arXiv subjects

Dragan Mihailovic

Publications and source records attributed to Dragan Mihailovic.

At least 37 records · Page 2Linked to original sources

arXiv2019

Intertwined chiral charge orders and topological stabilization of the light-induced state of a prototypical transition metal dichalcogenide

The fundamental idea that the constituents of interacting many body systems in complex quantum materials may self-organise into long range order under highly non-equilibrium conditions leads to the notion that entirely new and unexpected functionalities might be artificially created. However, demonstrating new emergent order in highly non-equilibrium transitions has proven surprisingly difficult. In spite of huge recent advances in experimental ultrafast time-resolved techniques, methods that average over successive transition outcomes have so far proved incapable of elucidating the emerging spatial structure. Here, using scanning tunneling microscopy, we report for the first time the charge order emerging after a single transition outcome in a prototypical two-dimensional dichalcogenide 1T-TaS$_2$ initiated by a single optical pulse. By mapping the vector field of charge displacements of the emergent state, we find surprisingly intricate, long-range, topologically non-trivial charge order in which chiral domain tiling is intertwined with unique unpaired dislocations which play a crucial role in enhancing the emergent states remarkable stability. The discovery of the principles that lead to metastability in charge-ordered systems open the way to designing novel emergent functionalities, particularly ultrafast all-electronic non-volatile cryo-memories.

cond-mat.str-el↗

arXiv2019

Theoretical Modeling of the Non-equilibrium Amorphous State in 1T-TaS$_2$

1T-TaS$_2$ is known for it's remarkably complex phase diagram and it's unique long-lived metastable hidden (H) state. Recently, a novel metastable state has been discovered using higher fluences for photoexcitation than in the case of the H state. The state has been dubbed as amorphous (A) due to it's similarity to glass. Expanding on the work of Brazovskii and Karpov, we show that the A state can be successfully modelled with classical interacting polarons on a two-dimensional hexagonal lattice. We have found that the polaron configuration of the A state corresponds to a frustrated screened Coulomb system, where there is no order-disorder phase transition.

cond-mat.str-el↗

arXiv2019

Configurational Electronic States in Layered Metallic Dichalcogenides

Mesoscopic irregularly ordered and even amorphous self-assembled electronic structures were recently reported in two-dimensional metallic dichalcogenides (TMDs), created and manipulated with short light pulses or by charge injection. Apart from promising new all-electronic memory devices, such states are of great fundamental importance, since such aperiodic states cannot be described in terms of conventional charge-density-wave (CDW) physics. In this paper we address the problem of metastable mesoscopic configurational charge ordering in TMDs with a sparsely filled charged lattice gas model in which electrons are subject only to screened Coulomb repulsion. The model correctly predicts commensurate CDW states corresponding to different TMDs at magic filling fractions $f_m=1/3,1/4,1/9,1/13,1/16$. Doping away from $f_m$ results either in multiple near-degenerate configurational states, or an amorphous state at the correct density observed by scanning tunnelling microscopy. Quantum fluctuations between degenerate states predict a quantum charge liquid at low temperatures, revealing a new generalized viewpoint on both regular, irregular and amorphous charge ordering in transition metal dichalcogenides.

