Signatures of room-temperature superconductivity emerging in two-dimensional domains within the new $\rm{Bi/Pb}$-based ceramic cuprate superconductors at ambient pressure

Remarkable progress in condensed matter physics is often driven by discoveries of new superconducting materials and superfluid (Fermi or Bose) liquids. In particular, the discovery of $\rm{La}$-based high-$T_{c}$ cuprate superconductors [1, 2, 3] and then the dramatic increase of the critical temperature $T_{c}$ of the superconducting transition in other discovered $\rm{Y}$-, $\rm{Bi}$- and $\rm{Hg}$-based cuprate superconductors to above 90 K [4], 110 K [5] and 130 [6] ushered in a new era of physics and technology. These important discoveries stimulated efforts to find new materials with even higher superconducting transition temperatures, possibly even close to room temperature. Various experiments showed [3, 7, 8] that the superconductivity is occurred in the bulk three-dimensional (3D) cuprate material rather than in the $\rm{CuO_{2}}$ plane. Therefore, the above values of $T_{c}$ observed in different high-$T_{c}$ cuprates correspond to the bulk superconducting transition temperatures.

Starting from 1993, there has been considerable interest in the increase of $T_{c}$ in some families of cuprate superconductors to a maximum value at different applied pressures [9, 10, 11] and the record values of $T_{c}\simeq 153-164$ K reported for $\rm{Hg}$-based cuprate superconductors are remained still unchanged. For a long time, despite intense research effors (see Refs.[12, 13, 14, 15, 16]), the increasing of $T_{c}$ up to room temperature in various high-$T_{c}$ cuprates was remained a difficult problem. So far, room-temperature superconductivity was not reported for high-$T_{c}$ cuprate materials even under pressure.

Recently, there has been growing interest in the discovery of room-temperature superconductivity in other classes of materials and the new so-called high-$T_{c}$ hydrides have been synthesized at very high pressures [17, 18, 19]. However, room-temperature superconductors discovered under high pressures will not have a wide-range practical applications. Therefore, in the last decade, new attempts were made to synthesize the promising ceramic cuprate superconductors with the onset temperature of the superconducting transition, $T_{c}^{onset}\gtrsim 150-180$ K at ambient (atmospheric) pressure, by using more advanced technology [20, 21]. Prominent among high-$T_{c}$ cuprate superconductors are the so-called $\rm{Bi/Pb}$-based ceramic cuprate compounds, such as $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ (with $n=2-30$), which may present an alternative path to realizing room-temperature superconductivity at atmospheric pressure.

For many decades, there has been speculation (see, e.g., Refs.[14, 18]) that Little’s model of high-$T_{c}$ superconductivity in one-dimensional organic polymers with polarizable side chains [22] and Ginzbur’s model of two-dimensional (2D) alternating conducting/insulating sandwich layers [23] may be possible routes to room-temperature superconductivity. Such early theoretical predictions are based on the Bardeen-Cooper-Schrieffer (BCS)-like theory of Fermi-liquid superconductivity. Still, most researchers trying to find the new high-$T_{c}$ superconductors make predictions on the basis of the BCS-like and Migdal-Eliashberg theories of Fermi-liquid superconductivity regarding the possibility of increasing the critical temperature $T_{c}$ up to room temperature (see Refs.[15, 16, 17]). However, the high-$T_{c}$ cuprates undergoing a $\lambda$- superconducting transition at $T_{c}$ [24, 25] just like $\lambda$-transition in liquid ⁴He can be in the regime of Bose-liquid superconductivity [26, 27] and the BCS-like theories are incapable of predicting the actual $T_{c}$ in such unconventional superconductors.

