Unconventional charge order in a co-doped high-Tc superconductor

Abstract

Charge-stripe order has recently been established as an important aspect of cuprate high-T_c superconductors. However, owing to the complex interplay between competing phases and the influence of disorder, it is unclear how it emerges from the parent high-temperature state. Here we report on the discovery of an unconventional ordered phase between charge-stripe order and (pseudogapped) metal in the cuprate La_1.8-x Eu_0.2 Sr_x CuO₄ . We use three complementary experiments--nuclear quadrupole resonance, nonlinear conductivity and specific heat--to demonstrate that the order appears through a sharp phase transition and exists in a dome-shaped region of the phase diagram. Our results imply that the new phase is a state, which preserves translational symmetry: a charge nematic. We thus resolve the process of charge-stripe development in cuprates, show that this nematic phase is distinct from high-temperature pseudogap and establish a link with other strongly correlated electronic materials with prominent nematic order.

ARTICLE

Received 23 Feb 2016 | Accepted 1 Aug 2016 | Published 8 Sep 2016

Charge-stripe order has recently been established as an important aspect of cuprate high-Tc superconductors. However, owing to the complex interplay between competing phases and the inuence of disorder, it is unclear how it emerges from the parent high-temperature state. Here we report on the discovery of an unconventional ordered phase between charge-stripe order and (pseudogapped) metal in the cuprate La1.8 xEu0.2SrxCuO4. We use three complementary experimentsnuclear quadrupole resonance, nonlinear conductivity and specic heatto demonstrate that the order appears through a sharp phase transition and exists in a dome-shaped region of the phase diagram. Our results imply that the new phase is a state, which preserves translational symmetry: a charge nematic. We thus resolve the process of charge-stripe development in cuprates, show that this nematic phase is distinct from high-temperature pseudogap and establish a link with other strongly correlated electronic materials with prominent nematic order.

DOI: 10.1038/ncomms12775 OPEN

Unconventional charge order in a co-doped high-Tc

superconductor

D. Pelc1, M. Vukovi1, H.-J. Grafe2, S.-H. Baek2 & M. Poek1

1 Faculty of Science, Department of Physics, University of Zagreb, Bijenika 32, Zagreb HR 10000, Croatia. 2 IFW Dresden, Institute for Solid State Research, P.O. Box 270116, Dresden D-01171, Germany. Correspondence and requests for materials should be addressed to D.P. (email: mailto:[email protected]

Web End [email protected] )or to M.P. (email: mailto:[email protected]

Web End [email protected] ).

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Cuprate high-temperature superconductors display a staggering complexity of electronic behaviour, which is the subject of intense research and controversy. One of the

central questions of cuprate physics is the role of competing electronic ordering tendencies in determining both normal-state and superconducting properties. Recently, charge-stripe order has emerged as a ubiquitous phenomenon in cuprates17 and charge stripes have been proposed to play a role in almost all of the important features of the cuprate phase diagram: the mysterious pseudogap phase8,9, Fermi surface reconstruction10 and even superconducting pairing11. However, rm experimental evidence for these claims is difcult to obtain, because charge stripes are sensitive to disorder, which is a prominent feature of all cuprate materials12. Thus, true long-range charge-stripe order is never established and a complex glassy/uctuating stripe dynamics emerges9,13,14. Adding to the intrigue, several theoretical studies predict that charge stripes appear through unconventional precursor phases1518, such as a charge nematic, which do not break translational symmetry at all. Although electron nematic ordering has been discussed extensively in other strongly correlated materials such as pnictide superconductors19,20 and quantum Hall systems21, its existence in the cuprates is less clear2226. Thus, gaining insight into the physics of the emergence of charge stripes from a parent high-temperature metallic state becomes of great interest.

Here we combine an unusual experimental technique nonlinear conductivitywith two established onesnuclear quadrupole resonance (NQR) and specic heatto study the appearance of charge stripes in the cuprate La2 x yEuySrxCuO4

with y 0.2 (LESCO). We use LESCO as a model system because

of its fortunate arrangement of structural and charge-related transitions. It is well known that lanthanum cuprates in their low-temperature tetragonal (LTT) phases have prominent charge and spin stripe order; the archetypal example is La2 xBaxCuO4

(LBCO), where superconductivity is strongly suppressed by stripe order around x 1/8, and separate charge and spin stripe

ordering temperatures are observed in neutron scattering, transport and magnetic susceptibility2729. However, in LBCO the static stripes only exist in the LTT phase and abruptly disappear on heating above the tetragonal-to-orthorhombic (LTT/LTO) transition, precluding any investigation of their intrinsic melting mechanism in the tetragonal setting. We therefore use the europium co-doped LESCO as a representative system for studying charge-stripe physics, owing to static charge stripes disappearing spontaneously far below the LTT/LTO transition temperature5,3032.

