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Dragana Popović

Publications and source records attributed to Dragana Popović.

At least 19 recordsLinked to original sources
arXiv2026

Mott Intermittency at the Metal-Insulator Boundary

The resistivity maximum at a temperature $T=T_{\mathrm{max}}$ is a recurring feature of bandwidth-tuned Mott systems, yet its meaning remains controversial: is it a coherence-incoherence crossover of an electronically homogeneous metal, or does it mark the onset of transport through a mixed landscape of metallic and insulating regions? Even more debated is whether a true phase-coexistence regime survives in the relevant parameter range, or whether apparent inhomogeneity is merely extrinsic. Here we address these questions by moving beyond temperature sweeps and probe charge transport in the time domain. Near $T=T_{\mathrm{max}}$, we find that the resistance of a model system, a quasi-two-dimensional Mott spin liquid material, exhibits clear random-telegraph switching between discrete levels over long timescales. The statistics of the switching - sharp two-level behavior with thermally activated dwell times - point to a mesoscopic "current-controlling" region that dynamically toggles between metallic and insulating states, intermittently opening and closing the dominant conduction channel. This characteristic fluctuating dynamics provides direct evidence for intrinsic metal-insulator coexistence and establishes $T\sim T_{\mathrm{max}}$ as the regime of Mott intermittency, where transport is governed by stochastic domain switching rather than quasiparticle decoherence.

cond-mat.str-el↗
arXiv2025

Failed superconductivity in a Mott spin liquid material

A central challenge for understanding unconventional superconductivity in most strongly correlated electronic materials is their complicated band structure and presence of competing orders. In contrast, quasi-two-dimensional organic spin liquids are single-band systems with superconductivity arising near the bandwidth-tuned Mott metal-insulator transition in the absence of other orders. Here, we study chemically substituted $κ$-organics in which superconducting fluctuations emerge in the phase coexistence region between the Mott insulator and the Fermi liquid. Using magnetotransport and ac susceptibility measurements, we find that global superconductivity fails to set in as temperature $T\rightarrow 0$. Our results indicate instead the presence of superconducting domains embedded in the metallic percolating cluster that undergo a magnetic field-tuned quantum superconductor-to-metal phase transition. Surprisingly, albeit consistent with the percolation picture, universal conductance fluctuations are seen at high fields in macroscopic samples. The observed interplay of the intrinsic inhomogeneity and quantum phase fluctuations provides a new insight into failed superconductivity, a phenomenon seen in various conventional and unconventional superconductors, including cuprates.

cond-mat.supr-con↗
arXiv2025

Field induced density wave in a kagome superconductor

On the kagome lattice, electrons benefit from the simultaneous presence of band topology, flat electronic bands, and van Hove singularities, forming competing or cooperating orders. Understanding the interrelation between these distinct order parameters remains a significant challenge, leaving much of the associated physics unexplored. In the kagome superconductor KV3Sb5, which exhibits a charge density wave (CDW) state below T = 78 K, we uncover an unpredicted field-induced phase transition below 6 K. The observed transition is marked by a hysteretic anomaly in the resistivity, nonlinear electrical transport, and a change in the symmetry of the electronic response as probed via the angular dependence of the magnetoresistivity. These observations surprisingly suggest the emergence of an unanticipated broken symmetry state coexisting with the original CDW. To understand this experimental observation, we developed a theoretical minimal model for the normal state inside the high-temperature parent CDW phase where an incommensurate CDW order emerges as an instability sub-leading to superconductivity. The incommensurate CDW emerges when superconducting fluctuations become fully suppressed by large magnetic fields. Our results suggest that, in kagome superconductors, quantum states can either coexist or are nearly degenerate in energy, indicating that these are rich platforms to expose new correlated phenomena.

cond-mat.str-el↗
arXiv2024

Persistence of vortexlike phase fluctuations in underdoped to heavily overdoped Bi-2201 cuprates

The mechanism that controls the superconducting (SC) transition temperature $T_{\mathrm{c}}^{0}$ as a function of doping is one of the central questions in cuprate high-temperature superconductors. While it is generally accepted that $T_{\mathrm{c}}^{0}$ in underdoped cuprates is not determined by the scale of pairing but by the onset of global phase coherence, the role of phase fluctuations in the overdoped region has been controversial. Here, our transport measurements in perpendicular magnetic fields ($H$) on underdoped Bi-2201 reveal immeasurably small Hall response for $T>T_{\mathrm{c}}(H)$ as a signature of SC phase with vortexlike phase fluctuations. We find that the extent of such a regime in $T$ and $H$ is suppressed near optimal doping but becomes strongly enhanced in heavily overdoped Bi-2201. Our results thus show that vortexlike phase fluctuations play an important role in the field-tuned SC transition in the heavily overdoped region, in contrast to conventional mean-field Bardeen-Cooper-Schrieffer description. The unexpected nonmonotonic dependence of phase fluctuations on doping provides a new perspective on the SC transition in cuprates.

