fabricating nanowires using BSCCO is challenging due to their extreme chemi-

cal sensitivity. For patterning narrow structures on a planar surface, ion milling or selective

etching has been a standard route [11]. A similar approach has been taken where a well-

focused high-intensity ion beam is irradiated on a selected region of a thin superconductor

to modify its superconductivity [12]; this locally damages the superconductor making it an

insulator, as shown schematically in Figure 1a. The above-mentioned conventional methods

of patterning superconductivity by locally irradiating high energetic ion beam cause inho-

mogeneity and disorder in the nanowire, which can compromise the detector’s performance.

Thus there is a need for an alternative approach to circumvent this issue.

In this letter, we integrate the two aspects – an alternative scheme for patterning super-

conducting nanostructure and the use of a few unit cells thick high-Tcvan der Waals super-

conductor to fabricate nanowires of high-Tccuprate superconductor BSCCO. An excellent

strategy to realize a high coupling eﬃciency detector involving superconducting nanowires is

to make a long meander geometry [13]. To this end, as a proof of concept, we study two sets

of nanowire devices with diﬀerent aspect ratios – a long nanowire (NW1) with 30 µmlength,

700 nm width, and 9 nm thickness and a short nanowire (NW2) with 3µmlength, 300 nm

width, and 12 nm thickness. We perform detailed transport characterization and photore-

sponse of the nanowires and show these nanowires can be used as a sensitive bolometer for

visible range. The Long nanowire (NW1) shows localized region of maximum photovoltage,

which changes its location along the nanowire length upon changing the bias current. We

observe that the responsivity of the superconducting bolometer scales with the normal state

a b c

Figure 1. Alternative approach in patterning superconducting nanowires compared

to existing method. (a) Schematic of conventional technique of patterning superconductivity.

Exposure to high intensity ion beam locally damages superconducting property making it an insu-

lator. (b) Schematic of our approach to pattern superconductivity locally in BSCCO. Deposition

of Cr on selected region makes underneath BSCCO insulating. (c) Patterning BSCCO in nanowire

geometry. Two separated Cr lines are deposited on BSCCO which deﬁnes the nanowire. Arrow

shows the supercurrent (Is) path through the device.

resistance of the nanowire. Furthermore, our measurements on short nanowire (NW2) with

smaller cross-section suggest the presence of phase slips.

II. Method

As shown in our earlier study [14], we selectively deposit Cr on the surface of freshly

cleaved few unit cells thick ﬂake of BSCCO, thereby making underneath superconductor

insulating, schematically shown in Figure 1b. Following this strategy, we deposit two Cr

lines on exfoliated thin BSCCO with a gap in between the Cr lines to deﬁne the nanowire,

as shown in Figure 1c. A capping layer of Au was deposited on the Cr lines to protect the

Cr lines from being oxidized.

Working with very thin layers of BSCCO is challenging as the dopant oxygen atoms dif-

fuse out of the material [15] over time, eventually making it insulating [16,17]. Various

capping layers have been used to minimize this eﬀect in past studies [18,19]. Exposure to

chemicals while making electrical contacts by standard lithographic process degrades the

device quality [20]. To minimize device fabrication time and to avoid chemical treatments,

we directly deposit 70 nm thick gold contacts to the freshly exfoliated BSCCO ﬂake through

a pre-aligned SiN mask [21]. Through a second SiN mask, we then deposit two 15 nm/10 nm

40 60 80 100 120

T (K)

10−3

10−2

10−1

100

101

Rnanowire (kΩ)

Nanowire

BSCCO

10−1

100

101

102

RBSCCO (Ω)

−0.3 −0.2 −0.1 0.0 0.1 0.2 0.3

I (mA)

dV/dI(kΩ)

60 64 68

T (K)

0.15

0.20

Ir(mA)

Figure 2. Superconducting nanowire device architecture and its electrical character-

ization. (a) Schematic of the nanowire device and the measurement scheme. The electrodes are

designed in such a way that we can simultaneously measure response of the nanowire (by blue

coloured voltmeter) and the remaining extended BSCCO (by brown coloured voltmeter). This

serves as an inbuilt check on quality of the pristine BSCCO from which nanowire is patterned. (b)

Simultaneous measurement of four probe resistances versus temperature of the device. Diﬀerent

coloured curves correspond to the response measured by the same coloured voltmeter indicated

in (a). The shaded regions indicate width of the superconducting transitions of pristine BSCCO

(brown) and the nanowire (blue). We identify Tcof the pristine BSCCO as the temperature where

dR/dT is maximum. Electrode pair across nanowire involve contribution from some part of the

extended ﬂake as well. Resistance at Tcof BSCCO is thus taken as the normal state resistance

Rnof the nanowire. The data corresponds to a 3 unit cells (9 nm) thick BSCCO. (c) Diﬀerential

resistance (dV/dI) vs dc biasing current of the nanowire. Two curves are for two sweep directions

of biasing current. Retrapping (Ir) and switching (Is) currents are identiﬁed as discussed in the

text. The inset shows evolution of Irwith temperature and its ﬁt with hotspot model (red dashed

curve).

of Cr/Au lines to deﬁne nanowire geometry, as shown schematically in Figure 1c. Anodic

bonding method [9] has been used to exfoliate large area (∼100 µmlateral dimension) ﬂakes

from optimally doped bulk Bi-2212 crystal. After the entire fabrication process, we imme-

diately transfer the device to a high vacuum closed-cycle cryostat. Electrical measurements

are done using low-frequency ac lock-in detection technique in four-probe conﬁguration. Op-

tical micrographs and SEM images of the nanowires are shown in Supporting Information

(Figure S1).

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Nanowirebolometerusinga2Dhigh-temperaturesuperconductorSanatGhosh,1,DigambarA.Jangade,1andMandarM.Deshmukh1,y1DepartmentofCondensedMatterPhysicsandMaterialsScience,TataInstituteofFundamentalResearch,HomiBhabhaRoad,Mumbai400005,India1AbstractSuperconductingnanowiresareveryimportantduetotheirapplicat...

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Nanowire bolometer using a 2D high-temperature superconductor Sanat Ghosh1Digambar A. Jangade1and Mandar M. Deshmukh1y 1Department of Condensed Matter Physics and Materials Science

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