Our work provides unprecedented experimental insight into the dynamics and thermodynamics of the process of charge-stripe formation in the cuprates. We show that indeed the stripes develop through an unconventional precursor phase consistent with a charge nematic. This phase is found to be insensitive to disorder, appearing at a true thermodynamic phase transition, in agreement with recent theoretical predictions for nematics9. In contrast, the transition into the charge-stripe phase is smeared out by disorder, resulting in glassiness and short-range stripe correlations. Importantly, we nd that the nematic order closely follows the dome-shaped charge-stripe appearance in the phase diagram, implying that it is separate from the high-temperature pseudogap in LESCO.

ResultsNuclear magnetic resonance. NQR experiments on copper, specically measurements of NQR signal intensity in dependence on temperature, provide the rst indication that an additional phase exists between charge order and the metallic state in

LESCO. A decrease of Cu NQR signaltermed the wipeout effecthas been previously observed in charge-ordered phases of several cuprates, including LBCO and LESCO13,3337. In our experiments, strong wipeout also begins at the charge-order onset temperatures TCO obtained by resonant X-ray scattering31but in addition a wipeout plateau is observed, extending 1020 K above TCO and ending at a temperature TEN, signicantly below the structural transition (Fig. 1). For doping x 0.125, we have

performed an additional Cu nuclear magnetic resonance (NMR) measurement in a eld of 11T, perpendicular to the crystalline c axis to obtain pure exponential spinspin relaxation38. Owing to a measuring frequency B4 times higher than in NQR, the NMR wipeout results have signicantly better signal-to-noise ratios and the measurement conrms the existence of an intermediate phase (Fig. 1). Interestingly, the wipeout fraction in 11T is systematically lower than in pure NQR by a factor of B1.6, implying that the strong in-plane eld modies the uctuations responsible for wipeout. A similar inuence is observed39 in La NMR of LBCO-1/8. The detailed investigation of magnetic eld effects is left for future discussion.

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Figure 1 | Copper NQR signal wipeout. NQR measurements of 63Cu signal

intensity in three La1.8 xEu0.2SrxCuO4 samples, compensated for spinspin relaxation and Boltzmann temperature dependence, and normalized to high-temperature values. Lines are guides to the eye and error bars are s.d. of the spinspin decay exponential least-square ts. The intermediate phase above the charge-ordering temperature is clearly seen as a plateau in the wipeout fraction; the logarithmic scale is noteworthy. Empty squares forx 0.125 are an additional Cu NMR measurement in an external eld of 11T,

also displaying a wipeout plateau. The structural orthorhombic-tetragonal (LTO-LTT) transition is at B130 K for all samples and the charge-stripe ordering temperatures TCO are from a resonant X-ray study31.

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Although Cu wipeout has been studied before in LESCO and related compounds, to our knowledge the plateau we see here has not been observed before. There are several possible reasons for this: the use of powders instead of single crystals, insufcient resolution and the dependence of wipeout on magnetic eld, which has not been systematically investigated. Here, both temperature and wipeout resolution is signicantly higher than in previous studies, we only work with single crystals to eliminate powder sample/grain boundary issues13,33,36,37 and we have carefully avoided or compensated for spurious effects, which could modify the signal intensity. Structural changes cannot inuence the results (the LTO/LTT transition is close to 130 K in the entire investigated doping range31,40 and no signicant intensity change is seen at TLTT), spinspin relaxation is compensated, for using exponential ts (see Supplementary Figs 1 and 2), the NQR lineshape remains essentially the same throughout our temperature range13,34 (see Supplementary Fig. 3) and skin depth changes at MHz frequencies are of the order of 1% in our range of temperatures, too small to account for the signal change (see Supplementary Fig. 4 and Methods for more details on these effects and procedures). However, we stress that the microscopic mechanism causing wipeout is a matter of debate and without a deeper understanding of its origin, wipeout remains a blunt instrument for studying electronic order. Here, its main purpose is to show that two distinct characteristic temperatures are always present, allowing us to construct a phase diagram.