cond-mat.str-el↗

arXiv2018

Ultrafast jamming of electrons into an amorphous entangled state

New emergent states of matter in quantum systems may be created under non-equilibrium conditions if - through many body interactions - its constituents order on a timescale which is shorter than the time required for the system to reach thermal equilibrium. Conventionally non-equilibrium ordering is discussed in terms of symmetry breaking, nonthermal order-disorder, and more recently quenched topological transitions. Here we report a fundamentally new and unusual metastable form of amorphous correlation-localized fermionic matter, which is formed in a new type of quantum transition at low temperature either by short pulse photoexcitation or by electrical charge injection in the transition metal dichalcogenide 1T-TaS2. Scanning tunnelling microscopy (STM) reveals a pseudo-amorphous packing of localized electrons within the crystal lattice that is significantly denser than its hexagonally ordered low-temperature ground state, or any other ordered states of the system. Remarkably, the arrangement is not random, but displays a hyperuniform spatial density distribution commonly encountered in classical jammed systems, showing no signs of aggregation or phase separation. Unexpectedly for a localized electron system, tunnelling spectroscopy and multi- STM-tip surface resistance measurements reveal that the overall state is gapless and conducting, which implies that localized and itinerant carriers are resonantly entangled. The amorphous localized electron subsystem can be understood theoretically to arise from strong correlations between polarons sparsely dispersed on a 2D hexagonal atomic lattice, while itinerant carriers act as a resonantly coupled reservoir distinct in momentum space.

cond-mat.str-el↗

arXiv2017

Real-time observation of the coherent transition to a metastable emergent state in 1T-TaS$_2$

The transition to a hidden metastable state in 1T-TaS2 is investigated in real time using coherent time-resolved femtosecond spectroscopy. Relying on spectral differences between phonon modes in the equilibrium states and in the metastable state, and temperature-tuning the metastable state lifetime, we perform stroboscopic measurements of the electronic response and switching of coherent oscillation frequency through the transition. Very fast coherent switching of the collective mode frequency is observed (400 fs), comparable to the electronic timescale (300 fs). A slower, 4.7 ps process is attributed to lattice relaxation. The observations are described well by a fast electronic band structure transformation into the metastable state, consistent with a topological transition.

cond-mat.str-el↗

arXiv2017

Inter-site pair superconductivity: origins and recent validation experiments

The challenge of understanding high-temperature superconductivity has led to a plethora of ideas, but 30 years after its discovery in cuprates, very few have achieved convincing experimental validation. While Hubbard and t-J models were given a lot of attention, a number of recent experiments appear to give decisive support to the model of real-space inter-site pairing and percolative superconductivity in cuprates. Systematic measurements of the doping dependence of the superfluid density show a linear dependence on superfluid density - rather than doping - over the entire phase diagram, in accordance with the model's predictions. The doping-dependence of the anomalous lattice dynamics of in-plane Cu-O mode vibrations observed by inelastic neutron scattering, gives remarkable reciprocal space signature of the inter-site pairing interaction whose doping dependence closely follows the predicted pair density. Symmetry-specific time-domain spectroscopy shows carrier localization, polaron formation, pairing and superconductivity to be distinct processes occurring on distinct timescales throughout the entire superconducting phase diagram. The three diverse experimental results confirm non-trivial predictions made more than a decade ago by the inter-site pairing model in the cuprates, remarkably also confirming some of the fundamental notions mentioned in the seminal paper on the discovery of high-temperature superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2017

A new high-temperature quantum spin liquid with polaron spins

The existence of a quantum spin liquid (QSL) in which quantum fluctuations of spins are sufficiently strong to preclude spin ordering down to zero temperature was originally proposed theoretically more than 40 years ago, but its experimental realisation turned out to be very elusive. Here we report on an almost ideal spin liquid state that appears to be realized by atomic-cluster spins on the triangular lattice of a charge-density wave (CDW) state of 1T-TaS$_2$. In this system, the charge excitations have a well-defined gap of $\sim 0.3$ eV, while nuclear magnetic quadrupole resonance and muon spin relaxation experiments reveal that the spins show gapless quantum spin liquid dynamics and no long range magnetic order down to 70~mK. Canonical $T^{2}$ power-law temperature dependence of the spin relaxation dynamics characteristic of a QSL is observed from 200~K to $T_f= 55$ K. Below this temperature we observe a new gapless state with reduced density of spin excitations and high degree of local disorder signifying new quantum spin order emerging from the QSL.