In this work, we present theoretical and experimental results of room-temperature superconductivity emerging in different 2D domains within the newly derived $\rm{Bi/Pb}$-based ceramic cuprate superconductors containing many grain boundaries, interfaces and mutiplate blocks. We show that, in these ceramic high-$T_{c}$ materials, besides bulk 3D superconductivity there is also strongly enhanced 2D superconductivity occurring well above the bulk $T_{c}$ in the 3D-2D crossover region. We argue that the unconventional Cooper pairs in high-$T_{c}$ cuprates behave like bosons and condense below certain critical temperatures into 3D and 2D Bose superfluids in 3D and 2D domains. We examine the possibility of the existence of 3D and 2D superconducting phases in ceramic cuprate superconductors predicted by the theory of 3D and 2D Bose superfluids. We find that superconducting transition temperature in 2D domains is much higher than that in 3D domains and the highest critical temperature $T_{c}$ for some high-$T_{c}$ cuprates, e.g., $\rm{Bi/Pb}$-based ceramic cuprate superconductors can reach up to room temperature in 2D domains. We report experimental signatures of room-temperature superconductivity emerging at different grain boundaries and interfaces and in multiplate blocks within the ceramic cuprate superconductors $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ (with $n=2-30$), synthesized by using the new melt technology under the influence of concentrated solar energy. We claim that the occurrence of such a room-temperature superconductivity in these high-$T_{c}$ materials is evidenced by the observations of a sharp drop in the resistance and a pronounced partial Meissner effect at around 300 K and ambient pressure.

The cuprate compounds are typical polar materials and charge-transfer-type Mott-Hubbard insulators [28]. Therefore, in these materials the charge carriers (holes or electrons) introduced by doping in a polar crystal strongly interact with optical phonons [29] and their self-trapping is favorable just like the self-trapping of holes in ionic crystal of alkali halides [30, 31]. In doped high-$T_{c}$ cuprates, the Cooper pairing of self-trapped carriers (polarons) results in the formation of tightly-bound (bosonic) Cooer pairs [32]. These unconventional Cooper pairs in 3D high-$T_{c}$ cuprates condense into a 3D Bose superfluid below the bulk $T_{c}$ [32, 33]. We now consider the ceramic cuprate superconductors consisting of 3D and 2D domains. Then the actual $T_{c}$ in 3D domains is determined from the self-consisting solutions of the integral equations of a 3D Bose superfluid [34]

where $\gamma_{B}=D_{B}\tilde{V}_{B}\sqrt{\xi_{BA}}$ is the interboson coupling constant in a 3D Bose superfluid, $\rho_{B}$ is the density of attracting (superfluid) bosons, $D_{B}=m_{B}^{3/2}/\sqrt{2}\pi^{2}\hbar^{3}$ is the density of states, $m_{B}$ is the mass of free bosons, $\tilde{\mu}_{B}$ is the renormalized chemical potential of an interacting Bose gas, $\tilde{V}_{B}=V_{BA}-V_{BR}/[1+D_{B}V_{BR}(\sqrt{\xi_{BR}}-\sqrt{\xi_{BA}})]$ is the effective interaction potential between bosons, $\xi_{BA}$ and $\xi_{BR}$ are the cutoff parameters for the attractive $V_{BA}$ and repulsive $V_{BR}$ parts of the interboson interaction potential, $\Delta_{B}$ is the coherence parameter (i.e. superfluid order parameter) of condensed bosons.

The critical temperature $T_{c}^{2D}$ of the superconducting transition in 2D domains is determined from the self-consistent solutions of the integral equations of a 2D Bose superfluid [34]

where $\gamma_{B}=D_{B}\tilde{V}_{B}$ is the interboson coupling constant in a 2D Bose superfluid, $\rho_{B}$ is the density of 2D superfluid bosons, $D_{B}=m_{B}/2\pi\hbar^{2}$.

Numerical solutions of Eqs. (1) and (2) determining the bulk $T_{c}=T^{3D}_{c}$ in 3D superconducting domains can be obtained for arbitrary $\gamma_{B}$. While the approximate analytical solutions of these equations are obtained for $\gamma_{B}<1$ [34]. In particular, the bulk $T_{c}$ at $\gamma_{B}>0.3$ can be determined approximately from the expression

where $T^{*}_{BEC}=3.31\hbar^{2}\rho^{2/3}_{B}/k_{B}m^{*}_{B}$ is the renormalized Bose-Einstein condensation temperature in a 3D Bose-liquid, in which $m_{B}$ is replaced by the renormalized mass $m^{*}_{B}$ of interacting bosons defined similarly in Ref.[35], $c=\pi^{3/2}/3.918$.

The solutions of Eqs. (3) and (4) determining the critical temperature $T^{2D}_{c}$ in 2D superconducting domains are obtained analytically [34] and the superconducting transition temperature at grain boundaries and interfaces and in multiplate blocks within the ceramic high-$T_{c}$ cuprate materials for arbitrary $\gamma_{B}$ is determined from the following relation:

where $T^{*}_{0}=2\pi\hbar^{2}\rho_{B}/k_{B}m^{*}_{B}$.