A close relationship between stripe order and the intermediate phase can be inferred from the behaviour of transition temperatures at different doping concentrations. We dene the intermediate phase transition temperature TEN as the midpoint of the wipeout step (the values agree with specic heat results, to be discussed below). TEN approximately follows the dependence of

TCO on Sr concentration (Fig. 2). Another appealing feature of the wipeout plateau is the dependence of its height (compared with high-temperature values) on doping (Fig. 2, empty squares), showing a trend different from the transition temperatures. It appears that the intermediate phase either merges with the stripe phase or vanishes altogether at xE0.17. The LTT structure becomes unstable at similar dopings31,40, although previous measurements do give indications of stripe order persisting up to xB0.2 (refs 33,36).

As a local probe, NQR cannot tell us more about the long-range properties of the intermediate phase or whether it is a distinct state of electronic matter at all. As there is no sign of the intermediate phase in X-ray scatteringwhich needs broken translational symmetry and coherence lengths of at least a few nanometreswe conclude that the order is either short ranged or possesses higher symmetry than charge stripes. This leads to two possible scenarios: phase separation, wherein nanoscale stripe-ordered bubbles are mixed with regions of pseudogapped metal, or a lower symmetry phase (stripe liquid), with restored long-range translational symmetry, but rotational symmetry still broken. To decide between the two possibilities we perform two additional experiments on the sample with x 1/8nonlinear

conductivity and heat capacity measurements.

Nonlinear conductivity. Nonlinear conductivity is generally understood as the leading order correction to conventional Ohms law:

j sE s3E3 ::: 1 where j is the current density, s the linear conductivity and s3 the leading nonlinear correctionif time-reversal or inversion symmetry is not broken, only odd powers of E can enter relation (1).

Relation (1) is written in a schematic way, as s3 is in general a fourth rank tensor. In our experiment the sample is oriented so that the electric elds and nonlinear currents lie in the CuO2 planes; thus, we only detect planar components of the s3 tensor (see Supplementary Note 1 for details). Owing to high linear conductivity of doped cuprates, measuring s3 without signicant unwanted heating effects is challenging. We have therefore developed a contactless pulse method41, wherein currents are induced in the sample at some frequency o (typically B20 MHz)

and the response at 3o (which is proportional to s3) is detected. Results of such a measurement are shown in Fig. 3, with two characteristic features clearly resolved: charge order below B75 K, where a nonlinear signal is expected due to either stripe pinning dynamics42,43 or glassiness44,45, and the dramatic peak at TEN. We stress that the linear conductivity is smooth and almost featureless (see Supplementary Fig. 4). Yet, small deviations from high-temperature behaviour were previously detected close to TEN even in linear transport measurements on LESCO10,46

possibly another consequence of the unconventional order.

Specic heat. The peak in s3 is highly suggestive of a diverging susceptibility at a phase transition. If a phase transition indeed occurs at TEN, it must have a signature in the specic heat, regardless of its microscopic nature. We succeeded in detecting such a signature in a sensitive differential calorimetry experiment. The results for the sample with x 0.125 are shown in Fig. 4, with

two visible featuresthe structural transition at TLTTB130 K and

a distinct peak at TEN. A comparison of nonlinear conductivity and DCp (inset) displays the concordance between transition temperatures. It is noteworthy that the DCp in the inset of Fig. 4 is a different, higher resolution measurement than the one in the

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Figure 2 | Phase diagram of La1.8 xEu0.2SrxCuO4. The intermediate phase, as obtained from NQR wipeout and specic heat, approximately follows the charge-stripe onset temperatures TCO (green circles, from resonant X-ray; ref. 31) for the samples investigated. The structural transition temperatures TLTT (grey diamonds) are from ref. 40 and signicantly higher than either TCO or TEN. The transition temperatures TEN (red circles) are determined as the midpoint of the wipeout steps, with the error bars denoting the widths of the steps. These TEN from NQR agree with transition temperatures seen in specic heat, for two samples (empty triangles). The height of the wipeout plateau (empty squares) has a different trend with increasing Sr content, indicating that the intermediate phase disappears at a critical doping slightly above x 0.17, which

approximately coincides with the critical tilt angle of CuO octahedra beyond which the tetragonal (LTT) phase is unstable. Error bars are s.d. from the means of the wipeout steps.