cond-mat.str-el↗

arXiv2016

Clocking the Onset of Bilayer Coherence in a High-$\mathrm{T_C}$ Cuprate

In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature $\mathrm{T_C}$. Here we show via a combination of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that signatures of such a state can be obtained via three independent observables in an underdoped sample of NdBa$_2$Cu$_3$O$_{6+δ}$. The pseudogap correlations were disentangled from the response of laser-broken pairs by clocking their characteristic time-scales. The onset of a superconducting precursor state was found at a temperature $\mathrm{T_{ONS}}$ $>$ $\mathrm{T_C}$, consistent with the temperature scale identified via static optical spectroscopy. Furthermore, the temperature evolution of the coherent vibration of the Ba ion, strongly renormalized by the onset of superconductivity, revealed a pronounced anomaly at the same temperature $\mathrm{T_{ONS}}$. The microscopic nature of such a precursor state is discussed in terms of pre-formed pairs and enhanced bilayer coherence.

cond-mat.supr-con↗

arXiv2016

Electroabsorption in MoS$_2$

To translate electrical into optical signals one uses the modulation of either the refractive index or the absorbance of a material by an electric field. Contemporary electroabsorption modulators (EAMs) employ the quantum confined Stark effect (QCSE), the field-induced red-shift and broadening of the strong excitonic absorption resonances characteristic of low-dimensional semiconductor structures. Here we show an unprecedentedly strong transverse electroabsorption (EA) signal in a monolayer of the two-dimensional semiconductor MoS2. The EA spectrum is dominated by an apparent linewidth broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary to the conventional QCSE, the signal increases linearly with the applied field strength and arises from a linear variation of the distance between the strongly overlapping exciton and trion resonances. The achievable modulation depths exceeding 0.1 dBnm-1 bear the scope for extremely compact, ultrafast, energy-efficient EAMs for integrated photonics, including on-chip optical communication.

cond-mat.mtrl-sci↗

arXiv2016

Ultrafast optical spectroscopy of strongly correlated materials and high-temperature superconductors: a non-equilibrium approach

In the last two decades, non-equilibrium spectroscopies have evolved from avant-garde studies to crucial tools for expanding our understanding of the physics of strongly correlated materials. The possibility of obtaining simultaneously spectroscopic and temporal information has led to insights that are complementary to (and in several cases beyond) those attainable by studying the matter at equilibrium. Multiple phase transitions and new orders arising from competing interactions are benchmark examples where the interplay among electrons, lattice, and spin dynamics can be disentangled because of the different timescales that characterize the recovery of the initial ground state. The nature of the broken-symmetry phases and of the bosonic excitations that mediate the electronic interactions, eventually leading to superconductivity or other exotic states, can be revealed by observing the sub-picosecond dynamics of impulsively excited states. Recent experimental developments have made possible to monitor the time-evolution of both the single-particle and collective excitations under extreme conditions, such as those arising from strong and selective photo-stimulation. Here, we review the most recent achievements in the experimental and theoretical studies of the non-equilibrium electronic, optical, structural and magnetic properties of correlated materials. The focus will be mainly on the prototypical case of correlated oxides that exhibit unconventional superconductivity or other exotic phases, even though the discussion will extend also to other topical systems. The necessity of extending the actual experimental capabilities and the numerical and analytic tools to microscopically treat the non-equilibrium phenomena beyond the simple phenomenological approaches represents one of the most challenging new frontier in physics.

cond-mat.supr-con↗

arXiv2016

Three-dimensional resistivity switching between correlated electronic states in 1T-TaS2