We use the analytical expressions (5) and (6) to examine the possibility of the existence of distinct 3D and 2D superconducting phases in ceramic high-$T_{c}$ materials and to predict the existence of two distinctly different regimes of high-$T_{c}$ superconductivity in them and the possible route to room-temperature superconductivity. In order to determine the characteristic superconducting transition temperatures in the bulk and the 3D-2D crossover region of high-$T_{c}$ cuprates, we estimate $T_{c}$ and $T^{2D}_{c}$ by assuming that the mass $m_{p}$ of polaronic carriers in 3D domains is of the order of $2m_{e}$ [36] (where $m_{e}$ is the mass of free electrons) and in 2D domains is somewhat larger than $2m_{e}$. It is natural to assume that, in high-$T_{c}$ cuprates, the preformed bosonic (polaronic) Cooper pairs have the mass $m_{B}=2m_{p}$ and only the interacting bosons with the renormalized mass $m^{*}_{B}>m_{B}$ can condense into Bose superfluids at $T\lesssim T_{c}$ (in 3D domains) and $T\lesssim T^{2D}_{c}$ (in 2D domains). By taking $m_{p}\simeq 2m_{e}$, $m_{B}=2m_{p}$, $m^{*}_{B}=1.05m_{B}$ and $\rho_{B}\simeq 4\cdot 10^{19}$ $cm^{-3}$ for 3D superconducting domains in ceramic cuprate materials, we find $T^{*}_{BEC}\simeq 81.7$ K. Using Eq.(5) we further estimate the bulk $T_{c}$ by assuming that two bosonic Cooper pairs interact with each other by means of exchange optical phonons having relatively low energy $\hbar\omega_{0}\simeq 0.03$ eV and the cutoff energy $\xi_{BA}$ for the attractive part of the interboson interaction potential can be replaced by $\hbar\omega_{0}$. Then, at $\gamma_{B}=0.7$ the analytical expression (5) for the bulk $T_{c}$ predicts the value of $T_{c}\simeq 1.508T^{*}_{BEC}\simeq 123$ K. We can now estimate the superconducting transition temperature in 2D domains by assuming that $m_{p}\simeq 3m_{e}$, $m_{B}=2m_{p}$, $m^{*}_{B}=1.05m_{B}$ and $\rho_{B}\simeq 2\cdot 10^{13}$ $cm^{-2}$. Under these assumptions, we obtain $T^{*}_{0}\simeq 176$ K. If we take the same value of $\gamma_{B}=0.7$ for a 2D Bose superfluid in the analytical expression (6), we find $T^{2D}_{c}\simeq 1.105T^{*}_{0}\simeq 195$ K. The above predictions of the theory of Bose-liquid superconductivity in high-$T_{c}$ cuprates indicate that the expected value of $T^{2D}_{c}$ in the 3D-2D crossover region will be much higher than the expected value of $T_{c}$ in the bulk. Therefore, the 3D domains in ceramic high-$T_{c}$ cuprates become non-superconducting above the bulk $T_{c}(=T^{3D}_{c})$, but high-$T_{c}$ superconductivity is still maintained in 2D domains in a wide temperature range above $T_{c}$. Such a remnant 2D superconductivity persisting at grain boundaries and interfaces and in multiplate blocks within the ceramic high-$T_{c}$ cuprate materials might be encouraging in achieving room-temperature superconductivity at atmospheric pressure. In order to make this argument more quantitative, the results of our numerical calculations of the critical superconducting transition temperature $T^{2D}_{c}$ as a function of $\gamma_{B}$ are presented in Fig.1. As may be seen in Fig.1, at some values of $\rho_{B}$ and $m_{B}$, the critical temperature $T^{2D}_{c}$ strongly depending on $\gamma_{B}$ eventually reaches to room-temperature at a certain value of $\gamma_{B}\gtrsim 0.8$.