cond-mat.supr-con↗
arXiv2023

Screening the Coulomb interaction leads to a prethermal regime in two-dimensional bad conductors

The absence of thermalization in certain isolated many-body systems is of great fundamental interest. Many-body localization (MBL) is a widely studied mechanism for thermalization to fail in strongly disordered quantum systems, but it is still not understood precisely how the range of interactions affects the dynamical behavior and the existence of MBL, especially in dimensions $D>1$. By investigating nonequilibrium dynamics in strongly disordered $D=2$ electron systems with power-law interactions $\propto 1/r^α$ and poor coupling to a thermal bath, here we observe MBL-like, prethermal dynamics for $α=3$. In contrast, for $α=1$, the system thermalizes, although the dynamics is glassy. Our results provide important insights for theory, especially since we obtained them on systems that are much closer to the thermodynamic limit than synthetic quantum systems employed in previous studies of MBL. Thus, our work is a key step towards further studies of ergodicity breaking and quantum entanglement in real materials.

cond-mat.dis-nn↗
arXiv2021

Transport signatures of fragile-glass dynamics in the melting of the two-dimensional vortex lattice

In two-dimensional (2D) systems, the melting from a solid to an isotropic liquid can occur via an intermediate phase that retains orientational order. However, in 2D superconducting vortex lattices, the effect of orientational correlations on transport, and their interplay with disorder remain open questions. Here we study a 2D weakly pinned vortex system in amorphous MoGe films over an extensive range of temperatures ($\bm{T}$) and perpendicular magnetic fields ($\bm{H}$) using linear and nonlinear transport measurements. We find that, at low fields, the resistivity obeys the Vogel-Fulcher-Tamman (VFT) form, $\bm{ρ(T)\propto\exp[-{W}(H)/(T-T_0(H))]}$, characteristic of fragile glasses. As $\bm{H}$ increases, $\bm{T_0(H)}$ is suppressed to zero, and a standard vortex liquid behavior consistent with a $\bm{T=0}$ superconducting transition is observed. Our findings, supported also by simulations, suggest that the presence of orientational correlations gives rise to a heterogeneous dynamics responsible for the VFT behavior. The effects of quenched disorder become dominant at high $\bm{H}$, where a crossover to a strong-glass behavior is observed. This is a new insight into the dynamics of melting in 2D systems with competing orders.

cond-mat.supr-con↗
arXiv2021

Magnetic field reveals vanishing Hall response in the normal state of stripe-ordered cuprates

The origin of the weak insulating behavior of the resistivity, i.e. $ρ_{xx}\propto\ln(1/T)$, revealed when magnetic fields ($H$) suppress superconductivity in underdoped cuprates has been a longtime mystery. Surprisingly, the high-field behavior of the resistivity observed recently in charge- and spin-stripe-ordered La-214 cuprates suggests a metallic, as opposed to insulating, high-field normal state. Here we report the vanishing of the Hall coefficient in this field-revealed normal state for all $T<(2-6)T_{\mathrm{c}}^{0}$, where $T_{\mathrm{c}}^{0}$ is the zero-field superconducting transition temperature. Our measurements demonstrate that this is a robust fundamental property of the normal state of cuprates with intertwined orders, exhibited in the previously unexplored regime of $T$ and $H$. The behavior of the high-field Hall coefficient is fundamentally different from that in other cuprates such as YBa$_2$Cu$_3$O$_{6+x}$ and YBa$_2$Cu$_4$O$_{8}$, and may imply an approximate particle-hole symmetry that is unique to stripe-ordered cuprates. Our results highlight the important role of the competing orders in determining the normal state of cuprates.

cond-mat.supr-con↗
arXiv2021

Charge order dynamics in underdoped La$\mathbf{_{1.6-\textit{x}}}$Nd$\mathbf{_{0.4}}$Sr$\mathbf{_\textit{x}}$CuO$\mathbf{_{4}}$ revealed by electric pulses

The dynamics of the charge-order domains has been investigated in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$, a prototypical stripe-ordered cuprate, using pulsed current injection. We first identify the regime in which nonthermal effects dominate over simple Joule heating, and then demonstrate that, for small enough perturbation, pulsed current injection allows access to nonthermally-induced resistive metastable states. The results are consistent with pinning of the fluctuating charge order, with fluctuations being most pronounced at the charge-order onset temperature. The nonequilibrium effects are revealed only when the transition is approached from the charge-ordered phase. Our experiment establishes pulsed current injection as a viable and effective method for probing the charge-order dynamics in various other materials.