Recent demonstrations of controlled switching between different ordered macroscopic states by impulsive electromagnetic perturbations in complex materials have opened some fundamental questions on the mechanisms responsible for such remarkable behavior. Here we experimentally address the question of whether two-dimensional (2D) Mott physics can be responsible for unusual switching between states of different electronic order in the layered dichalcogenide 1T-TaS2, or it is a result of subtle inter-layer orbitronic re-ordering of its helical stacking structure. We report on the switching properties both in-plane and perpendicular to the layers by current-pulse injection, the anisotropy of electronic transport in the commensurate ground state, and relaxation properties of the switched metastable state. Contrary to recent theoretical calculations, which predict a uni-directional metal perpendicular to the layers, we observe a large resistivity in this direction, with a temperature-dependent anisotropy. Remarkably, large resistance ratios are observed in the memristive switching both in-plane (IP) and out-of-plane (OP). The relaxation dynamics of the metastable state for both IP and OP electron transport are seemingly governed by the same mesoscopic quantum re-ordering process. We conclude that 1T-TaS2 shows resistance switching arising from an interplay of both IP and OP correlations.

cond-mat.mes-hall↗

arXiv2014

Charge photogeneration in few-layer MoS2

The two-dimensional semiconductor MoS2 in its mono- and few-layer form is expected to have a significant exciton binding energy of several 100 meV, leading to the consensus that excitons are the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating efficient charge carrier photogeneration (CPG). Here we use continuous wave photomodulation spectroscopy to identify the optical signature of long-lived charge carriers and femtosecond pump-probe spectroscopy to follow the CPG dynamics. We find that intitial photoexcitation yields a branching between excitons and charge carriers, followed by excitation energy dependent hot exciton dissociation as an additional CPG mechanism. Based on these findings, we make simple suggestions for the design of more efficient MoS2 photovoltaic and photodetector devices.

cond-mat.mtrl-sci↗

arXiv2014

Ultrafast carrier localisation in the pseudogap state of cuprate superconductors from coherent quench experiments

A pseudogap (PG) was introduced by Mott to describe a state of matter which has a minimum in the density of states at the Fermi level, deep enough for states to become localized. It can arise either from Coulomb repulsion between electrons, or due to an incipient charge or spin order, or a combination of the two. These states are rapidly fluctuating in time with random phase, so they are hard to observe experimentally. Here we present the first coherent quench measurements of the dynamical transition to the pseudogap state in the prototype high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$, revealing a marked absence of incipient collective ordering beyond a few coherence lengths on short timescales at any level of doping. Instead we find evidence for sub-picosecond carrier localization favouring a picture of pairing resulting from the competing Coulomb interaction and strain, enhanced by a Fermi surface instability.

cond-mat.str-el↗

arXiv2014

The effect of strain on the thawing of the hidden state and other transitions in 1T-TaS2

We investigate the effect of 2-dimensional (in-plane) strain on the critical transition temperature TH from the photoexcited hidden state in 1T-TaS$_2$ thin films on different substrates. We also measure the effect of in-plane strain on the transition temperature $T_{c2}$ between the nearly commensurate charge-density wave state and the commensurate state near 200 K. In each case, the strain is caused by the differential contraction of the sample and the substrate, and ranges from 0.5 % compressive strain (CaF$_2$) to 2 % tensile strain (sapphire). Strain appears to have an opposite effect on the H state and the NC-C state transitions. TH shows a large and negative strain coefficient of dT$_H$/de = - 8900+/-500 K, while $T_{c2}$ is not strongly affected by tensile strain and shows a positive coefficient for compressive strain, which is opposite to the effect observed for hydrostatic pressure.

cond-mat.mtrl-sci↗

arXiv2014

Separating pairing from quantum phase coherence dynamics above the superconducting transition by femtosecond spectroscopy

In classical superconductors an energy gap and phase coherence appear simultaneously with pairing at the transition to the superconducting state. In high-temperature superconductors, the possibility that pairing and phase coherence are distinct and independent processes has led to intense experimental search of their separate manifestations, but so far without success. Using femtosecond spectroscopy methods we now show that it is possible to clearly separate fluctuation dynamics of the superconducting pairing amplitude from the phase relaxation above the critical transition temperature. Empirically establishing a close correspondence between the superfluid density measured by THz spectroscopy and superconducting optical pump-probe response over a wide region of temperature, we find that in differently doped Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} crystals the pairing gap amplitude monotonically extends well beyond T_{c}, while the phase coherence shows a pronounced power-law divergence as T\rightarrow T_{c}, thus showing for the first time that phase coherence and gap formation are distinct processes which occur on different timescales.