Here we point out a possibility that many grain boundaries parallel to each other, interfaces and parallel multiplate blocks in specially grown ceramic cuprate superconductors (see, e.g., experimental results presented in Sec.3) will be involved in alternating 3D non-superconducting/2D superconducting sandwich layers above the bulk $T_{c}$. As can be seen in Fig.2, the crossover from the bulk 3D regime to surface 2D regime of superconductivity in these alternating 3D/2D sandwich layers might be possible path to realizing room-temperature superconductivity in such distinctive ceramic high-$T_{c}$ materials. Apparently, there is some experimental evidence for the signatures of superconducting transition well above the bulk $T_{c}$ in different families of high-$T_{c}$ cuprates. In particular, experimental results on the anomalous resistive transitions between 125 and 260 K in the cuprate superconductors $\rm{Y_{3}Ba_{4}Cu_{7}O_{x}}$ (with the bulk $T_{c}\simeq 90$ K) [12] indicate that superconductivity above the bulk $T_{c}$ emerges at grain boundaries in accordance with the prediction of the above expression (6) for $T^{2D}_{c}$. Further, there is also experimental evidence for the signatures of superconductivity at temperatures much higher than the bulk $T_{c}$ in other high-$T_{c}$ cuprates, where resistive transitions were also observed at a temperature of about 260 K [13]. Concurrently, the possibility of traces of superconductivity at temperatures $T>>T_{c}$ in the systems $\rm{HgBa_{2}Ca_{n-1}Cu_{n}O_{2n+2+\delta}}$ and $\rm{Bi_{2}Sr_{2}CaCu_{2}O_{8}}$ was reported [13]. Thus, the above key experimental findings provided the confirmation of the predictions of the theory of 3D and 2D Bose superfluids regarding the possibility of superconductivity well above the bulk $T_{c}$ in $\rm{Y}$-, $\rm{Bi}$-and $\rm{Hg}$-based cuprate superconductors. Among these high-$T_{c}$ materials, some $\rm{Bi}$-based ceramic cuprate superconductors may be the promising systems to investigate the 3D-2D crossover regime of superconductivity, which might be relevant to room-temperature superconductivity. In the following, we report experimental evidence of 2D room-temperature superconductivity occurring in the 3D-2D crossover region within the newly derived $\rm{Bi/Pb}$-based ceramic cuprate superconductors, which contain many grain boundaries, interfaces and multiplate blocks.

The possibility of producing superconducting materials with the highest critical temperature to a considerable extent depends on the methods of their synthesizing. The solid-state reaction method is often used to synthesize the samples of high-$T_{c}$ cuprate superconductors. However, this method has the following shortcomings: the synthesis of samples is the multi-stage process and the preparation of well-textured samples is difficult. In this regard, the melt technologies of sample synthesis are promising in obtaining high-$T_{c}$ cuprate superconductors with higher critical temperature and have certain advantages over other method of synthesizing high-$T_{c}$ superconducting samples. For the synthesis of the samples of high-$T_{c}$ cuprate superconductors, the resistance and induction furnaces, optical furnace, laser and other energy sources were used for melting a mixture of starting materials. In the last years, the new melt technology for synthesizing $\rm{Bi/Pb}$-based ceramic cuprate superconductors were developed by using the concentrated solar energy for melting a mixture of source materials [37]. The advantages of this method in the synthesis of the samples of high-$T_{c}$ ceramic materials have been described elsewhere [20, 21]. The new melting technology based on the use of solar energy allows us to obtain the well-textured ceramic cuprate superconductors $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ (with $n=2-30$), in which the onset of superconductivity occurs at critical temperatures close to room temperature. The samples of these ceramic cuprate superconductors were prepared by a melt process in a large solar furnace (in Parkent).

Nominal compositions, $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ were prepared from adequate mixtures of precursors $\rm{Bi_{2}O_{3}}$, $\rm{PbO}$, $\rm{SrSO_{3}}$, $\rm{CaO}$ and $\rm{CuO}$. Mixtures of these starting source materials were then put in a solar furnace and melted by concentrated solar radiation with a radiant flux of about 420-480 $W/cm^{2}$ in this furnace, which is shown in Fig.3. The appropriate temperature gradient in the melt process is of order 1480 ⁰C. The samples were prepared in the form of bars or plates with the size $5\times 5\times 45mm$ and in the form of disks with a diameter of $14-26mm$ and a thickness of $2.3-5.4mm$. The microstructure of the prepared samples were studied using the microscopy solver Next, SEM and electron microscopy Zeiss. Phase analysis of the samples was performed by $X$-ray diffraction (DRON UM-1) technique (using $\rm{CuK_{\alpha}}$ radiation and $\rm{Ni}$ filler). Samples of the new $\rm{Bi/Pb}$-based cuprate superconductors contain many grain boundaries parallel to each other, twin boundaries, interfaces and multiplate blocks (see Fig.4).