cond-mat.str-el↗
arXiv2020

Low-Temperature 2D/2D Ohmic Contacts in WSe$_2$ Field-Effect Transistors as a Platform for the 2D Metal-Insulator Transition

We report the fabrication of hexagonal-boron-nitride (hBN) encapsulated multi-terminal WSe$_2$ Hall bars with 2D/2D low-temperature Ohmic contacts as a platform for investigating the two-dimensional (2D) metal-insulator transition. We demonstrate that the WSe$_2$ devices exhibit Ohmic behavior down to 0.25 K and at low enough excitation voltages to avoid current-heating effects. Additionally, the high-quality hBN-encapsulated WSe$_2$ devices in ideal Hall-bar geometry enable us to accurately determine the carrier density. Measurements of the temperature ($T$) and density ($n_s$) dependence of the conductivity $σ(T,n_s)$ demonstrate scaling behavior consistent with a metal-insulator quantum phase transition driven by electron-electron interactions, but where disorder-induced local magnetic moments are also present. Our findings pave the way for further studies of the fundamental quantum mechanical properties of 2D transition metal dichalcogenides using the same contact engineering.

cond-mat.mes-hall↗
arXiv2020

Unveiling the phase diagram of a striped cuprate at high magnetic fields: Hidden order of Cooper pairs

The interplay of charge orders with superconductivity in underdoped cuprates at high magnetic fields ($H$) is an open question, and even the value of the upper critical field ($H_{c2}$), a measure of the strength of superconductivity, has been the subject of a long-term debate. We combined three complementary transport techniques on underdoped La$_{1.8-x}$Eu$_{0.2}$Sr$_{x}$CuO$_{4}$ with a "striped" charge order and a low $H=0$ transition temperature $T_{c}^{0}$, to establish the $T-H$ phase diagram and reveal the ground states in CuO$_2$ planes: a superconductor, a wide regime of superconducting phase fluctuations (i.e. a vortex liquid), and a high-field normal state. The relatively high $H_{c2}$ is consistent with the opening of a superconducting gap above $T_{c}^{0}$, but only at $T\sim (2$-$3)T_{c}^{0}$, an order of magnitude below the pseudogap temperature. Within the vortex liquid, an unanticipated, insulatinglike region, but with strong superconducting correlations, begins to emerge already at $T\lesssim T_{c}^{0}$. The results suggest that the presence of stripes plays a crucial role in the freezing of Cooper pairs in this novel state. Our findings provide a fresh perspective on the pairing strength in underdoped cuprates, and introduce a new avenue for exploring the interplay of various orders.

cond-mat.supr-con↗
arXiv2020

Signatures of a pair density wave at high magnetic fields in cuprates with charge and spin orders

In underdoped cuprates, the interplay of the pseudogap, superconductivity, and charge and spin ordering can give rise to exotic quantum states, including the pair density wave (PDW), in which the superconducting (SC) order parameter is oscillatory in space. However, the evidence for a PDW state remains inconclusive and its broader relevance to cuprate physics is an open question. To test the interlayer frustration, the crucial component of the PDW picture, we performed transport measurements on La$_{1.7}$Eu$_{0.2}$Sr$_{0.1}$CuO$_{4}$ and La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$, cuprates with "striped" spin and charge orders, in perpendicular magnetic fields ($H_\perp$), and also with an additional field applied parallel to CuO$_2$ layers ($H_\parallel$). We detected several phenomena predicted to arise from the existence of a PDW, including an enhancement of interlayer SC phase coherence with increasing $H_\parallel$. Our findings are consistent with the presence of local, PDW pairing correlations that compete with the uniform SC order at $T_{c}^{0}< T<(2-6) T_{c}^{0}$, where $T_{c}^{0}$ is the $H=0$ SC transition temperature, and become dominant at intermediate $H_\perp$ as $T\rightarrow 0$. These data also provide much-needed transport signatures of the PDW in the regime where superconductivity is destroyed by quantum phase fluctuations.

cond-mat.supr-con↗
arXiv2019

Vortex phase diagram and the normal state of cuprates with charge and spin orders