cond-mat.supr-con↗

arXiv2013

Coherent topological defect dynamics and collective modes in superconductors and electronic crystals

The control of condensed matter systems out of equilibrium by laser pulses allows us to investigate the system trajectories through symmetry-breaking phase transitions. Thus the evolution of both collective modes and single particle excitations can be followed through diverse phase transitions with femtosecond resolution. Here we present experimental observations of the order parameter trajectory in the normal-superconductor transition and charge-density wave ordering transitions. Of particular interest is the coherent evolution of topological defects forming during the transition via the Kibble-Zurek mechanism, which appears to be measurable in optical pump probe experiments. Experiments on CDW systems reveal some new phenomena, such as coherent oscillations of the order parameter, the creation and emission of dispersive amplitudon modes upon the annihilation of topological defects, and mixing with weakly coupled finite-frequency (massive) bosons.

cond-mat.mes-hall↗

arXiv2004

Dynamic inhomogeneity, pairing and superconductivity in cuprates

In this review we examine the experimental evidence for dynamic inhomogeneity, defining the length, time and energy scales of the relevant elementary excitations. The dynamics of the objects below and above Tc are examined in detail with femtosecond spectroscopy and compared with magnetic and other measurements. The dynamically inhomogeneous state is described theoretically by considering an interaction between electrons, spins and the lattice. By symmetry, only electrons in degenerate states can couple to the lattice and spins to give an anisotropic, d-wave symmetry interaction. The proposed interaction acts on a mesoscopic length-scale, taking into account the interplay of Coulomb repulsion between particles and anisotropic elastic strain, and leads to the formation of bipolaron pairs and stripes. The predicted symmetry breaking associated with pairing and stripe formation are observed in numerous experiments. The phenomenology associated with the co-existence of pairs and clusters (stripes) is found to apply to many different experiments ranging from femtosecond dynamics to transport measurements. Excitations of the system are described quite well in terms of a 2-level system, although we find that a complete description may require a more complicated energy landscape due to presence of mesoscopic objects such as stripes or clusters. The formation of the superconducting state can be understood quantitatively to be the result of the establishment of phase coherence percolation across pairs and stripes.

cond-mat.supr-con↗

arXiv2002

Carrier relaxation dynamics in high temperature superconductors

In this report we review recent experimental results on photoexcited carrier relaxation dynamics on high temperature superconductors (HTSC) probed by a femtosecond time-resolved optical spectroscopy, and compare the results with the data obtained on quasi two dimensional charge density waves. In these experiments, a femtosecond laser pump pulse excites electron-hole pairs via an inter-band transition in the material. These hot carriers rapidly release their energy via electron-electron and electron-phonon collisions reaching states near the Fermi energy within ~100 fs. If an energy gap is present in the low-energy density of states (DOS), it inhibits the final relaxation step and photoexcited carriers accumulate above the gap causing a transient change in reflectivity arising from excited state absorption. The relaxation and recombination processes of photoexcited quasiparticles, governed by the magnitude, anisotropy and the T-dependence of the low energy gap, are monitored by measuring the resulting photoinduced absorption as a function of time after the photoexcitation. This way, the studies of carrier relaxation dynamics give us direct information of the T-dependent changes in the low energy DOS. The technique is particularly useful to probe the systems with spatial inhomogeneities, where different local environments give rise to different relaxation rates. The data on series of HTSC-s show evidence for the coexistence of two distinct relaxation processes, whose T-dependences seem to be governed by two different energy scales: a T-independent pseudogap and a mean-field-like T-dependent gap that opens at Tc. The data suggest the origin of the two-gap behavior is in the intrinsic microscopic spatial inhomogeneity of these materials.

cond-mat.supr-con↗