According to the $X$-ray diffraction data and observed resistive transitions, the synthesized samples of $\rm{Bi/Pb}$-based cuprate superconductors are multiphase materials, where different superconducting phases are formed at different critical temperatures. In particular, 3D superconducting phases with different bulk $T_{c}$ values are observed in the temperature range $100K\lesssim T_{c}\lesssim 130K$. Analyses of the microstructure of ceramic samples using electron microscopies showed that the new superconducting systems $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ consist of many small and large grains (i.e. nanocrystals), twin domains and closely packed parallel multiplate blocks, as shown in Fig.4. The microstructure of the chip of ceramic samples obtained from glass-crystalline plates and annealed at temperatures $T\simeq 843-850$ ⁰C for 3-94 h represent closely packed multilamellar blocks, which contain coupled stacks of many quasi-2D parallel layers with nano-sized thickness.

The samples of $\rm{Bi/Pb}$-based ceramic cuprate superconductors were used to study the superconducting properties in a wide temperature range from 77 to 300 K. The critical temperatures of the superconducting transitions were determined by the resistance-temperature measurements carried out using the standard four-probe technique with silver past contacts. The contact resistance was far less than $1\Omega$ in each case. The superconducting transition temperature in a series of the studied samples is defined by the maximum slope of the resistive transition in the resistance-temperature curves. Fig.5 shows the resistance-temperature dependence of the synthesized sample of $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ ($\rm{Bi/Pb}-2245$), where the onset of superconductivity in 2D domains seemingly occurs at the temperature $T^{onset}_{c}\simeq 210$ K well above the bulk $T_{c}\simeq 120$ K. In Fig.6, we show the resistance-temperature dependence for the synthesized sample of $\rm{Bi/Pb}-2267$. For this sample, the onset of superconductivity is observed at room temperature $T^{onset}_{c}\simeq 395$ K and a sharp step-like drop in the resistance is observed at around this temperature. The onset of superconductivity in the studied sample of $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ ($\rm{Bi/Pb}-221920$)occurs at $T^{onset}_{c}\simeq 310$ and a sharp drop of the resistance is observed at around this temperature (see Fig.7). Fig.8 shows the resistance-temperature curve for the synthesized sample of $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ ($\rm{Bi/Pb}-222930$). First, the resistance of this sample increases slightly from 350 K down to 295 K and then a sharp drop in the resistance is observed around the temperature $T^{onset}_{c}\simeq 295$. The superconducting transition exhibits a step at about 295 K.

Upon lowering the temperature, other superconducting transitions in the studied samples of $\rm{Bi/Pb}-2245$, $\rm{Bi/Pb}-2267$, $\rm{Bi/Pb}-221920$ and $\rm{Bi/Pb}-222930$ are observed at $T_{c}\simeq 100-130$ K. Our experimental results indicate that the bulk 3D superconductivity in these $\rm{Bi/Pb}$-based ceramic cuprate materials occurs only at more lower temperature than the onset temperature of 2D superconductivity emerging at different grain boundaries and 3D/2D interfaces and in multiplate blocks. The superconducting transition in the multiphase superconductors $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ occurs first at about room temperature and is manifested as a step-like resistive transition in the resistance-temperature curve. Such a resistive transition is indicative of the presence of more than one superconducting phase and the persistence of remnant 2D superconducting phase in these ceramic materials above the bulk $T_{c}$ up to room temperature. Here we notify that the 3D and 2D superconducting phases coexist below the bulk $T_{c}$ and the ceramic samples of the $\rm{Bi/Pb}$-based cuprate superconductors below this critical temperature predominantly consist of 3D superconducting phase.

Another key criterion for emerging of room-temperature superconductivity in the synthesized samples of $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ is the detection of the presence of a pronounced Meissner effect below a characteristic temperature of the step-like resistive transition occurring at around 300 K. We detected an incomplete Meissner effect in the most samples of these ceramic superconducting materials at room temperature.