The phase diagram of underdoped cuprates in a magnetic field ($H$) is the key ingredient in understanding the anomalous normal state of these high-temperature superconductors. However, the upper critical field ($H_{c2}$) or the extent of superconducting phase with vortices, a type of topological excitations, and the role of charge orders that are present at high $H$, remain under debate. We address these questions by studying stripe-ordered La-214, i.e. cuprates in which charge orders are most pronounced and zero-field transition temperatures $T_{c}^{0}$ are lowest; the latter opens a much larger energy scale window to explore the vortex phases compared to previous studies. By combining linear and nonlinear transport techniques sensitive to vortex matter, we determine the $T$-$H$ phase diagram, directly detect $H_{c2}$, and reveal novel properties of the high-field ground state. Our results demonstrate that, while the vortex phase diagram of underdoped cuprates is not very sensitive to the details of the charge orders, quantum fluctuations and disorder play a key role as $T\rightarrow 0$. The presence of stripes, on the other hand, seems to alter the nature of the anomalous normal state, such that the high-field ground state is a metal, as opposed to an insulator.

cond-mat.supr-con↗
arXiv2019

Tuning from failed superconductor to failed insulator with magnetic field

Do charge modulations compete with electron pairing in high-temperature copper-oxide superconductors? We investigated this question by suppressing superconductivity in a stripe-ordered cuprate compound at low temperature with high magnetic fields. With increasing field, loss of three-dimensional superconducting order is followed by reentrant two-dimensional superconductivity and then an ultra-quantum metal phase. Circumstantial evidence suggests that the latter state is bosonic and associated with the charge stripes. These results provide experimental support to the theoretical perspective that local segregation of doped holes and antiferromagnetic spin correlations underlies the electron-pairing mechanism in cuprates.

cond-mat.supr-con↗
arXiv2018

Collective Dynamics and Strong Pinning near the Onset of Charge Order in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$

The dynamics of charge-ordered states is one of the key issues in underdoped cuprate high-temperature superconductors, but static short-range charge-order (CO) domains have been detected in almost all cuprates. We probe the dynamics across the CO (and structural) transition in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$ by measuring nonequilibrium charge transport, or resistance $R$ as the system responds to a change in temperature and to an applied magnetic field. We find evidence for metastable states, collective behavior, and criticality. The collective dynamics in the critical regime indicates strong pinning by disorder. Surprisingly, nonequilibrium effects, such as avalanches in $R$, are revealed only when the critical region is approached from the charge-ordered phase. Our results on La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$ provide the long-sought evidence for the fluctuating order across the CO transition, and also set important constraints on theories of dynamic stripes.

cond-mat.str-el↗
arXiv2016

Current-voltage characteristics and vortex dynamics in highly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$

The temperature dependence of the nonlinear current-voltage ($I$-$V$) characteristics in highly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x=0.07$ and 0.08) thick films has been studied in both zero and perpendicular magnetic fields $H$. Power-law behavior of $V(I)$ is found for both $H=0$ and $H \neq 0$. The critical current $I_{c}$ was extracted, and its temperature and magnetic field dependences were studied in detail. The Berezinskii-Kosterlitz-Thouless physics dominates the nonlinear $I$-$V$ near the superconducting transition at $H=0$, and it continues to contribute up to a characteristic temperature $T_x(H)$. Nonlinear $I$-$V$ persists up to an even higher temperature $T_{h}(H)$ due to the depinning of vortices.

cond-mat.supr-con↗
arXiv2016

Metal-insulator transition in correlated two-dimensional systems with disorder

Experimental evidence for the possible universality classes of the metal-insulator transition (MIT) in two dimensions (2D) is discussed. Sufficiently strong disorder, in particular, changes the nature of the transition. Comprehensive studies of the charge dynamics are also reviewed, describing evidence that the MIT in a 2D electron system in silicon should be viewed as the melting of the Coulomb glass. Comparisons are made to recent results on novel 2D materials and quasi-2D strongly correlated systems, such as cuprates.

cond-mat.str-el↗
arXiv2016

Effective 2D thickness for the Berezinskii-Kosterlitz-Thouless-like transition in a highly underdoped La$_{2-x}$Sr$_x$CuO$_4$

The nature of the superconducting transition in highly underdoped thick films of La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.07$ and 0.08) has been investigated using the in-plane transport measurements. The contribution of superconducting fluctuations to the conductivity in zero magnetic field, or paraconductivity, was determined from the magnetoresistance measured in fields applied perpendicular to the CuO$_2$ planes. Both the temperature dependence of the paraconductivity above the transition and the nonlinear current-voltage ($I-V$) characteristics measured across it, exhibit the main signatures of the Berezinskii-Kosterlitz-Thouless (BKT) transition. The quantitative comparison of the superfluid stiffness, extracted from the $I-V$ data, with the renormalization-group results for the BKT theory, reveals a large value of the vortex-core energy. This finding is confirmed by the analysis of the paraconductivity obtained using different methods. The results strongly suggest that the characteristic energy scale controlling the BKT behavior in this layered system corresponds to the superfluid stiffness of a few layers.

cond-mat.supr-con↗