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Figure 3 | Third harmonic response in La1.675Eu0.2Sr0.125CuO4. The

nonlinear conductivity s3 is shown in dependence on temperature and normalized to the low-temperature value (circles), with the electric elds in the CuO2 planes. Error bars are s.d. from measurements at different electric elds (see Methods). A sharp peak is observed at the intermediate phase transition temperature TEN, in good agreement with nucelar quadrupole resonance wipeout measurements (squares; error bars are s.d. of the spinspin decay exponential least-square ts). The linear conductivity is featureless in the entire temperature range. Solid lines are a guide to the eye. The nonlinear signal closely follows the charge-stripe peak intensity (diamonds) obtained in a resonant X-ray study30 below TCO, but not at higher temperatures. This implies that the unconventional phase between TCO and TEN is qualitatively different from charge stripes and does not break translational symmetry.

80 100 120 140

Figure 4 | Differential heat capacity measurement in La1.675Eu0.2Sr0.125CuO4. Two features can be discernedthe

orthorhombic-tetragonal structural transition at TLTTB130 K and a

signature of the intermediate phase at TEN. The latter is strong evidence

that a true thermodynamic phase transition takes place close to TEN. Inset

shows a comparison between the quadrupolar correlation function |s3|(kT)3 (circles; error bars are s.d. from measurements at different electric elds)

and DCp (small circles) for temperatures in the vicinity of TEN. The DCp measurement in the inset is performed B10 times slower than in the main graph, enhancing the sharpness of the cusp at TEN. The solid line is a t to a logarithmic divergence ln|(T TEN)/TEN|, showing that no simple power law

(scaling) relation can describe the data.

main graph, demonstrating both the repeatability and sharpness of the peak at TEN. An additional measurement was performed on the sample with x 0.15, conrming the presence of the

transition (see Supplementary Fig. 5). The transition temperatures TEN from all three methodsNQR, nonlinear conductivity and specic heatagree very well, justifying our denition of TEN

as the midpoint of the wipeout step in Fig. 2. Notably, no sharp features are present at TCOB75 K: this is to be expected, as charge stripes are sensitive to disorder and only in a clean material would they occur through a true phase transition.

DiscussionOur combined experimental approach establishes the existence of a well-dened phase between charge-stripe order and the metallic state in LESCO. Furthermore, nonlinear response and specic heat strongly suggest that a distinct type of order exists in the intermediate phase. Namely, the ordering is insensitive to structural disorder present in all doped cuprates, in contrast to stripes: orthorhombic twin domains create pinning centres for stripe order, making it glassy and smearing out the stripe ordering transition2,9,13. The temperature TCO is thus not a true phase transition, but rather an onset temperature where translational-symmetry breaking stripe correlations rst appear. The intermediate ordering transition, on the contrary, remains thermodynamically well dened. Thus, the intermediate phase order clearly behaves in a qualitatively different way from charge stripes, either homogeneous or phase separated. Although the nonlinear signal close to TEN might also be a dynamical, glassy effect44, we believe that this is not the case, for several reasons as follows: the transition is sharp, which would be odd for a glass transition; within experimental sensitivity, we have not observed

hysteresis or ageing effects characteristic of glassy systems (including spin glasses, which do occur in cuprates at lower temperatures); and the peak-like signature in heat capacity can only mean a true phase transition.

Having established the existence of the intermediate ordering phase, we now show it is consistent with a charge nematic. A recent theory18 predicts a series of phase transitions between charge-stripe order and Fermi liquid (in a clean material), in line with our observations. In that scenario, the intermediate phase would correspond to a nematic stripe loop metala state with broken rotational symmetry, essentially a stripe liquid with proliferated double defects (stripe loops), with a preferred direction (see schematic depiction on Fig. 5). Related nematicity has been extensively discussed in strongly correlated systems5,9,15,16,19,20,22,23. In addition, a recent eld-theoretical investigation has demonstrated that even weak quenched disorder destroys long-range charge-stripe correlationsprecluding a sharp transition in the charge-stripe phasewhereas the charge nematic transition remains well-dened9, in agreement with our data. A charge nematic has been shown to induce spin stripe uctuations if the system is close to an antiferromagnetic instability47, which is the case in cuprates. Such strong local spin uctuations provide a natural explanation of the observed wipeout plateau34,35. Normally, the most direct sign of rotational symmetry breaking is the resulting conductivity anisotropy, as for example, in quantum Hall systems21, but in the tetragonal phase of LESCO the nematic ordering direction is expected to vary from one CuO2 plane to the next28, so that no macroscopic anisotropy appears in linear conductivity. However, under quite general conditions, an extended uctuation-dissipation relation connects the quantity |s3|(kT)3 to four-point, quadrupolar charge correlations45,48, which have the same symmetry as nematic uctuations (see Supplementary Note 1). In other words, the

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Charge stripe Nematic Loop metal?

