PHYSICAL REVIEW B 87, 245113 (2013)
Universal origin of unconventional 1/ f noise in the weak-localization regime
C. Barone,
1,2,*
F. Romeo,
1,2
A. Galdi,
2,3
P. Orgiani,
2
L. Maritato,
2,3
A. Guarino,
1,2
A. Nigro,
1,2
and S. Pagano
1,2,†
1
Dipartimento di Fisica “E.R. Caianiello”, Universit` a di Salerno, I-84084 Fisciano, Salerno, Italy
2
CNR-SPIN, UOS di Salerno, I-84084 Fisciano, Salerno, Italy
3
Dipartimento di Ingegneria dell’Informazione, Ingegneria Elettrica e Matematica Applicata, Universit` a di Salerno,
I-84084 Fisciano, Salerno, Italy
(Received 2 August 2012; revised manuscript received 13 March 2013; published 17 June 2013)
The transport properties of manganite thin films characterized by a weak-localization transition have been
studied. Detailed voltage noise measurements show a specific 1/f noise spectrum below the transition
temperature. A theoretical interpretation in terms of universal conductance fluctuations explains the nature
of the unconventional electric noise, suggesting a direct connection between the weak-localization phenomenon
and universal conductance fluctuations. The universal nature of the mechanism allows its detection in different
systems under the weak-localization regime.
DOI: 10.1103/PhysRevB.87.245113 PACS number(s): 72.15.Rn, 72.70.+m, 73.23.−b
I. INTRODUCTION
Low-temperature electrical transport in a disordered metal
is often influenced by quantum interference effects (QIEs)
arising from the phase coherence of the electronic wave
function over a distance L
(i.e., the coherence length), which
can be much longer than the elastic mean free path at
low temperatures or for large impurity concentration. One
of these effects is weak localization (WL), a reduction of
conductivity induced by the interference of phase coherent
carriers backscattered from impurities. WL is revealed by an
increase of resistivity at low temperature and by a peculiar
magnetoresistance behavior, related to the phase shift induced
by the magnetic field.
1
An even more intriguing effect is the observation of
universal conductance fluctuations (UCFs) in mesoscopic
one-dimensional (1D) samples at very low temperatures (i.e.,
when the sample size L is ≈L
and the transport is diffusive,
L,L
). In such “small” systems, the interference between
the electron trajectories determines a phase-dependent correc-
tion that does not average to zero and that depends on a specific
impurity configuration. A magnetic field increment shifts the
phases of the electrons, so that a different interference pattern
results. This is seen in the magnetoresistance fluctuations,
characterized at T = 0 by a root-mean-square amplitude of
∼e
2
/h, independent of sample size or degree of disorder.
2,3
A direct connection between these two phenomena related
to QIEs has been proposed but never demonstrated.
4–6
Indeed,
WL is usually detected in systems of any dimensionality,
1
and at temperatures easy to access (in the range 10–80 K
in oxides).
7–9
The UCF detection, instead, is limited to
very peculiar sample geometry (i.e., nanowires, 1D-confined
electron gases) and low temperature (typically 0.1–10 K).
3,10
In this respect, it is well known that the oscillations of the UCF
magnetoresistance decay in large samples, due to ensemble
averaging, and the amplitude of the fluctuations is reduced by
the temperature increase, due to phase breaking excitations
and energy level smearing. However, as already reported in
the scientific literature,
11–14
the existence of UCFs can be
probed in larger samples (L L
) and at temperatures higher
than 10 K by investigating temporal conductivity fluctuations
characterized by a power spectral density of 1/f type.
Electric noise spectroscopy, without the presence of an ex-
ternal magnetic field and with no limitation on operating tem-
peratures, has proved to be a very sensitive method for studying
the dynamic behaviors of the charge carriers and the kinetic
processes in several systems, such as double perovskites,
15
manganites,
16
and electron-doped cuprate superconductors.
7
In these latter compounds, a preliminary study has revealed
that WL gives rise to an unusual noise-spectral density of
1/f type. In particular, under the WL regime the measured
1/f noise shows a very weak temperature dependence and
a peculiar linear bias current scaling of the spectral density
S
V
. At temperatures above the WL transition, the standard
resistance fluctuation behavior is gradually recovered, yielding
S
V
∝ I
2
.
17
This is evident in Fig. 1(a), where the experimental
temperature and current dependencies of the voltage spectral
density are shown in the case of Nd
1.83
Ce
0.17
CuO
4+δ
(NCCO)
thin film samples, undergoing weak localization below a
crossover temperature T
min
∼ 84 K. In order to prove that
this is not a peculiar behavior of NCCO compounds, a
similar analysis has been performed on a different material,
i.e., La
0.7
Ba
0.3
MnO
3
(LBMO) manganite. The experimental
results are shown in Fig. 1(b) and qualitatively reveal the
same dependence, although with a reduction of the crossover
temperature of ∼50 K.
The aim of this paper is to explain the observed noise
behavior, by identifying a universal origin of this phenomenon
in terms of UCF induced noise. Due to the universal nature
of the proposed mechanism, such phenomenology is expected
to be relevant in low-dimensional systems where quantum
corrections arise due to residual phase coherence. A model for
the voltage noise spectral density based on the UCF physics
is described in Sec. II. Section III contains the experimental
results on LBMO thin films. In Sec. IV a discussion of the noise
properties is addressed, also estimating the relevant energy
scales involved in UCF processes. The conclusions are given
in Sec. V.
II. THEORETICAL FRAMEWORK
In disordered (macroscopic, i.e., of size L>L
) thin
film systems at low temperature, the phase coherence of
the sample over regions of typical size L
may induce a
245113-1 1098-0121/2013/87(24)/245113(6) ©2013 American Physical Society