In Fig.9, we illustrate the detection of such a well-defined Meissner effect, that is, the expulsion of magnetic flux from the $\rm{Bi/Pb}$-based ceramic cuprate superconductors at room temperature. Upon lowering the temperature, fractional Meissner effect in these materials increases gradually and remained always smaller than 100% even below the bulk $T_{c}$, very likely due to the presence of the insulating phase in underdoped and even in optimally doped high-$T_{c}$ cuprates [38, 39, 40]. In insulating domains, the Cooper pairs do not exist in $k$-space any longer and become localized pairs (i.e. bipolarons) in real space [41], as observed in $\rm{Bi}-2212$ [42]. The variations of the Meissner effect in ceramic high-$T_{c}$ cuprate superconductors reflect variations of the insulating volume and superconducting volumes of 3D and 2D domains. One can assume that a partial superconducting volume of 2D domains in the studied samples of $\rm{Bi/Pb}$-based cuprate superconductors and related partial Meissner effect detected at room temperature varies from 5% to 25%.

We have presented the theoretical and experimental results concerning the possibility of the existence of 3D and 2D superconducting states in specially synthesized multiphase ceramic cuprate superconductors $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$ (with $n=2-30$). These newly derived high-$T_{c}$ materials consist of 3D and 2D superconducting domains and are particularly interesting from the viewpoint of novel superconducting properties and room-temperature superconductivity of such systems. We have argued that the high-$T_{c}$ cuprates are unconventional (bosonic) superconductors, in which the tightly-bound (poloronic) Cooper pairs behave like bosons and condense into 3D and 2D Bose superfluids in 3D and 2D domains. We have shown that, in ceramic high-$T_{c}$ cuprate superconductors, besides bulk 3D superconductivity there is also strongly enhanced 2D superconductivity emerging in the 3D-2D crossover region and persisting well above the bulk $T_{c}$. We have examined the possibility of the existence of distinctly different 3D and 2D superconducting phases in these materials predicted by the theory of Bose-liquid superconductivity. We have determined the critical superconducting transition temperatures in 3D and 2D systems, which depend on the effective mass of interacting bosons, the density of superfluid bosons and the interboson coupling constant. We have found that the superconducting transition temperatures in bosonic cuprate superconductors is much higher in 2D domains than in 3D domains and the bulk superconductivity is destroyed above $T_{c}(=T^{3D}_{c})$, but 2D superconductivity persists up to the onset temperature $T^{onset}_{c}=T^{2D}_{c}>>T^{3D}_{c}$. We have predicted the possibility of realizing room-temperature superconductivity at different grain boundaries and 3D/2D interfaces and in multiplate blocks within the new ceramic cuprate superconductors. Apparently, some families of such cuprate superconductors synthesized by using advanced technologies might be particularly promising in the experimental search for room-temperature superconductivity at ambient pressure. We have used the new technology for synthesizing the ceramic cuprate superconductors with very high critical temperatures, ever observed previously, and finally discovered room-temperature superconductivity in the $\rm{Bi/Pb}$-based ceramic high-$T_{c}$ materials $\rm{Bi_{1.7}Pb_{0.3}Sr_{2}Ca_{n-1}Cu_{n}O_{y}}$, synthesized by using the new melt technology in a large solar furnace, with the highest critical temperatures $T^{onset}_{c}\simeq 295-310$ K at ambient pressure. The samples of $\rm{Bi/Pb}$-based cuprate superconductors contain grain boundaries parallel to each other, twin boundaries and interfaces and multilamellar blocks represening the alternating 3D non-superconducting/2D superconducting sandwich layers above the bulk $T_{c}$, which are favorable for realizing room-temperature superconductivity. The remnant 2D superconductivity persisting at different grain boundaries and 3D/2D interfaces and in multilamellar blocks up to high temperatures $T=T^{onset}_{c}\simeq 295-310$ K and the onset of room-temperature superconductivity in the synthesized samples of $\rm{Bi/Pb}$-based ceramic cuprate materials are evidenced by the observations of a sharp step-like drop in the resistance and a pronounced partial Meissner effect at around 300 K. Thus, by the use of the series of $\rm{Bi/Pb}$-based ceramic cuprate materials, we have succeeded in finding the new class of room-temperature superconductors at ambient pressure.

We would first like to thank B.S. Yuldashev for his attention and encouragement in the process of carrying-out this work. We would also like to thank our colleagues I. Khidirov, B.L. Oksengendler, U.T. Kurbanov and E.Kh. Karimbaev for valuable discussions and suggestions. This work was support by the Foundation of the Fundamental Research, Grant No $\Phi$-$\Phi$A-2021-433.