TCO

TEN

Figure 5 | Electronic liquid crystal physics in La1.8 xEu0.2SrxCuO4. Three proposed phases are depicted schematically, according to the stripe melting model of refs 17,18: the charge-stripe phase below TCO, the loop nematic

phase between TCO and TEN, and the (tentative) loop metal phase with strong nematic uctuations above TEN. Only joined stripe ends exist in both nematic and loop metal phase; other stripe lines are assumed to extend out of the pictures. In a realistic, disordered material, the phase transition at TCO

is smeared out and charge stripes become glassy, whereas the nematic order remains sharply dened.

connection between third harmonic response and nematic uctuations originates from the quadrupolar nature of nematic order16,45,48,49. All two-point correlations, and thus linear response components, are featureless, because in a nematic only the direction (and not the relative sign) of electronic density modulations is important. Consequently, nematic order couples to other probes sensitive to quadrupolar effects, such as Raman scattering50 and NQR spectroscopy51. Yet, the latter connection is not useful in La-based cuprates due to the great width of the Cu NQR lines and strong antiferromagnetic uctuations, only leaving us with the possibility to observe the indirect inuence of the nematic on NQR wipeout. The above symmetry-based reasoning is rather general: higher-order correlators also appear in spin glass/spin nematic physics, where translational symmetry remains unbroken as well. Therefore, |s3|(kT)3 is a measure of charge nematic uctuations and it diverges at the transition temperature TEN, strongly suggesting that the transition is into a nematic phase.

Curiously, the temperature dependence of |s3|(kT)3 does not follow any scaling relation (although the small range below TEN

precludes any strong conclusions there). Instead, it seems to possess a logarithmic divergence of the form s3

j jkT3

ln T TEN=TEN

j j (inset of Fig. 4). Although it is difcult to

reliably differentiate between a logarithmic dependence and possible corrections to scaling, either way we have an indication that any mean-eld description of uctuations above TEN breaks down. It is tempting to speculate that this is related to the most intriguing prediction of the theory of stripe melting17,18the existence of another exotic metallic phase between charge nematic and Fermi liquid, a stripe loop metal, which should exhibit strong quadrupolar uctuations18 and thus large s3.

The results of our NQR, nonlinear response and specic heat investigations taken together with resonant X-ray scattering30 enable us to form a complete picture of charge-stripe appearance, which we summarize in Fig. 5. Starting from high temperatures, nematic uctuations appear and diverge at the well-dened nematic phase transition temperature TEN. The nematic phase breaks orientational symmetry and it can be imagined as a stripe liquid with no translational symmetry breaking. This liquid then gradually freezes, with the rst appearance of broken translational symmetry at TCO (as seen by X-ray scattering). However, the disorder present in the material strongly inuences the stripe freezing, eliminating a sharp charge-stripe phase transition and making TCO a crossover temperature. Long-range stripe order is

never established, as evidenced by relatively small X-ray coherence lengths31 and glassy dynamics observed in lanthanum NMR14,39.

Our study clearly shows that charge stripes develop through a nematic phase. As LESCO was carefully chosen as a model material, this scenario could plausibly apply to cuprates in general, with some material-specic modications. Perhaps most notably, the frequently studied YBa2Cu3Oy also shows charge stripes in a dome-shaped region1,3,6, but the formation of nematic order is more difcult to establish, as YBa2Cu3Oy already has broken orientational symmetry due to CuO chains present in its structure. Furthermore, the recent observation of charge order4 in the tetragonal HgBa2CuO4 d (Hg-1201) might provide an

opportunity to further test the phase diagram suggested here.

Finally, our investigation suggests that the high-temperature pseudogap is distinct from the well-dened nematic order we observe here. Both nematic and charge-stripe ordering tendencies are most prominent around 1/8-doping, whereas the pseudogap line has a different doping trend and occurs at signicantly higher temperatures in the underdoped region. However, nematic effects have repeatedly been observed to appear together with the pseudogap2224, implying that a nematic component is present in the pseudogap state. Thus, within the scenario of Fig. 5, the pseudogap might be related to (or at least interact with) the high-temperature loop liquid, which is close to a nematic instability. More broadly, our nding of the unconventional nematic phase in LESCO provides a connection to other materials such as pnictides and demonstrates the power of using complementary experimental techniqueslocal probes such as NQR and unconventional transport measurements, such as nonlinear conductivityin detecting strange metallic phases. This could be employed in answering fundamental questions in diverse correlated electronic systems, including heavy fermions and quantum magnets.

Methods

Samples. For all experiments, high-quality single crystals of LESCO were used, with typical dimensions of a few mm3. They were grown by Udo Ammerahl by the travelling solvent oating zone technique under 3 bar oxygen pressure in the group of A. Revcolevschi, Laboratoire de Chimie des Solides, Universite Paris-Sud. The x- and y-values were determined by energy-dispersive X-ray analysis and were consistent with the nominal values. The same crystals were used in different previous investigations, including NMR32, X-ray5,30,31 and thermal transport/ Nernst effect52 studies.

NMR and NQR analysis. In obtaining the wipeout fractions, spinspin relaxation measurements were performed at the peak of the 63Cu A lines in LESCO. The entire lineshapes were not integrated, as it is known from previous work13 that they do not appreciably change in the temperature range 20200 K important for us. The Cu NQR lineshape remains the same all the way from TLTT down to very low temperatures, where it changes drastically due to spin order13. To additionally check this fact, we made an NMR measurement of the 63Cu quadrupolar satellite of LESCO-1/8 in an in-plane magnetic eld of 11T (Supplementary Fig. 3). The spectrum (and thus NQR parameters and linewidth) changes very little from 60 to 110 K, justifying our assumption of temperature-independent NMR/NQR lineshapes.

Spinspin relaxation was compensated, for using purely exponential ts, as we did not notice any signicant departures from exponential relaxation in all samples in the relevant temperature range. Supplementary Fig. 1 shows selected NQR spinspin relaxation curves in three LESCO samples, whereas the NMR measurements on the line shown in Supplementary Fig. 3 is in Supplementary Fig. 2. The signal-to-noise ratio in the NQR measurements is relatively poor due to skin depth effects and comparatively low frequency of B36 MHz. NMR results (at B140 MHz) have a much better sensitivity and hence smaller error bars in the wipeout determination. Within our experimental accuracy, we nd no Gaussian components in the spinspin relaxation in either NMR or NQR. Despite the low signal-to-noise in LESCO x 0.11 and x 0.125, the wipeout below TEN is large

enough to be reliably resolved. The NMR measurement conrms its existence with much better precision. Wipeout in Cu NMR for x 0.125 and NQR for x 0.15

was measured up to 170 K, to check whether the LTT transition inuences the wipeout fraction. No such inuence was found, in agreement with all previous investigations. The structural transition is close to 130 K in the entire investigated

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doping range31,40 and thus cannot inuence the wipeout step. Owing to high conductivity, the skin depth in LESCO is below 10 mm (at the NQR frequency of B36 MHz) in all investigated samples, making the NQR signal small. We stress that the changes in conductivity at MHz frequencies are of the order of 10% in our temperature range, too small for the change in skin depth to account for the modied NQR signal.

Nonlinear conductivity. The nonlinear conductivity was determined using a two coil radiofrequency (RF) inductive method. The setup is similar to the well-known microwave conductivity experiment53,54 but uses RF excitation elds. The sample was mounted on a sapphire rod in a vacuum tube, with two coils wound around the tube and immersed in liquid helium. RF pulses were supplied to one of the coils using an NMR spectrometer, whereas the third harmonic signal was detected on the other coil. The setup and measurement procedure are described in detail in ref. 41. In experiments on LESCO, we have used RF pulses at an excitation frequency of 18.8 MHz, with pulse lengths of 50 ms, repetition time 8 ms and maximum pulse power B100 W. Measurements were performed with the power varying from 10 to 100% full power and the low power points adjusted to match at all temperatures to avoid baseline drift. Error bars reect the accuracy of this adjustment.

Heat capacity. The specic heat of LESCO was measured using a sensitive differential thermal analysis (DTA) setup55. The principle of operation is simple: the sample of interest and a reference are mounted on resistance thermometers, which are connected to a large mass platform with weak heat links. If the temperature of the platform is sweeped linearly in time, the temperatures of the sample and reference will lag behind the platform temperature by an amount proportional to their respective specic heats. If the temperature difference between sample and reference is detected, a sensitive differential specic heat measurement can be obtained. This kind of DTA is especially suited for small and sharp features in the specic heata conventional method such as pulse calorimetry does not have the necessary resolution to detect the small features in LESCO-specic heat occurring at TEN. However, exact absolute specic heat values are hard to extract from the

DTA measurement, which is why we only focus on sharp features in the differential heat capacity. The resolution of the method is limited by the resolution of temperature difference determination, which can easily be made very accurate.

In our setup (Supplementary Fig. 6) we use a home-made sample probe inserted either in a variable temperature inset of a superconducting magnet or a liquid helium/nitrogen bath. Platinum resistors are used as temperature sensors in a bridge conguration, with lock-in amplier detection of the unbalance voltage. The sample is mounted on the small-prole platinum resistor with a small amount of Apiezon N grease to increase thermal contact. Samples, thermometers and the heating/cooling platform are held in high vacuum and the temperature of the platform is separately monitored and regulated with a heater and Cernox temperature sensor.

The setup was not calibrated to measure absolute values of Cp (hence, the arbitrary units in Fig. 4), as we are only interested in temperature-dependent features of Cp and not its absolute value. The measurement always includes a smooth background due to sample-specic heat, mismatch between the two platinum chips and possible other imperfections; for us it was important to observe sharp features at the structural transition TLTT and the hidden ordering

temperature TEN; thus, we modelled the background as a simple fourth-order polynomial and subtracted it. The peak at TLTT (which agrees precisely with values from X-ray scattering) is a convenient demonstration of the sensitivity of the method and at the same time clearly shows that the ordering at TEN is distinct from the structural effects in the vicinity of TLTT. We note that the charge ordering at

TCO is not a true phase transition in 214 cuprates due to disorder and thus should not have a sharp signature in the specic heat. It would most probably show up as a low and broad feature below TCO, virtually undetectable with our current setup.

The two DCp measurements shown in Fig. 4 were obtained in different conditions to check the repeatability and resolution of the experiment. The data in the main graph is from a measurement in a liquid nitrogen bath, with open-loop temperature sweep control (that is, a linear sweep of the heater power)this minimizes small temperature oscillations, which inuence the measurement. The typical heating rate in that experiment is B1 K min 1. The data in the inset are from a slow measurement with the entire probe in a variable temperature inset (in helium gas). The system was heated at a rate of B0.1 K min 1, with lock-in lter times of 100 s to increase signal-to-noise. The slow measurement is better suited to determine the sharpness of the transition at TEN, but is also much more sensitive to unwanted temperature oscillations and thus more difcult. It is also clear that no sharp features occur at or near the charge-stripe ordering temperature TCO, as should be expected due to disorder. An additional measurement of the specic heat of the sample with x 0.15 was performed in a nitrogen bath in a

range close to TEN. The results and comparison to the x 0.125 measurement

(obtained in similar conditions) are shown in Supplementary Fig. 5.

Data availability. The data supporting the ndings of this study are available within the article and its Supplementary Information le.

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Acknowledgements

We thank B. Bchner for providing the LESCO single crystals, N. Bonai for assistance with the specic heat measurements and M. Grbi, A. Duli and M.-H. Julien for comments. D.P., M.V. and M.P. acknowledge funding by the Croatian Science Foundation under grant number IP-11-2013-2729 and S.-H.B. acknowledges support by the DFG Research Grant Number BA 4927/1-1.

Author contributions

D.P. conceived the study, performed NQR, nonlinear conductivity and specic heat measurements, analysed data and wrote the paper. M.V. designed and constructed the nonlinear conductivity setup and performed measurements. H.-J.G. selected samples and participated in study design and data analysis. S.-H.B. was involved in designing the study and results analysis. M.P. performed NQR measurements and supervised the project. All authors took part in discussing results and editing the manuscript.

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How to cite this article: Pelc, D. et al. Unconventional charge order in a co-doped high-Tc superconductor. Nat. Commun. 7:12775 doi: 10.1038/ncomms12775 (2016).

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